Mutations in an exposed domain of Sindbis virus capsid protein result in the production of noninfectious virions and morphological variants.

A critical event in the process of Sindbis virus assembly is the interaction of the virus nucleocapsid with the target membrane which has been modified by virus envelope glycoproteins. A specific association between the endodomain of the pE2/E2 glycoprotein and an unidentified domain in the capsid protein is responsible for this association. We have examined the nature of this association by the production of a mutant which has two amino acid substitutions in a domain of the capsid protein known to be exposed on the surface of the assembled nucleocapsid (Ser180/Gly183). These substitutions result in a mutant which is defective in production of infectious virus, especially at low (28 degrees) and high (39.5 degrees) temperatures. Although noninfectious, the mutant virus is as efficient at mediating cell-virus-cell fusion from without as is an equal amount of wild-type virus particles, suggesting that the envelope glycoproteins are functional and that the loss of infectivity may be due to an event after membrane fusion, possibly nucleocapsid uncoating. At low temperature but not high temperature, the mutant produces normal size virus particles (T = 4) and larger particles with a predicted triangulation number of 7. These data suggest that, at low temperature, the mutant capsid protein may assume two different conformations capable of assembling either T = 4 or T = 7 nucleocapsids.