A non-enveloped virus requires a membrane lesion to deliver its genome into a target cell¹. For rotaviruses, membrane perforation is a principal function of the viral outer-layer protein, VP4^2,3. Here we describe the use of electron cryomicroscopy to determine how VP4 performs this function and show that when activated by cleavage to VP8* and VP5*, VP4 can rearrange on the virion surface from an ‘upright’ to a ‘reversed’ conformation. The reversed structure projects a previously buried ‘foot’ domain outwards into the membrane of the host cell to which the virion has attached. Electron cryotomograms of virus particles entering cells are consistent with this picture. Using a disulfide mutant of VP4, we have also stabilized a probable intermediate in the transition between the two conformations. Our results define molecular mechanisms for the first steps of the penetration of rotaviruses into the membranes of target cells and suggest similarities with mechanisms postulated for other viruses.

无包膜病毒需要膜损伤才能将其基因组传递到靶细胞¹。对于轮状病毒，膜穿孔是病毒外层蛋白 VP4 ^{2,3的主要功能}. 在这里，我们描述了使用电子低温显微镜来确定 VP4 如何执行此功能，并表明当通过裂解为 VP8* 和 VP5* 激活时，VP4 可以在病毒粒子表面从“直立”构象重新排列为“倒置”构象。反向结构将先前埋藏的“足”域向外投射到病毒粒子附着的宿主细胞膜中。病毒颗粒进入细胞的电子冷冻断层照片与这张照片一致。使用 VP4 的二硫化物突变体，我们还在两种构象之间的转变中稳定了可能的中间体。我们的结果定义了轮状病毒渗透到靶细胞膜的第一步的分子机制，并表明与其他病毒假定的机制相似。