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Chikungunya virus assembly and egress


Cryo-electron tomography (cryo-ET) provides three-dimensional structural information on complex molecules by acquiring, aligning and averaging a series of sequentially tilted two-dimensional images from thin, frozen samples5. To identify alphavirus assembly and budding mechanisms, Chmielewski and colleagues used cryo-ET to capture intermediates in the morphogenesis of the CHIKV virion in cultured cells. They analysed almost 2,000 individual CHIKV assembly events and defined 12 sequential steps in virion assembly. Enumeration of viral complexes at each step identified a limited number of particles at the early and intermediate stages of assembly. The majority of particles were grouped into the late steps of the assembly process in which the glycoprotein shell is largely complete. This distribution suggests that assembly starts quickly but slows as virions approach the budding step. The images confirmed the presence of preformed NC structures in the cytoplasm, but showed that these structures lack the uniform, icosahedral conformation that is needed to serve as a template for envelope protein organization. Instead, they identified a collaborative process, in which interactions between envelope spikes and NCs at the plasma membrane result in the concerted ordering of both the outer spikes of the virion and the inner NC core. In the absence of an NC core, lateral interactions between envelope spikes are sufficient to promote self-assembly into higher-order structures, including planar sheets and tubular structures, but assemblies of spike pentagons, such as those found on infectious virions, were only observed at sites of virion assembly with an associated NC structure.


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