In situ Alphavirus Assembly and Budding Mechanism Revealed by Cellular CryoET

Summary Chikungunya virus (CHIKV) is an alphavirus and the etiological agent for debilitating arthritogenic disease in humans. Previous studies with purified virions or budding mutants have not resolved the structural mechanism of alphavirus assembly in situ. Here we used cryogenic electron tomography (cryoET) imaging of CHIKV-infected human cells and subvolume classification to resolve distinct assembly intermediate conformations. These structures revealed that particle formation is driven by the spike envelope layer. Additionally, we showed that asymmetric immature nucleocapsids (NCs) provide scaffolds to trigger assembly of the icosahedral spike lattice, which progressively transforms immature NCs into icosahedral cores during virus budding. Further, cryoET of the infected cells treated with neutralizing antibodies (NAbs) showed that NAb-induced blockage of CHIKV assembly was achieved by preventing spike-spike lateral interactions that are required to bend the plasma membrane around NC cores. These findings provide molecular mechanisms for designing antivirals targeting spike-driven assembly/budding of viruses.