Effects of the cowpea chlorotic mottle bromovirus beta-hexamer structure on virion assembly.

The X-ray crystal structure of Cowpea chlorotic mottle bromovirus (CCMV) revealed a unique tubular structure formed by the interaction of the N-termini from six coat protein subunits at each three-fold axis of the assembled virion. This structure, termed the beta-hexamer, consists of six short beta-strands. The beta-hexamer was postulated to play a critical role in the assembly and stability of the virion by stabilizing hexameric capsomers. Mutational analyses of the beta-hexamer structure, utilizing both in vitro and in vivo assembly assays, demonstrate that this structure is not required for virion formation devoid of nucleic acids in vitro or for RNA-containing virions in vivo. However, the beta-hexamer structure does contribute to virion stability in vitro and modulates disease expression in vivo. These results support a model for CCMV assembly through pentamer intermediates.