Role of target membrane structure in fusion with influenza virus: effect of modulating erythrocyte transbilayer phospholipid distribution.

To study the role of the target membrane in influenza virus fusion we chose erythrocyte membranes whose phospholipid arrangement can readily be modified. The phospholipids of normal erythrocytes are arranged asymmetrically across the plasma membrane; phosphatidylcholine (PC) and sphingomyelin are predominantly on the outer surface, whereas others such as phosphatidylserine (PS) and phosphatidylethanolamine (PE) are predominantly restricted to the inner leaflet. However, erythrocytes can be lyzed and resealed under conditions where the asymmetric distribution of phospholipids is lost or retained. Low pH-induced fusion of the A/PR 8 strain of influenza virus, monitored spectrofluorometrically by the octadecylrhodamine dequenching assay, was more rapid with lipid-symmetric erythrocyte ghosts than with lipid-asymmetric ghosts or intact erythrocytes. Neither conversion of PS in the lipid-symmetric ghost membrane to PE by means of the enzyme PS decarboxylaze, nor incorporation of spin-labeled phospholipid analogs with PS, PC or PE headgroups into the outer leaflet of lipid-asymmetric erythrocytes altered rates or extents of fusion of A/PR 8 with the modified target. These results indicate that effects on influenza virus fusion are not associated with any particular phospholipid headgroup, but rather related to the packing characteristics of the target membrane.