Department of Biochemistry and Molecular Biology, Oklahoma State University, Stillwater, OK 74078.
Department of Microbiology, Immunology, and Molecular Genetics, University of Texas Health Science Center at San Antonio, San Antonio, TX 78229.
Proc Natl Acad Sci U S A. 2018 Jul 3;115(27):7028-7032. doi: 10.1073/pnas.1805855115. Epub 2018 Jun 18.
Cellular membranes are maintained as closed compartments, broken up only transiently during membrane reorganization or lipid transportation. However, open-ended membranes, likely derived from scissions of the endoplasmic reticulum, persist in vaccinia virus-infected cells during the assembly of the viral envelope. A group of viral membrane assembly proteins (VMAPs) were identified as essential for this process. To understand the mechanism of VMAPs, we determined the 2.2-Å crystal structure of the largest member, named A6, which is a soluble protein with two distinct domains. The structure of A6 displays a novel protein fold composed mainly of alpha helices. The larger C-terminal domain forms a unique cage that encloses multiple glycerophospholipids with a lipid bilayer-like configuration. The smaller N-terminal domain does not bind lipid but negatively affects lipid binding by A6. Mutations of key hydrophobic residues lining the lipid-binding cage disrupt lipid binding and abolish viral replication. Our results reveal a protein modality for enclosing the lipid bilayer and provide molecular insight into a viral machinery involved in generating and/or stabilizing open-ended membranes.
细胞膜保持封闭的隔室状态，只有在膜重组或脂质运输过程中才会短暂地破裂。然而，在痘病毒感染细胞中，病毒包膜组装过程中会持续存在从内质网断裂而来的无终止膜。一组病毒膜组装蛋白（VMAPs）被鉴定为该过程的必需蛋白。为了理解 VMAPs 的机制，我们测定了最大成员 A6 的 2.2Å 晶体结构，A6 是一种可溶性蛋白，具有两个不同的结构域。A6 的结构呈现出一种新颖的蛋白折叠，主要由α螺旋组成。较大的 C 末端结构域形成一个独特的笼状结构，可将多个甘油磷脂以类似于双层脂膜的结构包封在内。较小的 N 末端结构域不结合脂质，但通过 A6 负向影响脂质结合。结合笼状结构的关键疏水性残基的突变会破坏脂质结合并使病毒复制失活。我们的结果揭示了一种封闭双层脂膜的蛋白模式，并为涉及生成和/或稳定无终止膜的病毒机制提供了分子见解。
