ENTRY OF ENVELOPED VIRUSES INTO HOST CELLS: FUSION ACTIVITY OF THE INFLUENZA VIRUS HEMAGGLUTININ

Considerable progress has been achieved in understanding the mechanism of entry of enveloped viruses into host cells during the last few years. Viruses have long been known to be capabable of transporting their genome and accessory proteins into the cytosol of the host cell, thus causing infections of variable severity. It is clear that the medical consequences of viral diseases remain an important reason to study the cellular entry of viruses, particularly since a knowledge of the molecular details underlying this event may help to develop new antiviral strategies. Apart from their importance as pathogens, viruses, particularly enveloped viruses, have proven to be an excellent tool for studying protein synthesis, processing and sorted transport, thus greatly contributing to our current knowledge of the complex pathways of intracellular traffic. To infect a host cell, enveloped viruses have evolved a mechanism which involves membrane fusion. Although the molecular details of this process remain unclear, it has been well established that specific virally encoded membrane proteins are responsible for triggering the fusion reaction. Because of the simplicity of the viral membrane protein composition of most enveloped viruses, viral systems provide a valuable tool for studying protein-mediated membrane fusion. Elucidation of the molecular mechanisms underlying virus-cell fusion may lead to a better understanding of the ubiquitous intracellular fusion events where specific proteins are also involved.