Abstract
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 pseudohexagonal lattice forms inside IV to produce the viral core. This lattice, consisting of trimeric pillars, appears as a palisade in cross-section. Our measurements suggest that the length of this core is determined by the D13 lattice. During maturation, which involves a 50% reduction in volume, the viral membrane becomes corrugated as it adapts to the IMV core in a process that does not appear to require membrane removal. Our study suggests that the flexible viral membrane is controlled by the consecutive D13 and palisade lattices, which structurally define vaccinia assembly and maturation.
Significance statement Poxviruses such as variola virus (smallpox) and monkeypox induce disease in humans. Other poxviruses, including vaccinia virus and modified vaccinia Ankara (MVA), are used as vaccine vectors. Despite their importance, a structural understanding of how infectious poxvirus virions assemble in the cytoplasm of infected cells is lacking. In this study, we used cryo-electron tomography of frozen-hydrated vaccinia infected cells to structurally characterise the maturation of spherical immature virions into the infectious brick-shaped intracellular mature virus. These high-resolution in situ observations reveal new features concerning the viral membrane and the inner core of mature virions. The model that emerges suggests that two consecutive viral-derived protein lattices drive vaccinia assembly and maturation.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
The abstract has been re-written and supplementary movies have been added.
