Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Viroporins: structure and biological functions

Key Points

  • Viroporins are small virally encoded hydrophobic proteins that oligomerize in the membrane of host cells, leading to the formation of hydrophilic pores. This activity modifies several cellular functions, including membrane permeability, Ca2+ homeostasis, membrane remodelling and glycoprotein trafficking.

  • A classification scheme for viroporins is proposed on the basis of their structure and membrane topology. Thus, class I and class II viroporins are defined according to the number of transmembrane domains in the protein (one and two, respectively), and subclasses are defined according to their orientation in the membrane.

  • The main function of viroporins during viral replication is to participate in virion morphogenesis and release from host cells. In addition, some viroporins are involved in viral entry and genome replication.

  • The structure and activity of several viroporins, such as picornavirus protein 2B (P2B), influenza A virus matrix protein 2 (M2), hepatitis C virus p7 and HIV-1 viral protein U (Vpu), have been analysed in detail.

  • New members of this expanding family of viral proteins have been described, from both RNA and DNA viruses. In addition to having a common general structure, all of these new viroporins have the ability to increase membrane permeability.

  • Viroporins represent ideal targets to block viral replication and the spread of infection. Although a number of selective inhibitors of viroporin ion channels have been analysed in detail, optimized screening systems promise to provide new and more potent antiviral compounds in the near future.


Viroporins are small, hydrophobic proteins that are encoded by a wide range of clinically relevant animal viruses. When these proteins oligomerize in host cell membranes, they form hydrophilic pores that disrupt a number of physiological properties of the cell. Viroporins are crucial for viral pathogenicity owing to their involvement in several diverse steps of the viral life cycle. Thus, these viral proteins, which include influenza A virus matrix protein 2 (M2), HIV-1 viral protein U (Vpu) and hepatitis C virus p7, represent ideal targets for therapeutic intervention, and several compounds that block their pore-forming activity have been identified. Here, we review recent studies in the field that have advanced our knowledge of the structure and function of this expanding family of viral proteins.


The prevalence of pathogenic viruses and the shortage of effective treatment options for the diseases that they cause warrant further research into the basic physiology and pathogenic mechanisms of these viruses. Over the past decade, an expanding viral protein family has become the subject of much interest owing to its central role in the viral life cycle. These proteins, termed viroporins, are involved in viral genome replication and assembly, as well as virus particle entry into and release from infected cells. The functional activities of these proteins also have a profound impact on the host cell, disrupting a number of important physiological processes. Viroporins are encoded by a range of animal viruses of clinical interest, such as hepatitis C virus (HCV), HIV-1, influenza A virus (IAV), coronaviruses, picornaviruses such as poliovirus, and togaviruses1,2,3,4,5,6.

The existence of viroporins was initially suggested following the observation that virus-infected cells become permeable to ions and small molecules7. One possibility put forward at that time implicated the exiting virus particles themselves as the permeabilizing agent7,8. However, cloning and expression of selected viral genes in animal cells supported the idea that these viruses encode at least one protein that could permeabilize cellular membranes9,10,11,12,13. On the basis of such observations, the idea of a viroporin family of proteins was subsequently accepted1.

The discovery of viroporins and their common and unique structural and functional characteristics have been reviewed previously2. Viroporins share several features, including their small size and hydrophobicity as well as their ability to permeabilize membranes on oligomerization. In general terms, their main function is participation in the assembly of virus particles and their release from infected cells2. Typically, deletion of a viroporin-encoding gene from a viral genome drastically diminishes the formation of viral progeny and viral pathogenicity, underscoring the essential role of these proteins in the viral life cycle2.

In its infancy, viroporin research concentrated on a limited number of proteins, such as picornavirus protein 2B (P2B)10,11,13,14, IAV matrix protein 2 (M2)15,16,17,18 and togavirus protein 6K19,20,21. More recently, the excellent research on HIV-1 viral protein U (Vpu) and HCV p7 has greatly expanded our knowledge of the structure and function of these viroporins3,4,22,23,24,25,26,27,28,29,30, and we are only now beginning to appreciate the diversity of this protein family. Indeed, the architecture of the pores or ion channels that are formed varies between viroporins, as do the sequences of the protein regions that protrude from the pores, suggesting that each viroporin can interact specifically with other viral or cellular proteins2 (Fig. 1). Furthermore, the effects on cellular metabolism vary widely according to the viroporin considered2,3,5,6.

Figure 1: Classification of viroporins according to the number of transmembrane domains and the membrane topology of the constituent monomers.

a | Class I viroporins have a single membrane-spanning domain. The A and B subclasses contain proteins that are inserted into the membrane with either a lumenal amino terminus and cytosolic carboxyl terminus (class IA) or a cytosolic amino terminus and lumenal carboxyl terminus (class IB). In addition, class IA members are usually phosphorylated at the C terminus. Known viroporins of each subclass are shown. b | Class II viroporins form helix–turn–helix hairpin motifs that span the membrane. Subclass A members have lumenal N and C termini, whereas members of subclass B have cytosolic N and C termini. Known viroporins of each subclass are shown. CoV, coronavirus; E, envelope small membrane protein; HCV, hepatitis C virus; HRSV, human respiratory syncytial virus; IAV, influenza A virus; M2, matrix protein 2; PV, poliovirus; SH, small hydrophobic protein; SV, Sindbis virus; Vpu, viral protein U.

The central role of viroporins in promoting viral pathogenicity has stimulated particular interest owing to their potential as targets for antiviral agents4, and a number of compounds that interfere with the permeabilizing ability of viroporins and thereby inhibit virus production have been identified5,15,18,24,25,27,31,32,33. Moreover, viroporin-defective viruses are being explored as live attenuated vaccines34. In this Review, we focus on viroporins from RNA viruses — although these proteins are also encoded by a number of DNA viruses (Box 1) — and highlight recent findings pertaining to the classification, structure and mode of action of viroporins as well as their potential as antiviral targets.

Classification by architecture

Over the past few years, substantial advances have been made in our understanding of viroporin architecture. Current hypotheses suggest that viroporins form aqueous channels following insertion into the host cell membrane and subsequent oligomerization. We propose that viroporins be classified into two major groups, class I and class II, depending on whether they contain one or two transmembrane (TM) domains, respectively (Fig. 1). These classes can be further divided into subgroups according to membrane topology. Class IA viroporins contain a small (9–25 amino acids) domain at the amino terminus, which resides in the ER lumen, and a long cytosolic tail (50 amino acids) at the carboxyl terminus, which is prone to phosphorylation18,35. The class IB viroporins contain a short N terminus that is found in the cytosol and a long C-terminal tail that is located in the ER lumen. Class II viroporins have two TM domains that are connected by a loop of basic amino acids. The N and C termini of class IIA viroporins both face the ER lumen, whereas in class IIB viroporins the opposite orientation is adopted, with both termini located in the cytosol. Class II viroporins are usually derived from larger precursors by proteolytic processing2, and both termini form part of the peptidase cleavage site. A putative third class of viroporins, for those that contain three hydrophobic domains, has been proposed; this class would include the coronavirus protein 3a36, but further investigation is required to confirm that this protein is a genuine viroporin.

Viruses that are deficient in viroporins can be rescued in trans by their own viroporin and, in some instances, can be partially rescued by other, unrelated viroporins, although opposing results have been reported, probably owing to differences in the experimental approaches used32,37,38. However, in general, the interchange of viroporins between different species of animal viruses fails to completely rescue viral replication38, supporting the idea that these proteins interact in a specific manner with other viral or cellular proteins. In some instances, chimeric viroporins can promote viral release; for example, the TM domain of HIV-1 Vpu can be functionally substituted by the corresponding domain of IAV M2 (Ref. 39).

Finally, linking specific functional activities to either the aqueous channels or the cytosolic domain (or domains) of each viroporin is challenging. Throughout this Review, we make this distinction when possible, but it is important to note that this information is often unavailable. The use of selective inhibitors of these ion channels, as well as detailed analyses of viroporin variants, should help resolve this issue.

Cytopathic effects of viroporins

Membrane permeability and calcium homeostasis. Modification of host cell membrane permeability is a common feature of infections with most animal viruses1,2. Notably, transgenic expression of the individual viroporin genes of these viruses in host cells mimics this phenomenon1,2,40, and this has greatly facilitated the identification of new viroporins. In addition, transgenic expression of viroporins in bacterial, yeast and mammalian cells has helped in analysing their mode of action, as their expression increases the membrane permeability of all these cell types2,9,14. Even neighbouring cells that do not express the viroporin become indirectly permeabilized in cell culture41. Purified viroporins also have the ability to form ion channels in planar lipid bilayers or in liposomes6.

A range of assays are available to analyse changes in membrane permeability, including assays that are based on the entry of ions and small molecules into cells or their release into the culture medium. One of these assays — patch clamping of Xenopus laevis oocytes after microinjection of viroporin mRNAs — provided the first evidence of ion channel formation by IAV M2 (Ref. 15). Another approach, the hygromycin B test1, has become a widely applied, easy-to-use assay for assessing membrane permeability42,43,44,45,46. Such assays have revealed that compounds below a molecular mass of 800–1,500 Da enter cells or liposomes containing viroporins47. In addition, ions are redistributed according to their concentration gradient, leading to depolarization as a result of disruption of the membrane potential2 (Fig. 2a).

Figure 2: Cytopathic effects of viroporins and their functions during the viral life cycle.

The main host cell organelles that are targeted by viroporins, and the cytopathic effects that are induced by viroporins and their functions during the viral life cycle are represented. a | Alteration of plasma membrane potential. Viroporins that are located at the plasma membrane can dissipate the ionic gradient across the membrane, leading to depolarization. b | Alteration of cellular Ca2+ homeostasis. The poliovirus (PV) viroporin protein 2B (P2B) assembles pores in the ER membrane and induces the release of Ca2+ from the ER lumen into the cytosol. Uptake of Ca2+ by the mitochondria can lead to dissipation of the inner-mitochondrial-membrane potential (ΔΨm), permeabilization of the outer mitochondrial membrane and, finally, the release of cytochrome c. In the cytosol, cytochrome c promotes the formation of the so-called apoptosome, a molecular platform that is involved in apoptosis. c | Certain viroporins, such as P2B, polyprotein P2BC and P3A from PV or envelope small membrane protein (E) from coronavirus (CoV), induce intracellular membrane remodelling to generate new membrane vesicles (called the viroplasm) that serve as viral replication sites. d | Dissipation of the proton gradient in the Golgi and the trans-Golgi network. The viroporins influenza A virus (IAV) matrix protein 2 (M2) and hepatitis C virus (HCV) p7 reduce the acidification of vesicular acidic compartments by equilibrating the proton concentration with the cytosol. Alteration of the intracellular ionic gradient in the vesicular system impairs glycoprotein trafficking. e | During the viral replication cycle, viroporins play an essential part in assembly, budding and release of the viral progeny. ARV, avian reovirus; [Ca2+]i, intracellular Ca2+ concentration; HRSV, human respiratory syncytial virus; JCV, JC polyomavirus; NSP4, non-structural protein 4; RV, rotavirus; SARS-CoV, severe acute respiratory syndrome CoV; SH, small hydrophobic protein; SV, Sindbis virus; Vpu, viral protein U.

Viroporin-induced membrane permeabilization to ions and/or small solutes can occur through the formation of gated channels or size-limited pores2,48. For instance, the membrane-spanning IAV protein M2 forms tetrameric channels that selectively conduct protons49. Similarly, HIV-1 Vpu oligomers can function as ion-conducting channels50 that open in a voltage-dependent or voltage-independent manner51. When expressed in bacteria and mammalian cells, these proteins also induce a certain degree of membrane permeabilization to small solutes52,53. Weak ion selectivity together with the capacity to induce leakage of solutes into and out of the cell across a sealed membrane (known as pore–channel dualism48,51) have been reported for other viroporins, such as HCV p7 (Refs 52, 54), togavirus 6K19,20 and coronavirus envelope small membrane protein (E)55,56. Viroporin activity is probably regulated by differences in the components and structures of organellar and plasma membrane lipid bilayers48,57. As the helices of viroporins form integral oligomeric bundles, they can adapt to changes in the lipid bilayer thickness or to bilayer deformations by tilting and/or bending. The ensuing pore opening and closing, coupled with helix motions, constitute the gating mechanism57.

Another important aspect of viroporin function is the resultant increase in the intracellular Ca2+ concentration ([Ca2+]i) after entry of extracellular Ca2+ and/or Ca2+ leakage from intracellular stores, such as the mitochondria, ER and Golgi complex12,58,59,60 (Fig. 2b). An increased [Ca2+]i is observed when several viral proteins are expressed, including picornavirus P2B and rotavirus non-structural protein 4 (NSP4)11,60,61,62. Ca2+ is an important regulator of many cellular enzymes and processes, and an increase in [Ca2+]i has substantial repercussions for many cellular and viral processes63, including viral morphogenesis. An increased [Ca2+]i can also induce apoptosis64, a phenomenon that is observed when various viral proteins, including viroporins, are expressed in host cells52,58,65. Furthermore, many viral proteins interact with Ca2+, which modulates their function. In this respect, Ca2+ regulates viral gene expression, post-translational processing of viral proteins, and virion maturation and release63. In conclusion, the alteration in [Ca2+]i that is observed after infection with most animal viruses modulates several viral functions and plays a part in host cell death.

Membrane remodelling and glycoprotein trafficking. To promote their own proliferation, animal viruses that replicate their genomes in the cytoplasm can induce profound remodelling of intracellular membranes. Thus, many animal viruses induce the formation of new cytoplasmic vesicles in which genome replication takes place66. For several viruses, including picornaviruses, inhibition of the vesicular system by brefeldin A or by inhibitors of phospholipid synthesis abrogates viral genome replication40. In general, viroporins are not necessary for viral genome replication; however, the rotavirus and picornavirus viroporins are exceptions. Indeed, transgenic expression of some picornavirus proteins, such as P2B, P2C and in particular their polyprotein precursor, P2BC, leads to remodelling of the membrane and formation of the 'viroplasm' (Fig. 2c), a multivesicular body that is generated from the ER14,61,67,68,69. The double-membraned vesicles of the viroplasm are necessary for the replication of viral RNA and have an autophagic origin61,67. Co-expression of picornavirus P2BC and P3A leads to the formation of intracellular vacuoles that are morphologically similar to those observed in picornavirus-infected cells61. Whether these membrane-remodelling events are due to ion channel formation, are mediated by another aspect of viroporin function or are dependent on interactions with cellular proteins is currently unknown.

The pioneering work of Doedens and Kirkegaard10 revealed that the transgenic expression of poliovirus P2B or P3A not only increases membrane permeability to hygromycin B but also interferes with trafficking of vesicular stomatitis virus (VSV) glycoprotein. In cells that are infected with enveloped viruses such as VSV, viral structural glycoproteins travel through the vesicular system accompanied by viroporins21. Trafficking of viral and cellular glycoproteins can be delayed by viroporins such as IAV M2 or HCV p7 through a monensin-like activity16,32. Monensin is an ionophore that promotes proton redistribution in a manner that prevents acidification of intracellular compartments. This loss of acidification can prevent structural modifications or rearrangements of viral glycoproteins. Indeed, HCV p7 and IAV M2 reduce the acidification of intracellular vesicles and even cellular organelles16,32 (Fig. 2d). Thus, in some instances viroporin-mediated inhibition of intracellular-vesicle acidification may be important for the production of infectious viral particles32. Finally, HIV-1 Vpu also hampers trafficking of several cellular proteins, including tetherin (also known as BST2) and the receptor CD4, promoting their degradation by lysosomes (tetherin) or by the proteasome (CD4)70,71,72. Vpu impedes the trafficking of tetherin by interfering with protein sorting at the trans-Golgi network (TGN), redirecting the protein from the cytoplasmic membrane to endolysosomal compartments72. Both the TM and cytosolic domains of Vpu are involved in directing the degradation of these cellular proteins23,35,70,72 (see below).

Viroporins and the viral life cycle

Although viroporins participate in different steps of the viral life cycle, such as cell entry and genome replication, the main activity of viroporins is their involvement in virion assembly and the release from infected cells (Fig. 2e). Viruses that are defective in viroporins are unable to accomplish proper assembly and release from cells26,27,36,46,70,73,74.

In the case of non-enveloped animal viruses, the progressive increase in membrane permeability as a result of viroporin accumulation during infection leads to cell lysis and the subsequent release of the assembled virus particles1,46. The last step in the maturation of enveloped viruses is the budding of virus particles from the plasma membrane (for IAV, togaviruses and lentiviruses) or from intracellular vesicles (for flaviviruses and coronaviruses). In the case of both enveloped and non-enveloped viruses, the insertion of viroporins in the membrane leads to the dissipation of membrane potential. We propose that viroporin insertion breaks the chemoelectrical barrier by conducting ions across membranes, thus dissipating the membrane potential of the plasma membrane or of internal vesicles and thereby stimulating budding21 (Fig. 3). Therefore, in enveloped viruses, the dissipation of membrane potential can be coupled to viral release. In the process of budding, membrane fission begins with the formation of a membrane neck, which is followed by self-merging of the inner monolayer of the neck. Subsequent self-merging of the outer monolayer of the neck results in the virus particle pinching off from the plasma or vesicle membrane. Membrane depolarization is accompanied by a reduction in the surface charge density of the membrane, and it has been argued that this reduces the repulsion between contacting monolayers and therefore enhances the contacts that are required for fission75 (Fig. 3). In this manner, the production of viruses and vesicles, even those of non-viral origin, is promoted75,76. IAV M2 increases proton conductance across the membrane and may act as a Na+ antiporter77,78,79; however, there is no pH gradient at the plasma membrane, which is the site of IAV budding. M2 localizes at the neck of budding virions and is necessary for this process80.

Figure 3: Model of a viroporin promoting viral budding at the plasma membrane.

Viroporins localize at the plasma membrane in specific sites surrounding the neck of the budding virus particle, as described for the influenza A virus matrix protein 2 (M2) viroporin80. Viroporins alter membrane permeability by conducting the flux of different ions (for example, Na+ and K+) across the membrane in favour of their concentration gradients and so reducing the transmembrane potential, which is essentially determined by three factors: the concentration of ions inside and outside the cell; the permeability of the cell membrane to those ions (that is, the ion conductance) through specific ion channels; and the activity of electrogenic pumps (for example, the (Na++K+)ATPase and Ca2+ transport pumps) that require energy to maintain the ion gradients across the membrane. Depolarization of the membrane (that is, decreasing the imbalance of charges across the membrane) leads to a reduction in the charge density on the membrane surface. This phenomenon would result in a decrease in the electrical or contact repulsion between opposing monolayers of the membrane at the neck of the budding site (middle) and could provide the stimulus and even the energy to locally promote budding and release21,75, as occurs in depolarization-dependent exocytosis76.

There are reports linking IAV M2 to viral entry15,17. However, during entry, membrane pores are usually formed by viral glycoproteins or capsids, depending on whether the virions are enveloped or not. The formation of pores in endosomes dissipates the proton gradient and propels viral capsids into the cytosol, and we suggest that it is unlikely that the majority of known viroporins participate in this process81. In support of this, studies have shown that the entry and infectivity of IAV and HCV are not hindered by the presence of a defective M2 viroporin or in the absence of p7 viroporin, respectively (Refs 27, 32, 82). Furthermore, chimeric viroporins bearing the M2 TM domain are sensitive to rimantadine, an adamantane derivative that blocks viral budding but not entry into cells39. Conversely, a study that examined p7 inhibitor compounds demonstrated that HCV entry is partially impeded in the presence of some of these compounds, suggesting that HCV requires p7 for entry into the cell83. Although the data are conflicting, we propose that viroporin-mediated permeabilization is generally not required for viral entry, as this is induced by virus particles. Thus, the cell permeabilization that is induced by virus particles appears to have a different function to the permeabilization functions of viroporins81,84.

Pore-forming activity of viroporins

Picornavirus P2B. The family Picornaviridae includes several viral species of clinical interest, including poliovirus, human rhinoviruses and hepatitis A virus. These viruses contain a single-stranded RNA genome of positive polarity with a single ORF that encodes a large polyprotein. All of the mature viral proteins are derived from this polyprotein by proteolytic cleavage. The mature picornavirus P2B viroporin, or its precursor, P2BC, is thought to be responsible for Golgi and mitochondrial membrane permeabilization and an increase in plasma membrane permeability to small solutes and ions, both of which effects are observed during the late phase of picornavirus infection10,11,13. As a member of the class IIB viroporins, the helical TM domains of picornavirus P2B are connected by a highly polar short stretch of amino acids, forming a hairpin α–loop–α motif across the bilayer40,47,85,86,87. Research on the biogenesis of viroporins and their mechanisms of membrane insertion has been scarce to date. Recent in vitro translation experiments support the suggestion that poliovirus P2B can be inserted into the ER membrane by the SEC translocon as a helical hairpin with a cytosolic orientation for the N and C termini88. Furthermore, mutagenesis studies of the hairpin motif have demonstrated the functional importance of the hydrophobic-residue distribution that forms a partially amphipathic helix and is followed by an overall hydrophobic TM domain11,13,86,87,89,90. Both of these domains are required for Golgi targeting and plasma membrane permeabilization90.

Membrane-inserted P2B pores were first reconstituted and characterized using purified hybrid maltose-binding protein (MBP)–P2B and large unilamellar vesicles (LUVs)47,85. The observed LUV permeabilization pattern was consistent with the formation of P2B TM pores with a radius of 6 Å, allowing the free diffusion of small solutes (<1 kDa). This pore-forming activity of P2B was therefore reminiscent of the plasma membrane permeabilization process that was detected during the mid phase of poliovirus infection40. In terms of the composition of P2B pores, detergent-resistant tetramers are formed in the presence of lipids47, and additional evidence supports the formation of similar P2B homooligomers at cell membrane surfaces91. The biochemical data agree reasonably well with recent molecular dynamics simulations, which indicate that P2B can assemble in lipid bilayers as stable tetrameric aqueous pores with a diameter of 5–7 Å92.

In the absence of a functional translocation machinery (for example, SEC or TIM), P2B can insert directly into membranes and form pores following a mechanism similar to that used by certain cytolytic toxins93. Work with peptides and recombinant proteins actually suggests that picornavirus P2B functions as an intracellularly delivered toxin during late phases of picornavirus infection94. The N-terminal amphipathic helices promote transfer of the protein from aqueous solution into the membrane and then act as the building blocks of the permeating, oligomeric pore structures that are subsequently assembled94,95. Quantitative assessments of P2B-induced permeabilization indicate that one membrane-bound tetramer can permeabilize a single vesicle with a diameter of 100 nm (105 lipid molecules)85. Such low doses indicate that P2B and its derivatives are capable of forming true TM pores and are devoid of detergent-like membrane-destabilizing activity85,94. The ability of the N-terminal amphipathic helix to effectively conduct ions across natural membranes provides further support for the existence of P2B TM channels94.

HCV p7. HCV is a hepatotropic single-stranded, positive-sense RNA virus belonging to the family Flaviviridae and the genus Hepacivirus. HCV is one of the fastest mutating viruses, and about 170 million individuals throughout the world are believed to be infected96. Persistent infection is associated with a chronic liver disease that often progresses to cirrhosis and hepatocellular carcinoma96. The viroporin p7 consists of 63 amino acids and is encoded at the junction between the structural and non-structural region of a viral polyprotein precursor, which is cleaved by cellular and viral proteases97. p7 is a member of the class IIA viroporins and possesses two TM α-helices that are slightly tilted relative to the membrane bilayer and are connected by a cytosolic loop (residues 33–39) containing a dibasic motif38. In human hepatocarcinoma cells transfected with the full-length genome of a JFH1 (Japanese fulminant hepatitis 1) isolate, p7 localizes primarily to the ER, and a small proportion of p7 is also detected in mitochondria and at the plasma membrane when it is ectopically overexpressed98. Topological analysis of epitope-tagged p7 showed that both the N and C termini are exposed to the ER lumen. However, when p7 is expressed as its precursor, E2p7, the C terminus can adopt a topology in which it faces the cytosol98. The oligomerization of p7 monomers in artificial membranes results in the assembly of cation-selective ion channels that can be specifically blocked by different drugs, such as adamantanes, with varying efficacies4,24,25,99,100 (Table 1). Although p7 does not appear to be required for viral entry or RNA replication, it is needed for polyprotein processing98 and is essential for a late step in viral assembly and release of infectious virions26,27, as confirmed using a trans-complementation assay38. The essential role of p7 in the production of virus particles relies on both its viroporin activity and its interactions with other structural proteins (and possibly with NS2, a non-structural HCV protein)27. Indeed, IAV M2 can compensate for the ion channel activity of a fully intact but inactive p7, but cannot compensate for the activity of a p7 deletion mutant32,38. This discrepancy may be due to functions of p7 that are unrelated to ion channel formation; however, this has not yet been clarified. In hepatoma cells, active p7 channels dissipate the proton gradient of the acidic vesicular compartments, causing leakage of protons into the cytosol. This inhibition of acidification is required for the production of virus particles32.

Table 1 Viroporin inhibitors

IAV M2. Influenza viruses are responsible for acute respiratory disease, leading not only to human suffering but also to economic losses. IAV contains a single-stranded, negative-sense RNA genome that is divided into eight segments. The smallest segment encodes two proteins, M1 and M2, that are synthesized from two different mRNAs generated by alternative splicing. M2 belongs to the class IA viroporins and is a 97 amino acid integral membrane phosphoprotein consisting of three distinct regions: an extracellular 23-residue fragment that is important for incorporation into the virion, a 19–20-residue TM helix and a 54-residue cytosolic tail. The tail includes an amphipathic helix that is involved in cholesterol binding, membrane localization, budding and scission, and its extreme terminus interacts with M1 (Refs 5, 101). The TM helix is essential for tetramerization, proton channel formation and binding to adamantane drugs such as amantadine and rimantadine5. It has been proposed that M2 becomes activated in acidic endosomes during viral entry and triggers viral uncoating by conducting protons to the interior of the virus particle5,18. In spite of this, M2 channel activity is not essential for viral RNA replication in cell culture, but it is necessary for completion of the IAV life cycle in mice82. In some IAV strains, M2 channel activity also equilibrates the pH gradient between the TGN and the cytosol to prevent premature maturation of the viral glycoprotein haemagglutinin17,18. As a consequence of disrupting the ionic balance in the Golgi, M2 activates host inflammasomes, leading to the processing and release of pro-inflammatory cytokines102.

M2 is also required for viral assembly and release; its cytosolic tail in particular participates in genome packaging and facilitates virus production101. Moreover, in IAV strains lacking M2, infectivity is attenuated in both cell culture and mice34. During viral release, M2 is located at the neck of the budding virion. During viral egress, the C-terminal amphipathic helix alters membrane curvature in a cholesterol-dependent manner, and the protein also assists the membrane scission process independently of the host ESCRT (endosomal sorting complex required for transport) machinery80. However, cellular factors that interact with M2 can modulate this function, as is the case for annexin A6, which negatively regulates IAV infection by specifically interfering with the budding process103. It has also been reported that M2 acts as an inhibitor of autophagosome maturation104.

HIV-1 Vpu. HIV is the causative agent of AIDS, which affects approximately 35 million people worldwide. HIV-1, as well as some other primate lentiviruses (but not HIV-2), encodes Vpu, which is translated from a bicistronic mRNA that also encodes viral glycoprotein 160 (gp160)35. Vpu is a class IA viroporin that is mainly located at the ER, TGN, endosomes and, to a lesser extent, the plasma membrane. Depending on the viral isolate, Vpu consists of 77–86 amino acids comprising a short ER-lumenal N-terminal domain of approximately nine residues, a single TM helix and a cytosolic domain containing two shorter α-helices (residues 33–49 and 57–70, approximately)35,70. These two cytosolic helices are separated by a short flexible loop with two conserved Ser residues that are susceptible to phosphorylation by casein kinase II35,70. On oligomerization in membranes, Vpu is thought to form an oligomeric ion channel105. The cytosolic domain is crucial for protein trafficking through the vesicular system3,35,72. Thus, mutations in this region can cause Vpu to exhibit deficiencies in its interactions with other cellular proteins. For example, mutations located in the first α-helix revealed that this region is important for the interaction with CD4 and the subsequent degradation of this receptor35. More recently, an EXXXLV motif in the second α-helix has been shown to be required for efficient tetherin degradation106. In addition, the TM domain of Vpu interacts with cellular proteins, including CD4 and tetherin107,108. The presence of CD4 at the cell surface impedes viral budding, but Vpu induces rapid degradation of newly synthesized CD4 molecules in the ER via the ubiquitin–proteasome system70. In addition, Vpu counteracts tetherin, a host restriction factor that is induced by interferon and strongly inhibits the release of virions from the host cell surface23,30,72,109,110. In this manner, this viroporin antagonizes the innate immune response to facilitate morphogenesis and viral release. Moreover, Vpu may also enhance the release of other viruses, such as Sindbis virus or filoviruses37,50,110,111. Thus, the Vpu cytosolic and TM domains counteract several cellular proteins, and the ion channel activity of the TM domain is responsible for dissipating the membrane potential and disrupting the electrical barrier at the plasma membrane75 (see below).

In addition to the viroporins of the RNA viruses discussed in detail thus far, a number of other RNA viruses encode well-characterized viroporins; however, a detailed account of these proteins is beyond the scope of this review (Box 2).

Viroporin structure

IAV M2. The structures of some viroporins have been determined by X-ray crystallography, solution and solid-state NMR spectroscopy, and electron microscopy (Fig. 4). IAV M2 is the best characterized viroporin in terms of structure, and its TM domain has been studied by X-ray crystallography with sufficient resolution to facilitate mechanistic studies of its proton-conducting function112. In the tetrameric pore, side chains of the highly conserved residues His37 and Trp41 in each helix face the aqueous channel and are crucial for the transport of protons (Fig. 4a). The mechanism of M2-mediated proton conductance has been extensively studied in X. laevis oocytes, mammalian cells, lipid bilayers and vesicles17,18,113. M2 channel activation and proton selectivity are conferred by the four His37 residues, which interrupt the conductance pathway and can be protonated at low pH to enhance proton flow. Below the four His residues, the side chains of Trp41 are clustered at neutral pH and block the proton channel, thereby acting as a gate to facilitate asymmetrical conductance5 (Fig. 4a). A comparison of M2 structures solved at neutral (pH 7.5)77, mildly acidic (pH 6.5)112 and acidic (pH 5)114 pH suggests that the gate of aromatic residues opens at low pH, concomitant with a rearrangement of the TM helices. Accordingly, His protonation correlates with tilting and bending of the TM helices relative to the pore axis112. In addition, the structure of M2 TM helices in complex with adamantanes has been solved31,77,114. High-resolution studies identified a high-affinity binding site for the amantadine inhibitor close to the Ser31 cluster, a pore region surrounded by residues that are mutated in amantadine-resistant viruses114. More recent results from solid-state NMR spectroscopy suggest that amantadine binding physically obstructs the proton channel31.

Figure 4: Three-dimensional structures of selected viroporins.

Each viroporin structure is shown both oriented in the membrane bilayer and from a down-top view. a | High-resolution (1.65 Å) X-ray structure of the oligomers formed by a peptide representing the transmembrane region (residues 25–46) of the class IA viroporin matrix protein 2 (M2) from influenza A virus. Crystals were obtained at pH 6.5 in the presence of N-octylglucoside (Protein Data Bank (PDB) accession 3LBW). In the ribbon representation (right), constituent helices (H1–H4) are indicated, and side chains of His37 and Trp41 are depicted in blue and green, respectively. Structures were generated using Swiss-PdbViewer. b | Solid-state NMR structure of the transmembrane region (residues 2–30) from the class IA viroporin viral protein U (Vpu) from HIV-1, in lipid bilayers (PDB accession 1PI7). The oligomeric form was calculated using energy minimization protocols, and side chains were added to a backbone structure that was generated from solid-state NMR spectroscopy data. Ribbon representations (right) display the constituent helices (H1–H5) and side chains of Ile17 and Trp22 in yellow and green, respectively. Structures were generated using Swiss-PdbViewer. c | Density map contours of oligomers of p7, a class IIA viroporin from hepatitis C virus, solubilized in detergent. Simulated p7 monomers were fitted with their amino and carboxyl termini oriented towards the petal tips29 (right). Structures in part c are modified, with permission, from Ref. 29 © (2009) US National Academy of Sciences.

HIV-1 Vpu. The three-dimensional structure of the Vpu TM helix has been determined by solution and solid-state NMR spectroscopy in micelle and bilayer samples, respectively22 (Fig. 4b). The data confirm that Vpu TM helix monomers comprise slightly kinked α-helices spanning residues 8–25 with an average tilt of 13° with respect to the bilayer normal. Although the structure of a functional, oligomeric Vpu has not been solved to atomic resolution, analytical ultracentrifugation and photochemical crosslinking experiments indicate the presence of a variety of coexisting oligomers ranging at the very least from tetramers to heptamers115. These studies are corroborated by computational modelling of Vpu oligomers, which suggests that the predominant Vpu channel is a pentamer or larger oligomer105,116. Using the three-dimensional structure and its known orientation within the bilayer as a template, oligomeric forms of the Vpu pore were modelled based on energy minimization protocols22. The most stable pentameric structure places Trp22 out towards the lipid headgroups, with a kink at Ile17 directly in the pore (Fig. 4b).

HCV p7. Experimentally solved, high-resolution three-dimensional structures of complete HCV p7 monomers or oligomers are not presently available in the Protein Data Bank (PDB). However, the possible oligomeric state and general organization adopted by this viroporin have been determined by single-particle electron microscopy29. Detergent-solubilized hexamers were subjected to high-contrast staining, which allowed an efficient three-dimensional image reconstruction of the complex. The density map at a resolution of 16 Å, together with immunolabelling using Fab fragments specific for the N and C termini, revealed a conically shaped hexameric channel with protruding petals oriented towards the ER lumen29 (Fig. 4c). The petals comprise the N- and C-terminal sequences, and the overall diameter of the cone ranges from 3.2 nm at its narrowest point to 8.1 nm at its widest point.

Furthermore, high-resolution data pertaining to the region comprising Arg35–Ala63 (that is, a reverting bent helical region that is not implicated in channel formation) were obtained by 1H and 13C solution NMR spectroscopy in trifluoroethanol–water mixtures and have been deposited in the PDB (accession 2K8J)100. New experimental data confirm that the membrane-embedded section of p7 consists of two TM domains connected by a short interhelical basic loop117. Each protein monomer comprises seven distinct segments, including four dynamic helices (spanning residues 6–16, 17–27, 41–49 and 50–57) that differ in their segmental motions. Solid-state NMR analysis has revealed that the protein contains helices that are tilted by 25° and 10° relative to the bilayer normal, with the helix consisting of residues 50–57 being tilted by 10°. The observed smaller tilt angle of the second TM domain (helices 41–49 and 50–57) is consistent with the shorter length of this domain compared to that of the first TM domain (helices 6–16 and 17–27). A shorter TM domain would be expected to require a smaller tilt angle to span the lipid bilayer, whereas the longer (first) TM domain would accommodate a larger tilt angle, possibly in its second helical segment.

Viroporins as therapeutic targets

Viroporins are attractive targets for antiviral therapy, as they are essential for viral release. Furthermore, inhibition of their membrane-permeabilizing activity is possible in both artificial membranes and cell culture systems, thereby facilitating the drug discovery process. Thus, the use of classic viroporin inhibitors, as well as the development of new or highly specific drug derivatives, could potentiate the efficacy of current antiviral treatments (Table 1).

Amantadine was the first drug shown to block IAV uncoating by inhibiting the ion channel activity of M2 (Ref. 118). It was later reported that amantadine interacts directly with HCV p7 in artificial membranes24,119. However, in cell culture amantadine induces only moderate and genotype-dependent inhibition of infectious HCV production83. A major concern regarding the use of amantadine as a monotherapy to treat influenza is the rapid emergence of amantadine-resistant IAV variants120, and adamantanes were also recently shown to select for specific resistance mutations in HCV p7 (Ref. 121). The results of clinical trials suggest that toxicity and resistance explain the failure of amantadine to enhance a sustained antiviral response in patients infected with HCV4,122. Using an alternative approach, recent studies have demonstrated that immunizing mice with an IAV strain that lacks M2 protects against lethal doses of the highly pathogenic H5N1 virus; these results suggest that this virus has potential as a live attenuated vaccine34. Long alkyl iminosugar derivatives, such as N-nonyl-deoxynojirimycin (NN-DNJ) and N-nonyl-deoxygalactonojirimycin (NN-DGJ), inhibit the function of the HCV p7 ion channel in vitro and reduce the production of infectious viruses25,28,121. However, UT-231B, a derivative of NN-DNJ with α-glucosidase inhibition properties, was used in a clinical Phase II study but did not demonstrate satisfactory efficacy123. Amiloride derivatives, especially 5-(N,N-hexamethylene) amiloride (HMA), can also inhibit HCV p7, HIV-1 Vpu and severe acute respiratory syndrome coronavirus (SARS-CoV) E channel activity33,99,124. HMA also impairs HIV budding and SARS-CoV replication. However, the concentration that is required to block the function of the p7 channel is cytotoxic in cell culture83. A new p7 ion channel inhibitor, BIT225 (N-(5-(1-methyl-1H-pyrazol-4-yl)-napthalene-2-carbonyl)-guanidine), is a potential candidate for future treatments against HCV. In a Phase Ib/IIa trial, administration of BIT225 for a week induced a moderate reduction of the viral load in patients infected with bovine viral diarrhoea virus, an HCV-related virus125. It has also been reported that this compound can target HIV-1 Vpu and inhibit viral release from macrophages126. Recently, a classic anion exchanger inhibitor, DIDS (4,4′-diisothiocyano-2,2′-stilbenedisulphonic acid), was reported to partially suppress the chloride conductance that is mediated by the enterovirus P2B viroporin in X. laevis oocytes and to block virus production in human cells127.

Concluding remarks and future prospects

This Review focuses on the biological structure and function of archetypal members of the viroporin family. With the advent of new bioinformatic tools and the availability of infectious clones, the identification of new members of the viroporin family is progressing at an increasing rate. Currently, viroporins are also emerging as new potential targets for clinical intervention. In this regard, the establishment of liposome-based assays and the development of high-throughput screening systems will facilitate the discovery of new and more potent inhibitors of viral infection119,128. We foresee that these systems could be optimized for drug screening, potentially leading to the identification and isolation of new antiviral compounds. Selective inhibitors of viroporins are useful tools to unveil the multifunctional activities of each viroporin during viral infection. Furthermore, advances in molecular biology techniques have facilitated the cloning and high-yield expression of viroporins, thereby allowing the implementation of crystallization protocols114 and NMR techniques to elucidate the structure of these proteins and to aid in their molecular characterization22. We predict that the availability of high-resolution crystallographic pore structures will boost the rational design of specific viroporin inhibitors in the near future.


  1. 1

    Carrasco, L. Modification of membrane permeability by animal viruses. Adv. Virus Res. 45, 61–112 (1995).

    CAS  Article  Google Scholar 

  2. 2

    Gonzalez, M. E. & Carrasco, L. Viroporins. FEBS Lett. 552, 28–34 (2003).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  3. 3

    Hout, D. R. et al. Vpu: A multifunctional protein that enhances the pathogenesis of human immunodeficiency virus type 1. Curr. HIV Res. 2, 255–270 (2004).

    CAS  Article  Google Scholar 

  4. 4

    Griffin, S. D. Plugging the holes in hepatitis C virus antiviral therapy. Proc. Natl Acad. Sci. USA 106, 12567–12568 (2009).

    CAS  Article  Google Scholar 

  5. 5

    Wang, J., Qiu, J. X., Soto, C. & DeGrado, W. F. Structural and dynamic mechanisms for the function and inhibition of the M2 proton channel from influenza A virus. Curr. Opin. Struct. Biol. 21, 68–80 (2011).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  6. 6

    Fischer, W. B. & Hsu, H. J. Viral channel forming proteins—modeling the target. Biochim. Biophys. Acta 1808, 561–571 (2011).

    CAS  Article  Google Scholar 

  7. 7

    Carrasco, L. Membrane leakiness after viral infection and a new approach to the development of antiviral agents. Nature 272, 694–699 (1978). This study demonstrates that inhibitors of translation which are usually incapable of passing through the plasma membrane of cells selectively enter virus-infected cells and block protein synthesis.

    CAS  Article  Google Scholar 

  8. 8

    Carrasco, L. & Smith, A. E. Sodium ions and the shut-off of host cell protein synthesis by picornaviruses. Nature 264, 807–809 (1976).

    CAS  Article  Google Scholar 

  9. 9

    Lama, J. & Carrasco, L. Expression of poliovirus nonstructural proteins in Escherichia coli cells. Modification of membrane permeability induced by 2B and 3A. J. Biol. Chem. 267, 15932–15937 (1992).

    CAS  Google Scholar 

  10. 10

    Doedens, J. R. & Kirkegaard, K. Inhibition of cellular protein secretion by poliovirus proteins 2B and 3A. EMBO J. 14, 894–907 (1995). This report describes how poliovirus infection blocks the secretory pathway. This effect is observed by transgenic expression of the non-structural proteins P2B or P3A.

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  11. 11

    Aldabe, R., Barco, A. & Carrasco, L. Membrane permeabilization by poliovirus proteins 2B and 2BC. J. Biol. Chem. 271, 23134–23137 (1996).

    CAS  Article  Google Scholar 

  12. 12

    Aldabe, R., Irurzun, A. & Carrasco, L. Poliovirus protein 2BC increases cytosolic free calcium concentrations. J. Virol. 71, 6214–6217 (1997).

    CAS  PubMed  PubMed Central  Google Scholar 

  13. 13

    van Kuppeveld, F. J. et al. Coxsackievirus protein 2B modifies endoplasmic reticulum membrane and plasma membrane permeability and facilitates virus release. EMBO J. 16, 3519–3532 (1997).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  14. 14

    Barco, A. & Carrasco, L. A human virus protein, poliovirus protein 2BC, induces membrane proliferation and blocks the exocytic pathway in the yeast Saccharomyces cerevisiae. EMBO J. 14, 3349–3364 (1995).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  15. 15

    Pinto, L. H., Holsinger, L. J. & Lamb, R. A. Influenza virus M2 protein has ion channel activity. Cell 69, 517–528 (1992). This article shows that the expression of IAV M2 in X. laevis oocytes results in the formation of ion channels that are inhibited by amantadine.

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  16. 16

    Sakaguchi, T., Leser, G. P. & Lamb, R. A. The ion channel activity of the influenza virus M2 protein affects transport through the Golgi apparatus. J. Cell Biol. 133, 733–747 (1996).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  17. 17

    Pinto, L. H. & Lamb, R. A. The M2 proton channels of influenza A and B viruses. J. Biol. Chem. 281, 8997–9000 (2006).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  18. 18

    Pielak, R. M. & Chou, J. J. Influenza M2 proton channels. Biochim. Biophys. Acta 1808, 522–529 (2011).

    CAS  Article  Google Scholar 

  19. 19

    Melton, J. V. et al. Alphavirus 6K proteins form ion channels. J. Biol. Chem. 277, 46923–46931 (2002).

    CAS  Article  Google Scholar 

  20. 20

    Sanz, M. A., Madan, V., Carrasco, L. & Nieva, J. L. Interfacial domains in Sindbis virus 6K protein. Detection and functional characterization. J. Biol. Chem. 278, 2051–2057 (2003).

    CAS  Article  Google Scholar 

  21. 21

    Sanz, M. A., Madan, V., Nieva, J. L. & Carrasco, L. in Viral Membrane Proteins: Structure, Function, and Drug Design (ed. Fischer, W.) 233–244 (Kluwer Academic/Plenum Publishers, 2005).

    Book  Google Scholar 

  22. 22

    Park, S. H. et al. Three-dimensional structure of the channel-forming trans-membrane domain of virus protein “u” (Vpu) from HIV-1. J. Mol. Biol. 333, 409–424 (2003).

    CAS  Article  Google Scholar 

  23. 23

    Neil, S. J., Zang, T. & Bieniasz, P. D. Tetherin inhibits retrovirus release and is antagonized by HIV-1 Vpu. Nature 451, 425–430 (2008). This work demonstrates that the HIV-1 viroporin Vpu counteracts the effects of tetherin, thereby facilitating the release of HIV-1 particles.

    CAS  Article  Google Scholar 

  24. 24

    Griffin, S. D. et al. The p7 protein of hepatitis C virus forms an ion channel that is blocked by the antiviral drug, Amantadine. FEBS Lett. 535, 34–38 (2003). This study describes the capacity of HCV p7 to form ion channels, confirming its identity as a genuine viroporin.

    CAS  Article  Google Scholar 

  25. 25

    Pavlovic, D. et al. The hepatitis C virus p7 protein forms an ion channel that is inhibited by long-alkyl-chain iminosugar derivatives. Proc. Natl Acad. Sci. USA 100, 6104–6108 (2003).

    CAS  Article  Google Scholar 

  26. 26

    Jones, C. T., Murray, C. L., Eastman, D. K., Tassello, J. & Rice, C. M. Hepatitis C virus p7 and NS2 proteins are essential for production of infectious virus. J. Virol. 81, 8374–8383 (2007).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  27. 27

    Steinmann, E. et al. Hepatitis C virus p7 protein is crucial for assembly and release of infectious virions. PLoS Pathog. 3, 962–971 (2007).

    CAS  Article  Google Scholar 

  28. 28

    Steinmann, E. et al. Antiviral effects of amantadine and iminosugar derivatives against hepatitis C virus. Hepatology 46, 330–338 (2007).

    CAS  Article  Google Scholar 

  29. 29

    Luik, P. et al. The 3-dimensional structure of a hepatitis C virus p7 ion channel by electron microscopy. Proc. Natl Acad. Sci. USA 106, 12712–12716 (2009). This article provides the first reported structural reconstruction of a full-length viroporin immersed in a membrane-like environment.

    CAS  Article  Google Scholar 

  30. 30

    Van Damme, N. et al. The interferon-induced protein BST-2 restricts HIV-1 release and is downregulated from the cell surface by the viral Vpu protein. Cell Host Microbe 3, 245–252 (2008).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  31. 31

    Cady, S. D. et al. Structure of the amantadine binding site of influenza M2 proton channels in lipid bilayers. Nature 463, 689–692 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  32. 32

    Wozniak, A. L. et al. Intracellular proton conductance of the hepatitis C virus p7 protein and its contribution to infectious virus production. PLoS Pathog. 6, e1001087 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  33. 33

    Ewart, G. D., Mills, K., Cox, G. B. & Gage, P. W. Amiloride derivatives block ion channel activity and enhancement of virus-like particle budding caused by HIV-1 protein Vpu. Eur. Biophys. J. 31, 26–35 (2002).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  34. 34

    Watanabe, S., Watanabe, T. & Kawaoka, Y. Influenza A virus lacking M2 protein as a live attenuated vaccine. J. Virol. 83, 5947–5950 (2009).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  35. 35

    Dube, M., Bego, M. G., Paquay, C. & Cohen, E. A. Modulation of HIV-1-host interaction: role of the Vpu accessory protein. Retrovirology 7, 114 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  36. 36

    Lu, W. et al. Severe acute respiratory syndrome-associated coronavirus 3a protein forms an ion channel and modulates virus release. Proc. Natl Acad. Sci. USA 103, 12540–12545 (2006).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  37. 37

    Gonzalez, M. E. & Carrasco, L. Human immunodeficiency virus type 1 VPU protein affects Sindbis virus glycoprotein processing and enhances membrane permeabilization. Virology 279, 201–209 (2001).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  38. 38

    Brohm, C. et al. Characterization of determinants important for hepatitis C virus p7 function in morphogenesis by using trans-complementation. J. Virol. 83, 11682–11693 (2009).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  39. 39

    Hout, D. R. et al. Substitution of the transmembrane domain of Vpu in simian–human immunodeficiency virus (SHlVKu1bMC33) with that of M2 of influenza A results in a virus that is sensitive to inhibitors of the M2 ion channel and is pathogenic for pig-tailed macaques. Virology 344, 541–559 (2006).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  40. 40

    Carrasco, L., Guinea, R., Irurzun, A. & Barco, A. in Molecular Biology of Picornaviruses (eds Semler, B. L. & Wimmer, E.) 337–354 (American Society for Microbiology Press, 2002).

    Google Scholar 

  41. 41

    Madan, V., Redondo, N. & Carrasco, L. Cell permeabilization by poliovirus 2B viroporin triggers bystander permeabilization in neighbouring cells through a mechanism involving gap junctions. Cell. Microbiol. 12, 1144–1157 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  42. 42

    Han, Z. Y. & Harty, R. N. The NS3 protein of bluetongue virus exhibits viroporin-like properties. J. Biol. Chem. 279, 43092–43097 (2004).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  43. 43

    Madan, V., Sanz, M. A. & Carrasco, L. Requirement of the vesicular system for membrane permeabilization by Sindbis virus. Virology 332, 307–315 (2005).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  44. 44

    Liu, D. X., Yuan, Q. & Liao, Y. Coronavirus envelope protein: a small membrane protein with multiple functions. Cell. Mol. Life Sci. 64, 2043–2048 (2007).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  45. 45

    Han, Z. Y., Licata, J. M., Paragas, J. & Harty, R. N. Permeabilization of the plasma membrane by Ebola virus GP2. Virus Genes 34, 273–281 (2007).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  46. 46

    Suzuki, T. et al. The human polyoma JC virus agnoprotein acts as a viroporin. PLoS Pathog. 6, e1000801 (2010). An instructive example of the thorough analysis that is required in order to specifically identify and classify new members of the viroporin family.

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  47. 47

    Agirre, A., Barco, A., Carrasco, L. & Nieva, J. L. Viroporin-mediated membrane permeabilization. Pore formation by nonstructural poliovirus 2B protein. J. Biol. Chem. 277, 40434–40441 (2002).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  48. 48

    Fischer, W. B. & Kruger, J. Viral channel-forming proteins. Int. Rev. Cell. Mol. Biol. 275, 35–63 (2009).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  49. 49

    Pinto, L. H. et al. A functionally defined model for the M2 proton channel of influenza A virus suggests a mechanism for its ion selectivity. Proc. Natl Acad. Sci. USA 94, 11301–11306 (1997).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  50. 50

    Schubert, U. et al. Identification of an ion channel activity of the Vpu transmembrane domain and its involvement in the regulation of virus release from HIV-1-infected cells. FEBS Lett. 398, 12–18 (1996).

    CAS  Article  Google Scholar 

  51. 51

    Mehnert, T. et al. Biophysical characterization of Vpu from HIV-1 suggests a channel-pore dualism. Proteins 70, 1488–1497 (2008).

    CAS  Article  Google Scholar 

  52. 52

    Madan, V., Castello, A. & Carrasco, L. Viroporins from RNA viruses induce caspase-dependent apoptosis. Cell. Microbiol. 10, 437–451 (2008).

    CAS  Google Scholar 

  53. 53

    Gonzalez, M. E. & Carrasco, L. The human immunodeficiency virus type 1 Vpu protein enhances membrane permeability. Biochemistry 37, 13710–13719 (1998).

    CAS  Article  Google Scholar 

  54. 54

    Clarke, D. et al. Evidence for the formation of a heptameric ion channel complex by the hepatitis C virus p7 protein in vitro. J. Biol. Chem. 281, 37057–37068 (2006).

    CAS  Article  Google Scholar 

  55. 55

    Madan, V., Garcia Mde, J., Sanz, M. A. & Carrasco, L. Viroporin activity of murine hepatitis virus E protein. FEBS Lett. 579, 3607–3612 (2005).

    CAS  Article  Google Scholar 

  56. 56

    Wilson, L., McKinlay, C., Gage, P. & Ewart, G. SARS coronavirus E protein forms cation-selective ion channels. Virology 330, 322–331 (2004).

    CAS  Article  Google Scholar 

  57. 57

    Phillips, R., Ursell, T., Wiggins, P. & Sens, P. Emerging roles for lipids in shaping membrane-protein function. Nature 459, 379–385 (2009).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  58. 58

    Chami, M., Ferrari, D., Nicotera, P., Paterlini-Brechot, P. & Rizzuto, R. Caspase-dependent alterations of Ca2+ signaling in the induction of apoptosis by hepatitis B virus X protein. J. Biol. Chem. 278, 31745–31755 (2003).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  59. 59

    de Jong, A. S. et al. Functional analysis of picornavirus 2B proteins: effects on calcium homeostasis and intracellular protein trafficking. J. Virol. 82, 3782–3790 (2008).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  60. 60

    Hyser, J. M., Collinson-Pautz, M. R., Utama, B. & Estes, M. K. Rotavirus disrupts calcium homeostasis by NSP4 viroporin activity. mBio 1, e00265-10 (2010).

    Article  CAS  PubMed  PubMed Central  Google Scholar 

  61. 61

    van Kuppeveld, F. J., Below, G. & Ehrenfeld, E. in The Picornaviruses (eds Ehrenfeld, E., Domingo, E. & Roos, R. P.) 181–193 (American Society for Microbiology Press, 2010).

    Google Scholar 

  62. 62

    Diaz, Y. et al. Expression of nonstructural rotavirus protein NSP4 mimics Ca2+ homeostasis changes induced by rotavirus infection in cultured cells. J. Virol. 82, 11331–11343 (2008).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  63. 63

    Zhou, Y., Frey, T. K. & Yang, J. J. Viral calciomics: interplays between Ca2+ and virus. Cell Calcium 46, 1–17 (2009).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  64. 64

    Hajnoczky, G., Davies, E. & Madesh, M. Calcium signaling and apoptosis. Biochem. Biophys. Res. Commun. 304, 445–454 (2003).

    CAS  Article  Google Scholar 

  65. 65

    Jacotot, E. et al. The HIV-1 viral protein R induces apoptosis via a direct effect on the mitochondrial permeability transition pore. J. Exp. Med. 191, 33–46 (2000).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  66. 66

    Hsu, N. Y. et al. Viral reorganization of the secretory pathway generates distinct organelles for RNA replication. Cell 141, 799–811 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  67. 67

    Suhy, D. A., Giddings, T. H. Jr & Kirkegaard, K. Remodeling the endoplasmic reticulum by poliovirus infection and by individual viral proteins: an autophagy-like origin for virus-induced vesicles. J. Virol. 74, 8953–8965 (2000).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  68. 68

    de Jong, A. S. et al. The coxsackievirus 2B protein increases efflux of ions from the endoplasmic reticulum and Golgi, thereby inhibiting protein trafficking through the Golgi. J. Biol. Chem. 281, 14144–14150 (2006).

    CAS  Article  Google Scholar 

  69. 69

    Kirkegaard, K. Subversion of the cellular autophagy pathway by viruses. Curr. Top. Microbiol. Immunol. 335, 323–333 (2009).

    CAS  Google Scholar 

  70. 70

    Ruiz, A., Guatelli, J. C. & Stephens, E. B. The Vpu protein: new concepts in virus release and CD4 down-modulation. Curr. HIV Res. 8, 240–252 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  71. 71

    Dube, M. et al. HIV-1 Vpu antagonizes BST-2 by interfering mainly with the trafficking of newly synthesized BST-2 to the cell surface. Traffic 12, 1714–1729 (2011).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  72. 72

    Martin-Serrano, J. & Neil, S. J. Host factors involved in retroviral budding and release. Nature Rev. Microbiol. 9, 519–531 (2011).

    CAS  Article  Google Scholar 

  73. 73

    Terwilliger, E. F., Cohen, E. A., Lu, Y. C., Sodroski, J. G. & Haseltine, W. A. Functional role of human immunodeficiency virus type 1 vpu. Proc. Natl Acad. Sci. USA 86, 5163–5167 (1989).

    CAS  Article  Google Scholar 

  74. 74

    Iwatsuki-Horimoto, K. et al. The cytoplasmic tail of the influenza A virus M2 protein plays a role in viral assembly. J. Virol. 80, 5233–5240 (2006).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  75. 75

    Hsu, K., Han, J., Shinlapawittayatorn, K., Deschenes, I. & Marban, E. Membrane potential depolarization as a triggering mechanism for Vpu-mediated HIV-1 release. Biophys. J. 99, 1718–1725 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  76. 76

    Huang, D. T., Chi, N., Chen, S. C., Lee, T. Y. & Hsu, K. Background K2P channels KCNK3/9/15 limit the budding of cell membrane-derived vesicles. Cell Biochem. Biophys. 61, 585–594 (2011).

    CAS  Article  Google Scholar 

  77. 77

    Schnell, J. R. & Chou, J. J. Structure and mechanism of the M2 proton channel of influenza A virus. Nature 451, 591–595 (2008).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  78. 78

    Leiding, T., Wang, J., Martinsson, J., DeGrado, W. F. & Arskold, S. P. Proton and cation transport activity of the M2 proton channel from influenza A virus. Proc. Natl Acad. Sci. USA 107, 15409–15414 (2010).

    CAS  Article  Google Scholar 

  79. 79

    Sharma, M. et al. Insight into the mechanism of the influenza A proton channel from a structure in a lipid bilayer. Science 330, 509–512 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  80. 80

    Rossman, J. S., Jing, X., Leser, G. P. & Lamb, R. A. Influenza virus M2 protein mediates ESCRT-independent membrane scission. Cell 142, 902–913 (2010). This study localizes IAV M2 to the neck of budding influenza viruses and describes its participation in virion scission from host cell membranes, in agreement with previously reported results.

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  81. 81

    Carrasco, L. Entry of animal viruses and macromolecules into cells. FEBS Lett. 350, 151–154 (1994).

    CAS  Article  Google Scholar 

  82. 82

    Watanabe, T., Watanabe, S., Ito, H., Kida, H. & Kawaoka, Y. Influenza A virus can undergo multiple cycles of replication without M2 ion channel activity. J. Virol. 75, 5656–5662 (2001).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  83. 83

    Griffin, S. et al. Genotype-dependent sensitivity of hepatitis C virus to inhibitors of the p7 ion channel. Hepatology 48, 1779–1790 (2008).

    CAS  Article  Google Scholar 

  84. 84

    Fernandez-Puentes, C. & Carrasco, L. Viral infection permeabilizes mammalian cells to protein toxins. Cell 20, 769–775 (1980).

    CAS  Article  Google Scholar 

  85. 85

    Nieva, J. L., Agirre, A., Nir, S. & Carrasco, L. Mechanisms of membrane permeabilization by picornavirus 2B viroporin. FEBS Lett. 552, 68–73 (2003).

    CAS  Article  Google Scholar 

  86. 86

    van Kuppeveld, F. J., Galama, J. M., Zoll, J., van den Hurk, P. J. & Melchers, W. J. Coxsackie B3 virus protein 2B contains cationic amphipathic helix that is required for viral RNA replication. J. Virol. 70, 3876–3886 (1996).

    CAS  PubMed  PubMed Central  Google Scholar 

  87. 87

    van Kuppeveld, F. J., Melchers, W. J., Kirkegaard, K. & Doedens, J. R. Structure-function analysis of coxsackie B3 virus protein 2B. Virology 227, 111–118 (1997).

    CAS  Article  Google Scholar 

  88. 88

    Martinez-Gil, L. et al. Membrane integration of poliovirus 2B viroporin. J. Virol. 85, 11315–11324 (2011).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  89. 89

    Barco, A. & Carrasco, L. Identification of regions of poliovirus 2BC protein that are involved in cytotoxicity. J. Virol. 72, 3560–3570 (1998).

    CAS  PubMed  PubMed Central  Google Scholar 

  90. 90

    de Jong, A. S. et al. Determinants for membrane association and permeabilization of the coxsackievirus 2B protein and the identification of the Golgi complex as the target organelle. J. Biol. Chem. 278, 1012–1021 (2003).

    CAS  Article  Google Scholar 

  91. 91

    van Kuppeveld, F. J., Melchers, W. J., Willems, P. H. & Gadella, T. W. Jr. Homomultimerization of the coxsackievirus 2B protein in living cells visualized by fluorescence resonance energy transfer microscopy. J. Virol. 76, 9446–9456 (2002).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  92. 92

    Patargias, G., Barke, T., Watts, A. & Fischer, W. B. Model generation of viral channel forming 2B protein bundles from polio and coxsackie viruses. Mol. Membr. Biol. 26, 309–320 (2009).

    CAS  Article  Google Scholar 

  93. 93

    Ojcius, D. M. & Young, J. D. Cytolytic pore-forming proteins and peptides: is there a common structural motif? Trends Biochem. Sci. 16, 225–229 (1991).

    CAS  Article  Google Scholar 

  94. 94

    Madan, V. et al. Plasma membrane-porating domain in poliovirus 2B protein. A short peptide mimics viroporin activity. J. Mol. Biol. 374, 951–964 (2007). The region from poliovirus viroporin P2B that assembles hydrophilic pores is defined by assessing the pore-forming activity of an overlapping peptide library of the P2B sequence. The peptide comprising the amphipathic α-helix of P2B permeabilizes liposomes as well as bacterial and mammalian cells and has ion channel activity when inserted into the plasma membrane.

    CAS  Article  Google Scholar 

  95. 95

    Sanchez-Martinez, S. et al. Functional and structural characterization of 2B viroporin membranolytic domains. Biochemistry 47, 10731–10739 (2008).

    CAS  Article  Google Scholar 

  96. 96

    Hoofnagle, J. H. Course and outcome of hepatitis C. Hepatology 36, S21–S29 (2002).

    Google Scholar 

  97. 97

    Moradpour, D., Penin, F. & Rice, C. M. Replication of hepatitis C virus. Nature Rev. Microbiol. 5, 453–463 (2007).

    CAS  Article  Google Scholar 

  98. 98

    Steinmann, E. & Pietschmann, T. Hepatitis C virus P7—a viroporin crucial for virus assembly and an emerging target for antiviral therapy. Viruses 2, 2078–2095 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  99. 99

    Premkumar, A., Wilson, L., Ewart, G. D. & Gage, P. W. Cation-selective ion channels formed by p7 of hepatitis C virus are blocked by hexamethylene amiloride. FEBS Lett. 557, 99–103 (2004).

    CAS  Article  Google Scholar 

  100. 100

    Montserret, R. et al. NMR structure and ion channel activity of the p7 protein from hepatitis C virus. J. Biol. Chem. 285, 31446–31461 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  101. 101

    McCown, M. F. & Pekosz, A. The influenza A virus M2 cytoplasmic tail is required for infectious virus production and efficient genome packaging. J. Virol. 79, 3595–3605 (2005).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  102. 102

    Ichinohe, T., Pang, I. K. & Iwasaki, A. Influenza virus activates inflammasomes via its intracellular M2 ion channel. Nature Immunol. 11, 404–410 (2010).

    CAS  Article  Google Scholar 

  103. 103

    Ma, H. et al. Human annexin A6 interacts with influenza A virus protein M2 and negatively modulates infection. J. Virol. 86, 1789–1901 (2012).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  104. 104

    Gannage, M. et al. Matrix protein 2 of influenza A virus blocks autophagosome fusion with lysosomes. Cell Host Microbe 6, 367–380 (2009).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  105. 105

    Cordes, F. S., Tustian, A. D., Sansom, M. S., Watts, A. & Fischer, W. B. Bundles consisting of extended transmembrane segments of Vpu from HIV-1: computer simulations and conductance measurements. Biochemistry 41, 7359–7365 (2002).

    CAS  Article  Google Scholar 

  106. 106

    Kueck, T. & Neil, S. J. A. Cytoplasmic tail determinant in HIV-1 Vpu mediates targeting of tetherin for endosomal degradation and counteracts interferon-induced restriction. PLoS Pathog. 8, e1002609 (2011).

    Article  Google Scholar 

  107. 107

    Magadan, J. G. & Bonifacino, J. S. Transmembrane domain determinants of CD4 downregulation by HIV-1 Vpu. J. Virol. 86, 757–772 (2012).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  108. 108

    Iwabu, Y. et al. HIV-1 accessory protein Vpu internalizes cell-surface BST-2/tetherin through transmembrane interactions leading to lysosomes. J. Biol. Chem. 284, 35060–35072 (2009).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  109. 109

    Perez-Caballero, D. et al. Tetherin inhibits HIV-1 release by directly tethering virions to cells. Cell 139, 499–511 (2009).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  110. 110

    Evans, D. T., Serra-Moreno, R., Singh, R. K. & Guatelli, J. C. BST-2/tetherin: a new component of the innate immune response to enveloped viruses. Trends Microbiol. 18, 388–396 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  111. 111

    Neil, S. J., Sandrin, V., Sundquist, W. I. & Bieniasz, P. D. An interferon-alpha-induced tethering mechanism inhibits HIV-1 and Ebola virus particle release but is counteracted by the HIV-1 Vpu protein. Cell Host Microbe 2, 193–203 (2007).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  112. 112

    Acharya, R. et al. Structure and mechanism of proton transport through the transmembrane tetrameric M2 protein bundle of the influenza A virus. Proc. Natl Acad. Sci. USA 107, 15075–15080 (2010). On the basis of the high-resolution structure of the M2 channel, the authors of this report suggest mechanisms for proton conductance and asymmetrical diffusion in the presence of a proton gradient.

    CAS  Article  Google Scholar 

  113. 113

    Moffat, J. C. et al. Proton transport through influenza A virus M2 protein reconstituted in vesicles. Biophys. J. 94, 434–445 (2008).

    CAS  Article  Google Scholar 

  114. 114

    Stouffer, A. L. et al. Structural basis for the function and inhibition of an influenza virus proton channel. Nature 451, 596–599 (2008).

    CAS  Article  Google Scholar 

  115. 115

    Lu, J. X., Sharpe, S., Ghirlando, R., Yau, W. M. & Tycko, R. Oligomerization state and supramolecular structure of the HIV-1 Vpu protein transmembrane segment in phospholipid bilayers. Protein Sci. 19, 1877–1896 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  116. 116

    Lopez, C. F., Montal, M., Blasie, J. K., Klein, M. L. & Moore, P. B. Molecular dynamics investigation of membrane-bound bundles of the channel-forming transmembrane domain of viral protein U from the human immunodeficiency virus HIV-1. Biophys. J. 83, 1259–1267 (2002).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  117. 117

    Cook, G. A. & Opella, S. J. Secondary structure, dynamics, and architecture of the p7 membrane protein from hepatitis C virus by NMR spectroscopy. Biochim. Biophys. Acta 1808, 1448–1453 (2011).

    CAS  Article  Google Scholar 

  118. 118

    Kendal, A. P. & Klenk, H. D. Amantadine inhibits an early, M2 protein-dependent event in the replication cycle of avian influenza (H7) viruses. Arch. Virol. 119, 265–273 (1991).

    CAS  Article  Google Scholar 

  119. 119

    StGelais, C. et al. Inhibition of hepatitis C virus p7 membrane channels in a liposome-based assay system. Antiviral Res. 76, 48–58 (2007).

    CAS  Article  Google Scholar 

  120. 120

    Govorkova, E. A. & Webster, R. G. Combination chemotherapy for influenza. Viruses 2, 1510–1529 (2010).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  121. 121

    Foster, T. L. et al. Resistance mutations define specific antiviral effects for inhibitors of the hepatitis C virus p7 ion channel. Hepatology 54, 79–90 (2011). Specific resistance mutations are used to define the mode of action for two classes of p7 inhibitors: adamantanes and alkylated imino sugars. Adamantanes prevent the channel opening that is mediated by low pH, whereas imino sugars prevent p7 oligomerization and channel formation.

    CAS  Article  Google Scholar 

  122. 122

    van Soest, H. et al. No beneficial effects of amantadine in treatment of chronic hepatitis C patients. Dig. Liver Dis. 42, 496–502 (2010).

    CAS  Article  Google Scholar 

  123. 123

    Pawlotsky, J. M., Chevaliez, S. & McHutchison, J. G. The hepatitis C virus life cycle as a target for new antiviral therapies. Gastroenterology 132, 1979–1998 (2007).

    CAS  Article  Google Scholar 

  124. 124

    Wilson, L., Gage, P. & Ewart, G. Hexamethylene amiloride blocks E protein ion channels and inhibits coronavirus replication. Virology 353, 294–306 (2006).

    CAS  Article  Google Scholar 

  125. 125

    Luscombe, C. A. et al. A novel Hepatitis C virus p7 ion channel inhibitor, BIT225, inhibits bovine viral diarrhea virus in vitro and shows synergism with recombinant interferon-α-2b and nucleoside analogues. Antiviral Res. 86, 144–153 (2010).

    CAS  Article  Google Scholar 

  126. 126

    Khoury, G., Ewart, G., Luscombe, C., Miller, M. & Wilkinson, J. Antiviral efficacy of the novel compound BIT225 against HIV-1 release from human macrophages. Antimicrob. Agents Chemother. 54, 835–845 (2010).

    CAS  Article  Google Scholar 

  127. 127

    Xie, S. Q. et al. DIDS blocks a chloride-dependent current that is mediated by the 2B protein of enterovirus 71. Cell Res. 21, 1271–1275 (2011).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  128. 128

    Gervais, C. et al. Development and validation of a high-throughput screening assay for the hepatitis C virus p7 viroporin. J. Biomol. Screen 16, 363–369 (2011).

    CAS  Article  Google Scholar 

  129. 129

    Raghava, S., Giorda, K. M., Romano, F. B., Heuck, A. P. & Hebert, D. N. The SV40 late protein VP4 is a viroporin that forms pores to disrupt membranes for viral release. PLoS Pathog. 7, e1002116 (2011).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  130. 130

    Wetherill, L. F. et al. High-risk human papillomavirus E5 oncoprotein displays channel-forming activity sensitive to small-molecule inhibitors. J. Virol. 86, 5341–5351 (2012).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  131. 131

    Machamer, C. E. & Youn, S. The transmembrane domain of the infectious bronchitis virus E protein is required for efficient virus release. Adv. Exp. Med. Biol. 581, 193–198 (2006).

    CAS  Article  Google Scholar 

  132. 132

    Ye, Y. & Hogue, B. G. Role of the coronavirus E viroporin protein transmembrane domain in virus assembly. J. Virol. 81, 3597–3607 (2007).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  133. 133

    Tan, Y. J., Lim, S. G. & Hong, W. Regulation of cell death during infection by the severe acute respiratory syndrome coronavirus and other coronaviruses. Cell. Microbiol. 9, 2552–2561 (2007).

    CAS  Article  Google Scholar 

  134. 134

    Plugge, B. et al. A potassium channel protein encoded by chlorella virus PBCV-1. Science 287, 1641–1644 (2000).

    CAS  Article  Google Scholar 

  135. 135

    Wang, K., Xie, S. Q. & Sun, B. Viral proteins function as ion channels. Biochim. Biophys. Acta 1808, 510–515 (2011).

    CAS  Article  Google Scholar 

  136. 136

    Gladue, D. P. et al. Classical swine fever virus p7 protein is a viroporin involved in virulence in swine. J. Virol. 86, 6778–6791 (2012).

    CAS  Article  PubMed  PubMed Central  Google Scholar 

  137. 137

    Mould, J. A. et al. Influenza B virus BM2 protein has ion channel activity that conducts protons across membranes. Dev. Cell 5, 175–184 (2003).

    CAS  Article  Google Scholar 

  138. 138

    Piller, S. C., Ewart, G. D., Premkumar, A., Cox, G. B. & Gage, P. W. Vpr protein of human immunodeficiency virus type 1 forms cation-selective channels in planar lipid bilayers. Proc. Natl Acad. Sci. USA 93, 111–115 (1996).

    CAS  Article  Google Scholar 

  139. 139

    Silic-Benussi, M., Marin, O., Biasiotto, R., D'Agostino, D. M. & Ciminale, V. Effects of human T-cell leukemia virus type 1 (HTLV-1) p13 on mitochondrial K+ permeability: a new member of the viroporin family? FEBS Lett. 584, 2070–2075 (2010).

    CAS  Article  Google Scholar 

Download references


L.C. is supported by the Spanish Ministerio de Ciencia e Innovacion (MICIIN) (grant BFU2009-07352), and J.L.N. is supported by the MICIIN (grant BIO2011-29792) and the Basque Government (grant GIU-06/42). The authors also acknowledge the institutional grant from the Fundacion Ramón Areces to the Centro de Biología Molecular Severo Ochoa at the Universidad Autónoma de Madrid, Spain, and the Spanish Science Research Council (CSIC). The authors thank M. E. González for critical reading of this manuscript.

Author information



Ethics declarations

Competing interests

The authors declare no competing financial interests.

Related links






Luis Carrasco's homepage


Patch clamping

A high-resolution electrophysiological technique that allows currents to be recorded through single ion channels. The technique uses glass pipettes to electrically isolate channel-containing membrane patches. Current flow can be recorded in cell-attached and excised membrane patches.

Hygromycin B test

An experiment to test membrane permeability. Hygromycin, an inhibitor of translation, does not easily pass through the plasma membrane of control cells, but after cell permeabilization by viroporins, the inhibitor is able to reach its target (the ribosome) and block protein synthesis.


A lipophilic compound that inserts into membranes and transports ions in favour of the electrochemical gradient.


A rigid molecule with a backbone based on three interconnected cyclohexane rings. Derivatives of adamantane (amantadine and rimantadine) have been shown to be effective inhibitors of viroporin activity in some cases.

Molecular dynamics simulations

Computational simulations that calculate the time-dependent motions of atoms in a molecular system. This method is used to obtain microscopic-level information about the fluctuations and conformational changes of proteins.


Protein complexes that are involved in the processing and release of pro-inflammatory cytokines during the innate immune response to viral infection.


A double-membraned vesicle that captures cellular components (including proteins, membrane fragments and whole organelles) and delivers them to lysosomes for degradation through a process called autophagy.

Asymmetrical conductance

The capacity of an ion channel to conduct preferentially outward or preferentially inward ion fluxes.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Nieva, J., Madan, V. & Carrasco, L. Viroporins: structure and biological functions. Nat Rev Microbiol 10, 563–574 (2012).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing