Mutations in the zinc-binding motif of the reovirus capsid protein sigma 3 eliminate its ability to associate with capsid protein mu 1

Reovirus capsid protein sigma 3 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 sigma 3 to bind dsRNA. We expressed wild-type and mutant sigma 3 molecules by in vitro transcription/translation to evaluate the importance of the zinc finger for other functions of sigma 3. sigma 3 molecules with mutations in the zinc finger did not form complexes with capsid protein mu 1 but bound dsRNA more efficiently than wild-type sigma 3 did. In contrast, a dsRNA-binding mutant was unimpaired in its ability to associate with mu 1. Studies with sigma 3 fragments support these findings and indicate that sequences critical for sigma 3's interaction with mu 1 lie in the amino terminus of the molecule. Our finding that mu 1 and dsRNA do not compete for identical binding sites on sigma 3 has implications for its function as a translational regulator in infected cells.