Kimura T, Ohyama A
Department of Microbiology, Kansai Medical University, Osaka, Japan.
J Gen Virol. 1988 Jun;69 ( Pt 6):1247-54. doi: 10.1099/0022-1317-69-6-1247.
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.
西尼罗河病毒的主要包膜蛋白（E）在略低于中性的最佳pH值下介导病毒包膜与靶细胞膜之间的融合。这种融合对于病毒进入机制至关重要，使病毒能够从酸性内体区室中逃逸。为了确定病毒E蛋白在融合反应中的作用，利用E蛋白的蛋白酶消化和病毒感染性测定，对E蛋白的构象变化以及病毒感染性的伴随变化进行了定量研究。结果表明，构象变化以pH依赖的方式发生，pH上限为7.0，在pH 6.4及以下时发生最大转变。这种转变迅速，酸化后15秒内达到最大值的一半。游离或细胞结合的病毒粒子暴露于酸性pH值会导致感染性以几乎相同的pH依赖方式丧失。基于这些发现，有人提出病毒进入巨噬细胞有两种不同模式，即感染性内吞作用和病毒与质膜的非感染性融合，细胞外介质的pH值决定了哪种模式占主导。讨论了这些观察结果对E蛋白在膜融合中的作用以及融合表位可能定位的影响。
