Genetic analyses of putative conformation switching and cross-species inhibitory domains in Microviridae external scaffolding proteins.

Putative conformational switching and inhibitory regions in the Microviridae external scaffolding protein were investigated. Substitutions for glycine 61, hypothesized to promote a postdimerization conformational switch, have dominant lethal phenotypes. In previous studies, chimeric alpha3/phiX174 proteins for structures alpha-helix 1 and loop 6/alpha-helix 7 inhibited phiX174 morphogenesis when expressed from high copy number plasmids. To determine if inhibition was due to overexpression, chimeric genes were constructed into the phiX174 genome. In coinfections with wild-type, protein ratios would be 1:1. The helix 1 chimera has a recessive lethal phenotype; thus, overexpression confers inhibition. In single infections, the mutant cannot form procapsids, suggesting that helix 1 mediates the initial recognition of structural proteins. The lethal chimeric helix 7 protein has a dominant phenotype. Alone, the mutant forms defective procapsids, suggesting a later morphogenetic defect. The results of second-site genetic analyses indicate that the capsid-external scaffolding protein interface is larger than revealed in the crystal structure.