Mutations in the zinc-binding motif of the reovirus capsid protein σ3 eliminate its ability to associate with capsid protein μ1

Reovirus capsid protein σ3 binds both double-stranded RNA (dsRNA) and zinc. Previous studies have revealed that the amino-terminal zinc finger is not required for the ability of σ3 to bind dsRNA. We expressed wild-type and mutant σ3 molecules by in vitro transcription/translation to evaluate the importance of the zinc finger for other functions of σ3. σ3 molecules with mutations in the zinc finger did not form complexes with capsid protein μ1 but bound dsRNA more efficiently than wild-type σ3 did. In contrast, a dsRNA-binding mutant was unimpaired in its ability to associate with μ1. Studies with σ3 fragments support these findings and indicate that sequences critical for σ3's interaction with μ1 lie in the amino terminus of the molecule. Our finding that μ1 and dsRNA do not compete for identical binding sites on σ3 has implications for its function as a translational regulator in infected cells.