Virion swelling is not required for cotranslational disassembly of cowpea chlorotic mottle virus in vitro

The mechanism by which virions of cowpea chlorotic mottle virus (CCMV) disassemble and allow for translation of the virion RNA is not well understood. Previous models have suggested that virion swelling is required to expose the virion RNA for translation in a process referred to as cotranslational disassembly (M. Brisco, R. Hull, and T. M. A. Wilson, Virology 148:210-217, 1986; J. W. Roenhorst, J. W. M. van Lent, and B. J. M. Verduin, Virology 164:91-98, 1988; J. W. Roenhorst, J. M. Verduin, and R. W. Goldbach, Virology 168:138-146, 1989). Previous work in our laboratory has identified point mutations in the CCMV coat protein which result in virions with altered swelling characteristics (J. Fox, F. G. Albert, J. Speir, and M. J. Young, Virology 227:229-233, 1997; J. M. Fox, X. Zhao, J. A. Speir, and M. J. Young, Virology 222:115-122, 1996). The wild-type and mutant CCMV virions were used to correlate virion swelling with the ability of virion RNA to be translated in a cell-free wheat germ extract. Mutant virions unable to swell (cpK42R) are as infectious as wild-type virions in vivo, and the levels of translated encapsidated virion RNA are similar to those of wild-type virions in vitro. Mutant virions capable of swelling but not of disassembling in vitro (cpR26C) are noninfectious and have severely reduced levels of translation of the encapsidated virion RNA in vitro. These studies suggest that virion swelling is not required for the cotranslational disassembly of CCMV. Additionally, the results indicate that there is a pH-dependent structural transition in the virion, other than swelling, that results in the RNA's being exposed for translation in vitro. An alternative model suggesting that cotranslational disassembly of CCMV involves presentation of the virion RNA through the virion fivefold axis is proposed.