INFLUENZA HEMAGGLUTININ-MEDIATED MEMBRANE-FUSION - INFLUENCE OF RECEPTOR-BINDING ON THE LAG PHASE PRECEDING FUSION

Fusion of influenza virus with liposomes is triggered by low pH, resul ting in a conformational change in the fusion protein (HA) and-the ins ertion of fusion peptides, from HA into the Liposomal membrane. Fusion does not take place immediately after insertion but is preceded by a lag phase, the duration of which, as we have found previously, depends on the presence of ganglioside receptors in the liposomal membrane [S tegmann, T., White, J. M., and Helenius, A. (1990) EMBO J. 9, 4231-424 1]. Here we have investigated why that is the case. Surprisingly, the 2-4-fold shorter lag phase observed with phosphatidylcholine (PC)/phos phatidylethanolamine (PE)/ganglioside liposomes was ndt due to slower or more readily reversible binding of the virus to PC/PE liposomes lac king receptors. Nevertheless, using liposomes with various glycolipids as targets, it was found that specific HA-receptor interactions were required for a shorter lag, and not just the negative charge of the ga ngliosides, or the presence of ceramide lipid tails in the Liposomal m embrane. Receptor binding also,did not facilitate the conformational c hange in HA. Surprisingly, however, it was found that after an incubat ion of the virus at low pH in the absence of target membranes at 0 deg rees C for several minutes, the binding and fusion activity of virus u sing PC/PE liposomes, but not PC/PE/ganglioside Liposomes as targets, was decreased. The population of virus that did still bind to and fuse with the PC/PE liposomes after low pH preincubation did so after a si gnificantly increased lag time. Binding of virus to Liposomes without receptors is solely due to insertion of viral fusion peptides into the liposomal membrane, suggesting that the availability of fusion peptid es is decreased after low pH preincubation. Zn accordance with this su ggestion, if the Liposomal Lipid bilayers were in the gel phase, bindi ng of virus to PC liposomes but not to PC/ganglioside liposomes was st rongly inhibited, and the lag phase was about 9 times shorter for lipo somes with receptors. Therefore, these results suggest that gangliosid e receptors shorten the lag phase because they facilitate insertion of fusion peptides into the target membrane.