Endoplasmic reticulum-Golgi intermediate compartment membranes and vimentin filaments participate in vaccinia virus assembly.

Vaccinia virus (VV) has a complex morphogenetic pathway whose first steps are poorly characterized. We have studied the early phase of VV assembly, when viral factories and spherical immature viruses (IVs) form in the cytoplasm of the infected cell. After freeze-substitution numerous cellular elements are detected around assembling viruses: membranes, ribosomes, microtubules, filaments, and unidentified structures. A double membrane is clearly resolved in the VV envelope for the first time, and freeze fracture reveals groups of tubules interacting laterally on the surface of the viroplasm foci. These data strongly support the hypothesis of a cellular tubulovesicular compartment, related to the endoplasmic reticulum-Golgi intermediate compartment (ERGIC), as the origin of the first VV envelope. Moreover, the cytoskeletal vimentin intermediate filaments are found around viral factories and inside the viroplasm foci, where vimentin and the VV core protein p39 colocalize in the areas where crescents protrude. Confocal microscopy showed that ERGIC elements and vimentin filaments concentrate in the viral factories. We propose that modified cellular ERGIC membranes and vimentin intermediate filaments act coordinately in the construction of viral factories and the first VV form through a unique mechanism of viral morphogenesis from cellular elements.