The sequences of reovirus serotype 3 genome segments M1 and M3 encoding the minor protein mu 2 and the major nonstructural protein mu NS, respectively.

The sequences of the M1 and M3 genome segments of reovirus serotype 3 strain Dearing, which encode protein mu 2, a minor capsid, component, and protein mu NS, one of the two nonstructural proteins, are reported. They are 2304 and 2235 base pairs long, respectively, and proteins mu 2 and mu NS comprise 736 and 719 amino acids, respectively. This completes the sequencing of the reovirus serotype 3 genome: it comprises 23,549 basepairs. Neither protein mu 2 nor protein mu NS possesses any sequence similarity to any protein sequence in gene banks, nor any of the commonly recognized motifs indicative of specialized function. Protein mu 2 has a higher alpha-helix content (36%) than other capsid proteins; for it, the ratio of amino acids in alpha-helix/beta-sheet configuration is 1.2, whereas that of typical reovirus capsid proteins ranges from 0.5 to 0.9. Thus it is not a typical capsid protein. Protein mu NS has a very high alpha-helix content (about 50%; alpha-helix/beta-sheet ratio 2.5), which is very similar to that of the other nonstructural reovirus protein, protein sigma NS. The C-terminal regions of mu NS and various myosins exhibit periodic sequence similarity elements indicative of helical structure. Protein mu NS exists in two forms in infected cells: protein mu NS and a protein, mu NSC, which lacks a region of about 5 kDa at its N-terminus. Pulse-chase analysis in vivo suggests that protein mu NSC is not a cleavage product of protein mu NS; further, protein mu NSC is formed along with protein mu NS in in vitro protein synthesizing systems, whereas protein mu 1C, the cleavage product of protein mu 1, is not. It is likely, therefore, that protein mu NSC is a primary translation product, formed either by ribosomes reading through the first initiation codon of m1 messenger RNA at position 14 and initiating at codon 42, or by de novo internal initiation at this codon.