Mechanism of fusion of Sendai virus: role of hydrophobic interactions and mobility constraints of viral membrane proteins. Effects of polyethylene glycol.

The mechanism of Sendai virus fusion was investigated by studying the effect of the dehydrating agent polyethylene glycol (PEG) on the interaction of the virus with erythrocyte membranes. The initial rate of virus fusion, monitored continuously by a fluorescence membrane fusion assay, increases approximately 5-fold in the presence of small amounts (4%, w/v) of PEG. The polymer did not trigger a massive nonspecific fusion event, as the limited number of virus particles that fuse per erythrocyte ghost remains unaltered. A mass action kinetic analysis reveals that the binding rate constant increases approximately 1.5-fold; however, the fusion rate constant is enhanced by about an order of magnitude. The results demonstrate that hydrophobic interaction forces dominate the actual fusion step of the virus. Below about 22 degrees C, the viral membrane proteins appear to be clustered, as revealed by temperature-dependent fluorescence measurements of fluorescently tagged viral proteins. Clustering is not modulated by the presence of PEG, and fusion at those conditions is not observed. It is concluded that in addition to hydrophobic interactions, constraints in the mobility of the viral membrane proteins codetermine the fusogenic capacity of the virus. Such constraints have to be relieved in order to allow the occurrence of the hydrophobic interactions. PEG primarily affects the surface properties of the viral membrane, including the properties of the membrane glycoproteins. We hypothesize that during virus-target membrane interaction but prior to the actual fusion reaction, the fusion protein may undergo a conformational change, triggered by an enhancement in hydrophobic environment, which accounts for the need to establish close, i.e. fusion-susceptible intermembrane contact between virus and target membrane.