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Journal of Virology, June 2008, p. 5774-5780, Vol. 82, No. 12
0022-538X/08/$08.00+0     doi:10.1128/JVI.02758-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

Scaffolding Proteins Altered in the Ability To Perform a Conformational Switch Confer Dominant Lethal Assembly Defects

James E. Cherwa Jr., Asako Uchiyama, and Bentley A. Fane^*

The BIO5 Institute, University of Arizona, Tucson, Arizona 85719

Received 28 December 2007/ Accepted 28 March 2008

In the X174 procapsid crystal structure, 240 external scaffolding protein D subunits form 60 pairs of asymmetric dimers, D₁D₂ and D₃D₄, in a non-quasi-equivalent structure. To achieve this arrangement, -helix 3 assumes two different conformations: (i) kinked 30° at glycine residue 61 in subunits D₁ and D₃ and (ii) straight in subunits D₂ and D₄. Substitutions for G61 may inhibit viral assembly by preventing the protein from achieving its fully kinked conformation while still allowing it to interact with other scaffolding and structural proteins. Mutations designed to inhibit conformational switching in -helix 3 were introduced into a cloned gene, and expression was demonstrated to inhibit wild-type morphogenesis. The severity of inhibition appears to be related to the size of the substituted amino acid. For infections in which only the mutant protein is present, morphogenesis does not proceed past the first step that requires the wild-type external scaffolding protein. Thus, mutant subunits alone appear to have little or no morphogenetic function. In contrast, assembly in the presence of wild-type and mutant subunits is blocked prematurely, before D protein is required in a wild-type infection, or channeled into an off-pathway reaction. These data suggest that the wild-type protein transports the inhibitory protein to the pathway. Viruses resistant to the lethal dominant proteins were isolated, and mutations were mapped to the coat and internal scaffolding proteins. The affected amino acids cluster in the atomic structure and may act to exclude mutant subunits from occupying particular positions atop pentamers of the viral coat protein.

Published ahead of print on 9 April 2008.

Present address: Department of Plant Pathology, Cornell University, Ithaca, NY 14853.