Abstract
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.
Highlights
Enveloped virus assembly is a highly coordinated process that requires budding the cell membrane and incorporating all necessary components into the viral particle for subsequent cell infection
Alphavirus particles assemble into two icosahedral protein layers: the glycoprotein spike shell embedded in a lipid envelope and the inner nucleocapsid (NC) core
Contradictory mechanisms were proposed for the assembly of twolayered icosahedral alphavirus particles, largely centered around whether capsid proteins (Cps) pre-assemble into icosahedral NCs5,6 that serve as the structural templates for spike incorporation or if spikes drive coassembly of NC and spike lattices and transmit symmetry to initially non-icosahedral NCs2
Summary
Enveloped virus assembly is a highly coordinated process that requires budding the cell membrane and incorporating all necessary components into the viral particle for subsequent cell infection. This process is relatively well understood for viruses whose assembly and budding are driven solely by capsid or matrix proteins, such as retroviruses and filoviruses[1]. Without direct visualization of NCs and spikes prior to and during the assembly/budding process in virus-infected cells, the mechanistic roles of each protein layer in particle assembly and final release of completed viruses from the cell membrane remains poorly defined
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