Effect of X31 influenza virus fusion on phosphatidylserine asymmetry in erythrocytes.

Influenza virus fusion is mediated by its fusion protein, hemagglutinin (HA). HA undergoes a low pH dependent conformational change that results in insertion into the cell membrane bilayer, formation of a fusion pore, and merging of membrane lipids and establishment of cytoplasmic continuity. Erythrocytes, which can serve as targets of influenza virus fusion, display an asymmetric transbilayer arrangement of their phospholipids. The effect of influenza virus fusion on erythrocyte phosphatidylserine asymmetry was determined. Influenza virus were bound to erythrocytes containing the fluorescent membrane probe NBD-PS in the inner leaflet. Induction of fusion by exposure to a low pH environment resulted in movement of PS to the outer leaflet of the cell as well as hemolysis. Insertion of the fusion protein into erythrocytes and subsequent fusion can be distinguished from hemolysis by examining the interaction of a soluble form of HA (BHA) with cells and by monitoring viral fusion at low temperatures. No hemolysis was observed under either condition. BHA binding and insertion into cells did not affect the asymmetry of PS. Incubation of influenza virus fusion at pH 5, 0 degrees C resulted in complete fusion but no outward movement of PS was observed. These findings suggest the viral fusion pore does not involve a rearrangement of the transbilayer phospholipid organization of the target membrane.