Avian and mammalian reoviruses use different molecular mechanisms to synthesize their μNS isoforms

Previous reports revealed that the M3 gene of both avian and mammalian reoviruses express two isoforms of the non-structural protein μNS in infected cells. The larger isoforms initiate translation at the AUG codon closest to the 5' end of their respective m3 mRNAs, and were therefore designated μNS. In this study we have performed experiments to identify the molecular mechanisms by which the smaller μNS isoforms are generated. The results of this study confirmed the previous findings indicating that the smaller mammalian reovirus μNS isoform is a primary translation product, the translation of which is initiated at the internal AUG-41 codon of mammalian reovirus m3 mRNA. Our results further revealed that the smaller avian reovirus μNS isoform originates from a specific post-translational cleavage site near the amino terminus of μNS. This cleavage produces a 55 kDa carboxy-terminal protein, termed μNSC, and a 17 kDa amino-terminal polypeptide, designated μNSN. These results allowed us to extend the known avian reovirus protein-encoding capacity to 18 proteins, 12 of which are structural proteins and six of which are non-structural proteins. Our finding that avian and mammalian reoviruses use different mechanisms to express their μNSC isoforms suggests that these isoforms are important for reovirus replication.