The Reovirus Mutant tsA279 L2 Gene Is Associated with Generation of a Spikeless Core Particle: Implications for Capsid Assembly

Abstract

Previous studies which used intertypic reassortants of the wild-type reovirus serotype 1 Lang and the temperature-sensitive ( ts ) serotype 3 mutant clone tsA279 identified two ts lesions; one lesion, in the M2 gene segment, was associated with defective transmembrane transport of restrictively assembled virions (P. R. Hazelton and K. M. Coombs, Virology 207:46–58, 1995). In the present study we show that the second lesion, in the L2 gene segment, which encodes the λ2 protein, is associated with the accumulation of a core-like particle defective for the λ2 pentameric spike. Physicochemical, biochemical, and immunological studies showed that these structures were deficient for genomic double-stranded RNA, the core spike protein λ2, and the minor core protein μ2. Core particles with the λ2 spike structure accumulated after temperature shift-down from a restrictive to a permissive temperature in the presence of cycloheximide. These data suggest the spike-deficient, core-like particle is an assembly intermediate in reovirus morphogenesis. The existence of this naturally occurring primary core structure suggests that the core proteins λ1, λ3, and ς2 interact to initiate the process of virion capsid assembly through a dodecahedral mechanism. The next step in the proposed capsid assembly model would be the association of the minor core protein μ2, either preceding or collateral to the condensation of the λ2 pentameric spike at the apices of the primary core structure. The assembly pathway of the reovirus double capsid is further elaborated when these observations are combined with structures identified in other studies.