The major envelope protein (E) of West Nile virus mediates fusion between the membranes of the viral envelope and the target cell at optimum pH values of just below neutrality. The fusion is critical for the entry mechanism, allowing virus to escape from the acidic endosomal compartment. To define the role of the viral E protein in the fusion reaction, the conformational change in E and concomitant change of viral infectivity were studied quantitatively, using protease digestion of the E protein and assay of viral infectivity. The results showed that the conformational change occurred in a pH-dependent manner with an upper threshold of pH 7.0 and maximum conversion occurring at pH 6.4 and below. The conversion was rapid and reached a half-maximal value within 15 s after acidification. The exposure of free or cell-bound virions to acid pH resulted in the loss of infectivity in an almost identical pH-dependent manner. Based on these findings, it is suggested that there are two distinct viral modes of entry into macrophages, i.e. infectious endocytosis and non-infectious viral fusion with plasma membranes, with the pH of the extracellular medium determining which of these predominates. The implications of these observations for the role of the E protein in membrane fusion and the probable localization of fusion epitopes are discussed.


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