Vaccinia Virus H7-Protein Organizes the Viral Scaffold Protein-Essential for Membrane and Virus Assembly

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 62kDa 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 21kDa 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 17kDa 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 organize into a regular network, 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.