some cases, as with influenza virus, the fusion peptide occurs We have developed a micropipette aspiration assay to observe at the N-terminus of the fusion protein; in other cases, as the lysis of large (20-30 mm diameter) vesicles aspirated using with Semliki Forest virus, the fusion peptide occurs in the micropipettes. Single membrane lysis events can be seen with the middle of the sequence. light microscope and are followed using fluorescence assays and The mechanism by which influenza virus infects a cell video microscopy. In this study, we have examined the ability of has been particularly widely studied. The virus binds to sialic two analogs of the fusion peptide from influenza virus hemagglutiacid on the target cell surface, and the entire virus is endocynin to induce the lysis of large unilamellar egg phosphatidylcholine tosed into the cell. The endocytosed vesicle combines with vesicles, as a function of peptide concentration and pH. X31 is a an endosome, and the pH of the compartment is reduced.
wild-type peptide from one strain of Influenza A, and E5 is an analogue which has several residues replaced by glutamate resi-At the lower pH, influenza virus hemagglutinin undergoes dues. Both peptides were found to induce lysis of large vesicles in a conformational change, exposing the fusion peptide. There a pH-dependent manner. Both peptides exhibited maximal activity are many proposed models for the mechanism by which at pH 5, measured in terms of both rate and extent of lysis. E5 influenza hemagglutinin causes fusion (7,(11)(12)(13)(14)(15)(16)(17)53), but was active at much lower concentrations than X31. Our results the details have been difficult to elucidate experimentally. with both peptides are compared to results published from other The recent solution of the structure of a portion of the laboratories. α§ 1997 Academic Press hemagglutinin molecule at low pH (16) has shown that the