Review Article | Published:

Pathways of clathrin-independent endocytosis

Nature Reviews Molecular Cell Biology volume 8, pages 603612 (2007) | Download Citation



There are numerous ways that endocytic cargo molecules may be internalized from the surface of eukaryotic cells. In addition to the classical clathrin-dependent mechanism of endocytosis, several pathways that do not use a clathrin coat are emerging. These pathways transport a diverse array of cargoes and are sometimes hijacked by bacteria and viruses to gain access to the host cell. Here, we review our current understanding of various clathrin-independent mechanisms of endocytosis and propose a classification scheme to help organize the data in this complex and evolving field.

Key points

  • In addition to the classical clathrin-dependent mechanisms of endocytosis, there are several pathways that do not use a clathrin coat and are, therefore, referred to as clathrin-independent (CI) mechanisms.

  • CI mechanisms of uptake have gained much attention with the realization that they have important roles in the regulation of cell growth and development as well as important implications in the study of certain diseases and pathogens.

  • Two well-known CI mechanisms are the caveolar pathway and fluid-phase endocytosis. However, there is much debate concerning the number of distinct CI mechanisms that exist, the best cargo molecules for the study of a particular pathway, and the underlying protein machinery that regulates these pathways.

  • To organize the extensive literature on CI endocytosis for the purpose of arriving at a mechanistic understanding, this Review classifies CI mechanisms as follows: first, on whether or not they are dynamin dependent and, second, according to the involvement of the small GTPases CDC42, RhoA or ARF6.

  • Protein-based mechanisms (for example, ubiquitylation) and lipid-based mechanisms (for example, nanoscale clustering of lipid-tethered proteins) may both function in the selection of cargo for CI endocytosis.

  • The mechanism of budding in the dynamin-independent pathways remains elusive; however, recent theoretical studies provide testable ideas in this area.

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We are extremely grateful to members of the Mayor and Pagano laboratories for comments on the manuscript, and to H.M. Thompson (Mayo Clinic College of Medicine) for expert editorial assistance and for generating the figure in Box 1. We apologize to colleagues whose work has not been directly cited owing to space limitations. Research in S.M.'s laboratory was supported by a Senior Research Fellowship from The Wellcome Trust, the Department of Biotechnology (India), the Human Frontier Science Program, and intramural funds from the National Centre for Biological Sciences, India. Research in R.P.'s laboratory was supported by grants from the U.S. National Institutes of Health and the National Niemann–Pick Disease Foundation, and intramural funds from the Mayo Foundation.

Author information


  1. National Centre for Biological Sciences, UAS-GKVK Campus, Bangalore 560065, India.

    • Satyajit Mayor
  2. Mayo Clinic College of Medicine, Departments of Medicine, Biochemistry and Molecular Biology, 200 First Street Southwest, Rochester, Minnesota 55905, USA.

    • Richard E. Pagano


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The authors declare no competing financial interests.

Supplementary information

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  1. 1.

    Supplementary information S1 (table)

    Pharmacological inhibitors or dominant negative proteins used to distinguish clathrin-independent mechanisms of endocytosis


Rab GTPase

A Ras family GTPase that regulates membrane-trafficking events in eukaryotic cells. Different Rab proteins are specific for different transport pathways and different subcellular compartments.


A mainly actin-dependent endocytic mechanism that is used to internalize large amounts of fluid and growth factors; vesicles mediating this form of endocytosis are usually uncoated and >500 nm in diameter.


An actin-based endocytic mechanism. It is mediated by cup-like membrane extensions that are used to internalize large particles such as bacteria.


A large GTPase of 100 kDa. It mediates many, but not all, forms of endocytosis as well as vesicle formation from various intracellular organelles through its ability to tubulate and sever membranes.


50–80 nm flask-shaped pits that form in the plasma membrane and are enriched in caveolins, sphingolipids and cholesterol.


An intracellular compartment that is involved in the intracellular transport of simian virus-40 from caveolae to the endoplasmic reticulum.

Glycosyl phosphatidylinositol-anchored protein

A protein that is anchored to the extracellular membrane leaflet through the glycosyl phosphatidylinositol lipid modification rather than through a transmembrane protein domain.

Dominant negative protein

A mutant version of a protein that, when expressed, results in an inhibitory phenotype, usually by competing with interaction partners and thereby suppressing the function of the endogenous wild-type protein.

Carboxypeptidase E

An endopeptidase that is found in secretory vesicles and can activate neuropeptides. It may also function as a sorting receptor that sorts cargoes into the regulated secretory pathway.


A post-translational modification that is added to some receptor tyrosine kinases following ligand binding. It mediates internalization and subsequent sorting at the level of endosomes.

E3 ubiquitin ligase

An enzyme that functions downstream of or in combination with a ubiquitin-conjugating enzyme (E2) to attach ubiquitin molecules to a target protein, marking the protein for subsequent recognition by ubiquitin-binding domains.

Ubiquitin interacting motif

A protein motif of 24 amino acids that interacts with ubiquitin and, in some instances, is also necessary for the ubiquitylation of proteins that contain this motif.

Fluorescence resonance energy transfer

(FRET). A fluorescence-based method for detecting interactions between proteins that are <10 nm apart in a cell. This method is dependent on the spectral overlap between donor and acceptor chromophores and uses the transfer of (non-radiative) energy from an initially excited donor molecule to subsequently excite an acceptor molecule.

Transient confinement zone

A region of the cell membrane that is defined in single-particle diffusion studies of mobile molecules as an area where mobile species are likely to spend more time than expected from the analysis of the trajectory of diffusing species.


The entire repertoire of kinases encoded by the genome of an organism.

Guanine nucleotide dissociation inhibitor

A protein that inhibits the removal of guanosine diphosphate from the nucleotide-binding pocket of a GTP-binding protein, thus keeping it in the inactive state.


Soluble N-ethylmaleimide sensitive factor attachment protein receptor. A family of coiled-coil proteins that operate in paired complexes (vesicle SNAREs and target SNAREs) to mediate the fusion of donor and acceptor membranes, usually between a vesicle and an organelle or two vesicles.

C-terminal binding protein 3/brefeldin A-ribosylated substrate

(CtBP3/BARS). A member of the CtBP transcription co-repressor family of proteins. It is involved in the dynamin-independent fission of vesicles from the plasma membrane as well as from the Golgi apparatus.

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