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

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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 pH2–6. This pH-dependent fusion activity has been studied in vitro using as targets cultured cells7–9, erythrocytes9,10 and liposomes11–13. 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 activity9,12,14–16. 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 HA217,18. 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.