Sequences of avian reovirus M1, M2 and M3 genes and predicted structure/function of the encoded μ proteins

We report the first sequence analysis of the entire complement of M-class genome segments of an avian reovirus (ARV). We analyzed the M1, M2 and M3 genome segment sequences, and sequences of the corresponding μA, μB and μNS proteins, of two virus strains, ARV138 and ARV176. The ARV M1 genes were 2283 nucleotides in length and predicted to encode μA proteins of 732 residues. Alignment of the homologous mammalian reovirus (MRV) μ2 and ARV μA proteins revealed a relatively low overall amino acid identity (∼30%), although several highly conserved regions were identified that may contribute to conserved structural and/or functional properties of this minor core protein (i.e. the MRV μ2 protein is an NTPase and a putative RNA-dependent RNA polymerase cofactor). The ARV M2 genes were 2158 nucleotides in length, encoding predicted μB major outer capsid proteins of 676 amino acids, more than 30 amino acids shorter than the homologous MRV μ1 proteins. In spite of the difference in size, the ARV/MRV μB/μ1 proteins were more conserved than any of the homologous proteins encoded by other M- or S-class genome segments, exhibiting percent amino acid identities of ∼45%. The conserved regions included the residues involved in the maturation- and entry- specific proteolytic cleavages that occur in the MRV μ1 protein. Notably missing was a region recently implicated in MRV μ1 stabilization and in forming “hub and spokes” complexes in the MRV outer capsid. The ARV M3 genes were 1996 nucleotides in length and predicted to encode a μNS non-structural protein of 635 amino acids, significantly shorter than the homologous MRV μNS protein, which is attributed to several substantial deletions in the aligned ARV μNS proteins. Alignments of the ARV and MRV μNS proteins revealed a low overall amino acid identity (∼25%), although several regions were relatively conserved.