Multiple forms of the sigma 3 protein of reovirus: occurrence and binding properties

The sigma 3 protein of reovirus serotype 3 is present in different forms differing in isoelectric point, as already described by other authors. Proteolytic digestion of each of these forms generates similar peptides. The different forms are present in the mature virus and have affinity for double-stranded RNA and for ribosomes. The observations that the different forms have the same functional properties and are absent in reovirus serotype 1 suggest that the presence of multiple forms is dispensable. The different forms can be synthesized by in vitro translation of mRNA extracted from infected cells. However, only one form is clearly made when mRNA synthesized in vitro, using the cloned S4 gene encoding sigma 3 as a template, is translated in vitro. Similarly the cloned S4 gene expressed in vivo produces only one form of the protein. These results indicate that the other forms are not derived by post-translational modification. The different forms are possibly due to the presence of mutant viruses.