Effects of the cowpea chlorotic mottle bromovirus beta-hexamer structure on virion assembly - PubMed (original) (raw)

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

D Willits et al. Virology. 2003.

Abstract

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.

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Figures

Fig. 1

Fig. 1

Structural features of CCMV coat protein and virus particles. (A) Ribbon diagram of the CCMV coat protein. The β-strand involved in the β-hexamer formation (amino acids 29–33) is labeled. (B) Enlarged views of a hexamer viewed down the three-fold axis with the β-hexamer emphasized. (C) Side view of the β-hexamer motif.

Fig. 2

Fig. 2

Electron micrographs of CCMV β-hexamer deletion mutants assembled under in vitro and in vivo conditions. (A) Purified NΔβ27–35 coat protein in vitro assembled into T = 3 particles in the absence of viral RNA. (B) Purified NΔβ327–35 coat protein in vitro assembled in the presence of viral RNA showing aberrant assembly products. (C) β-hexdel T = 3 particles purified from protoplasts or plants.

Fig. 3

Fig. 3

(Top) Shaded-surface representations of the image reconstructions of in vitro assembled CCMV wild-type (WT) and NΔβ27–35 empty particles. (Bottom) Same as (Top) but with front half of particles removed to show the particle interiors indicating the presence (WT) or absence (NΔβ27–35) of β-hexamer density at the pseudo-six-fold axes (arrow).

Fig. 4

Fig. 4

Typical symptom expression of cowpea infected with the β-hexdel mutant virus as compared to wild-type virus.

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