Cryo-ET of infected cells reveals that a succession of two lattices drives vaccinia virus assembly

During its cytoplasmic replication, vaccinia virus assembles non-infectious spherical immature virions (IV) coated by a viral D13 lattice. Subsequently, IV mature into infectious brick-shaped intracellular mature virus (IMV) that lack D13. Here, we performed cryo-electron tomography of frozen-hydrated vaccinia-infected cells to structurally characterise the maturation process in situ. During IMV formation a new viral core forms inside IV with a wall consisting of trimeric pillars arranged in a new pseudohexagonal lattice. This lattice appears as a palisade in cross-section. During maturation, which involves a 50% reduction in virion volume, the viral membrane becomes corrugated as it adapts to the newly formed viral core in a process that does not appear to require membrane removal. Our study suggests that the length of this core is determined by the D13 lattice and that the consecutive D13 and palisade lattices control virion shape and dimensions during vaccinia assembly and maturation.