Structural transition of the immature poxvirus scaffold from preassembly to the scaffold lattice.

During poxvirus morphogenesis, the external scaffold defines the shape and size of immature virions. However, its structural characterization has been hindered by the pleomorphic nature of authentic virus particles. Here, we present single-particle cryo-electron microscopy and cryo-electron tomography structures of two distinct in vitro assemblies of the vaccinia virus scaffold protein D13 that recapitulate oligomeric states previously observed in situ. These structures reveal a dramatic transition from rod-like oligomers, representing a pre-assembly state, to a fully formed scaffold, triggered by the docking of the N-terminal peptide of the viral membrane protein A17 into the base cavity of D13. We propose that this interaction destabilizes the pre-assembly conformation and initiates scaffold formation upon viral membrane recruitment, ultimately leading to the formation of immature virions. Our findings provide structural insights into the early stages of poxvirus morphogenesis and establish a mechanistic link between conformational changes in D13 and scaffold assembly.