In situ cryo-electron tomography of vaccinia virus exit from infected cells

Poxvirus-infected cells release newly assembled virions via Golgi-mediated envelopment and subsequent exocytosis at the plasma membrane, prior to cell lysis. During exocytosis, the viral outer membrane fuses with the plasma membrane but retains a distinct identity, beneath which septins and clathrin are independently recruited. Clathrin enhances actin-dependent viral spread, while septins suppress virus release from the cell. Here, we used cryo-electron tomography and structured illumination microscopy (SIM) to study vaccinia egress. Our 3D analysis reveals that Golgi-mediated envelopment is a highly adaptable process that involves remodelling of the enfolding membrane and a final pore sealing step. Filaments that are ~ 4-nm wide, which are likely septins based on their structural features and our SIM imaging, run parallel to the inner surface of the plasma membrane beneath virions attached to the cell surface. In contrast, clathrin induces plasma membrane invaginations at distinct subdomains beneath the virus. We propose that actin assembly at these clathrin subdomains would provide a template for subsequent virus-induced actin polymerization, therefore enhancing it.