Haemagglutinin of influenza virus expressed from a cloned gene promotes membrane fusion

Abstract

Enveloped animal viruses enter and infect cells by a process involving fusion of the viral membrane with a cellular membrane. In some cases (for example, Sendai virus¹), the fusion event occurs at the plasma membrane; for many other viruses including influenza, entry occurs through the membranes of intracellular vesicles such as endosomes or lysosomes, where the fusion is triggered by the endogenous low pH^2–6. This pH-dependent fusion activity has been studied in vitro using as targets cultured cells^7–9, erythrocytes^9,10 and liposomes^11–13. Fusion is a function of the viral surface glycoproteins¹² and occurs at a threshold pH that is characteristic of each virus species and strain⁸. In the case of influenza virus, there is strong evidence that the haemagglutinin glycoprotein has a key role in both virus infectivity and fusion activity^9,12,14–16. Both processes require a post-translational proteolytic cleavage of the haemagglutinin precursor, HA0, into the active form of the molecule, HA, which consists of two disulphide-bonded subunits, HA1 and HA2^17,18. A new N-terminus is generated on the HA2 subunit, the C-terminus of which is embedded in the virus membrane¹⁹. At the low pH values required for fusion the cleaved HA undergoes a conformational change exposing this previously buried hydrophobic N-terminus of HA2²⁰, possibly enabling it to interact with the target membrane. While it has been established that HA is necessary for fusion, it is unclear whether HA alone is sufficient or whether other viral proteins are involved²¹. Here we use cells expressing HA from a cloned copy of the HA gene inserted into a recombinant simian virus 40 (SV40) vector²² to demonstrate that the HA molecule displays fusion in the absence of any other influenza virus-encoded components. These results open the possibility of using HA-mediated membrane fusion as a system to deliver foreign molecules into mammalian cells.