Vaccinia virus H7-protein is required for the organization of the viral scaffold protein into hexamers

Abstract

Viruses of the giant virus family are characterized by a structurally conserved scaffold-capsid protein that shapes the icosahedral virion. The vaccinia virus (VACV) scaffold protein D13, however, transiently shapes the newly assembled viral membrane in to a sphere and is absent from the mature brick-shaped virion. In infected cells D13, a 62 kDa polypeptide, forms trimers that arrange in hexamers and a honey-comb like lattice. Membrane association of the D13-lattice may be mediated by A17, an abundant 21 kDa viral membrane protein. Whether membrane binding mediates the formation of the honey-comb lattice or if other factors are involved, remains elusive. Here we show that H7, a 17 kDa protein conserved among poxviruses, mediates proper formation of D13-hexamers, and hence the honey comb lattice and spherical immature virus. Without H7 synthesis D13 trimers assemble into a large 3D network rather than the typical well organized scaffold layer observed in wild-type infection, composed of short D13 tubes of discrete length that are tightly associated with the endoplasmic reticulum (ER). The data show an unexpected role for H7 in D13 organization and imply that formation of the honey-comb, hexagonal, lattice is essential for VACV membrane assembly and production of infectious progeny. The data are discussed with respect to scaffold proteins of other giant viruses.

Poxvirus under the eyes of electron microscope

“…For example, cryo-ET examinations of deletion mutants of viral membrane assembly proteins (VMAPs), a group of proteins including L2, A30.5, A11, A6 and H7, demonstrated that one or more VMAPs interact for ER membrane hijacking and the separation of viral membrane from its origin (Suarez et al 2017;Weisberg et al 2017). Recent study proposed H7 is directly relevant in hexameric organization of D13 scaffold protein, suggesting complex interactions among VMAPs, D13, A17 and A14 (Tonnemacher et al 2022).…”