FUSION OF INFLUENZA-VIRUS WITH SIALIC ACID-BEARING TARGET MEMBRANES

We have monitored the fusion of intact A/PR/8/34 influenza virus with glycophorin-bearing liposomes and with ganglioside- (GD1a-) containing liposomes. The lipid bilayers of the glycophorin-bearing liposomes ha d several compositions, including pure dioleoylphosphatidylethanolamin e (DOPE), pure egg phosphatidylethanolamine (EPE), and pure dioleoylph osphatidylcholine (DOPC). Examination of the temperature dependence of fusion for these and other compositions showed that even if the lipid s are competent to form inverted hexagonal phases (H-II), there is no enhancement of the fusion rate constant at the L(alpha)-H-II phase tra nsition temperature of the lipids, T-H. Thus, the H-II phase transitio n is not involved in the HA-mediated fusion mechanism. However, this m echanism is sensitive to lipid composition, in that PC bilayers fused more slowly than PE-containing bilayers above 20 degrees C. These resu lts show that the HA-mediated fusion mechanism depends primarily upon specific lipid-protein interactions, although the fundamental paramete rs of lipid phase stability (interstice stabilization and monolayer sp ontaneous radius of curvature) may also be important. The fact that H- II phase-competent lipid bilayers in the glycophorin liposomes do not enhance the HA-mediated fusion rate strongly suggests that substantial bilayer-bilayer contact is not involved in HA-mediated fusion. Previo usly, we have shown that glycoprotein-bearing liposomes bind to HA-exp ressing cells specifically through HA-glycophorin interactions and tha t fusion is mediated by HAs not bound to glycophorin. Thus, with respe ct to the target membrane, the fusion site involves just the lipid bil ayer. Our results with GD1a-containing liposomes strongly suggest that HAs bound to this sialic acid-bearing molecule are likewise incapable of participating in the fusion site. This could be due to a diminishe d lateral mobility of the HAs simultaneously bound to both closely app osed membranes. Finally, we find that the low-pH-induced viral inactiv ation is inhibited by binding to either glycophorin- or GD1a-containin g target membranes.