Kinetics and Extent of Fusion of Influenza Virus and Sendai Virus with Liposomes

Abstract

The kinetics and extent of low-pH induced fusion of influenza virus and Sendai virus with large unilamellar liposomes were investigated with fluorescence assays for lipid mixing. The model views the overall fusion reaction as a sequence of a second-order process of virus-liposome aggregation, which is partially reversible, followed by the first-order fusion reaction itself. The calculations gave good simulations and predictions for the kinetics and extent of fusion at different virus/liposome concentrations and ratios, and provided a pattern of the dependence of viral fusion activity on liposome composition, i.e., acidic and neutral phospholipids, cholesterol and gangliosides. The fusion products consist of a single virus particle and several liposomes, indicating negative cooperativity between viral glycoproteins and those present in the fusion products. At pH 5.0 and 37°C very high rate constants for aggregation and fusion were obtained and essentially all of the virus particles were capable of fusing with cardiolipin liposomes. At higher pH values the rate constants of fusion were considerably smaller for all liposome compositions and the percent of virus particles capable of fusing was lowered, whereas the aggregation rate constants were only moderately affected. The fusion activity of influenza virus was reduced by its exposure to low pH, The inactivation reflected intramembrane events and was independent of virus concentrations. A mode of virus inactivation due to formation of clusters of glycoproteins in its membrane at low pH is consistent with our results.