The sequence similarity of reovirus sigma 3 protein to picornaviral proteases is unrelated to its role in mu 1 viral protein cleavage.

Recent data have shown that the reovirus sigma 3 protein is the only viral product required for proteolytic conversion of the viral mu 1 protein to mu 1C. In an effort to determine the importance of a sigma 3 region exhibiting sequence similarity to a functionally important region of picornaviral proteases, we analyzed proteins produced in COS cells cotransfected with expression vectors encoding mu 1 and either wild-type or mutated forms of sigma 3. This approach revealed that association with sigma 3 contributes to the stability of mu 1-related polypeptides (mu 1 + mu 1C), but conservation of the sigma 3 region related to picornaviral proteases is not required for proteolytic conversion of mu 1 to mu 1C.