A succession of two viral lattices drives vaccinia virus assembly

Abstract

During its cytoplasmic replication, vaccinia virus assembles non-infectious spherical immature virions (IVs) coated by a viral D13 lattice. Subsequently, IV mature into infectious brick-shaped intracellular mature virions (IMVs) that lack D13. Here, we performed cryo-electron tomography (cryo-ET) 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. As maturation occurs, which involves a 50% reduction in particle 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.

Palisade structure in intact vaccinia virions

“…S1). Importantly, the architecture of purified virions is identical to those imaged inside infected cells by cryo-ET ( 11 ). At the acquired magnification and defocus conditions, both leaflets of the viral membrane are discernible, whereas the inner wall of the core beneath the palisade appears as a densely packed layer that lacks membrane features ( Fig.…”

“…Subsequent proteolytic processing of the viral proteins A3, A10, and L4 by the protease I7 and the formation of disulfide bonds trigger the maturation of IVs into infectious intracellular mature virions (IMVs) ( 7 – 10 ). This maturation involves a dramatic structural reorganization of IV into brick-shaped IMV with a distinct internal core containing the genome ( 3 , 11 , 12 ). The core boundary is defined by a capsid-like structure with trimers projecting from an inner wall in a pseudohexagonal arrangement, which gives the appearance of a palisade ( 11 , 13 , 14 ).…”

“…This maturation involves a dramatic structural reorganization of IV into brick-shaped IMV with a distinct internal core containing the genome ( 3 , 11 , 12 ). The core boundary is defined by a capsid-like structure with trimers projecting from an inner wall in a pseudohexagonal arrangement, which gives the appearance of a palisade ( 11 , 13 , 14 ). During maturation, the viral membrane becomes tightly associated with the palisade by an unknown mechanism.…”

“…Moreover, A10 and A4 form a stable complex that can be cross-linked in purified IMVs ( 22 , 23 ). Recent observations suggest that three A10/A4 heterodimers are consistent with the mass and shape of a single palisade trimer, based on a low-resolution subtomogram-averaged map of the palisade and AlphaFold2 structural predictions ( 11 ). Altogether, current evidence suggests that A10 and/or A4 form the trimeric units that constitute the palisade in intact virions.…”

London calling: The 5th UK Cellular Microbiology Network Meeting

“…The next talk was from Samkeliso Lisa Blundell (from David Holden's lab, Imperial College London), describing the Salmonella effector SteD as a co‐activator of the E3 ubiquitin ligase WWP2. The last talk of this session was Miguel Hernandez‐Gonzalez (from Michael Way's lab, Francis Crick), who is imaging poxvirus assembly and exit from infected cells using state‐of‐the‐art microscopy techniques, including cryo‐electron tomography (Hernandez‐Gonzalez et al., 2023).…”