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. Here, we used cryo-electron tomography and structured illumination microscopy to study vaccinia egress. Our 3D analysis reveals that Golgi-mediated envelopment is a flexible process that involves remodelling of the enfolding membrane and a final step that seals a small pore. During subsequent 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. We found that septin filaments run parallel to the inner surface of the plasma membrane beneath virions attached to the cell surface. In contrast, clathrin induces the formation of plasma membrane invaginations in distinct subdomains. We propose that actin assembly at these subdomains provides a template for subsequent virus-induced actin polymerization.