Parameters affecting fusion between Sendai virus and liposomes. Role of viral proteins, liposome composition, and pH

Abstract

A kinetic and quantitative characterization of the fusion process between Sendai virus and phospholipid vesicles is presented. Membrane fusion was monitored in a direct and continuous manner by employing an assay which relies on the relief of fluorescence self-quenching of the probe octadecylrhodamine B chloride which was located in the viral membrane. Viral fusion activity was strongly dependent on the vesicle lipid composition and was most efficient with vesicles solely consisting of acidic phospholipids, particularly cardiolipin (CL). This result implies that the fusion of viruses with liposomes does not display an absolute requirement for specific membrane receptors. Incorporation of phosphatidylcholine (PC), rather than phosphatidylethanolamine (PE), into CL bilayers strongly inhibited fusion, suggesting that repulsive hydration forces interfere with the close approach of viral and target membrane. Virus-liposome fusion products were capable of fusing with liposomes, but not with virus. In contrast to fusion with erythrocyte membranes fusion between virus and acidic phospholipid vesicles was triggered immediately, did not strictly depend on viral protein conformation, and did not display a pH optimum around pH 7.5. On the other hand, with vesicles consisting of PC, PE, cholesterol, and the ganglioside GD1a, the virus resembled more closely the fusogenic properties that were seen with erythrocyte target membranes. Upon decreasing the pH below 5.0, the viral fusion activity increased dramatically. With acidic phospholipid vesicles, maximal activity was observed around pH 4.0, while with GD1a-containing zwitterionic vesicles the fusion activity continued to increase with decreasing pH down to values as low as 3.0. This indicates that the fusion susceptibility of the virus at low pH also depends on the molecular properties of the target membrane. Presumably, these properties determine the ability of the viral proteins to interact hydrophobically with the membranes, thus triggering fusion. Fusion between Sendai virus and CL liposomes at low pH was not affected when the virus had been pretreated with trypsin, suggesting that low pH induced fusion was largely mediated by the viral binding protein HN rather than the fusion protein F. It is concluded that, depending on the pH, different mechanisms are operational in the fusion process between Sendai virus and liposomes; i.e., fusion with liposomes can, at least partly, be accomplished in a manner which may not bear physiological significance.