Effects of the Cowpea chlorotic mottle bromovirus β-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 β-hexamer, consists of six short β-strands. The β-hexamer was postulated to play a critical role in the assembly and stability of the virion by stabilizing hexameric capsomers (Speir et al., 1995). Mutational analyses of the β-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 β-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.