Alteration of Sendai virus morphogenesis and nucleocapsid incorporation due to mutation of cysteine residues of the matrix protein

The matrix (M) protein of Sendai virus (SeV) has five cysteine residues, at positions 83, 106, 158, 251, and 295. To determine the roles of the cysteine residues in viral assembly, we generated mutant M cDNA possessing a substitution to serine at one of the cysteine residues or at all of the cysteine residues. Some mutant M proteins were unstable when expressed in cultured cells, suggesting that cysteine residues affect protein stability, probably by disrupting the proper conformation. In an attempt to generate virus from cDNA, SeV M-C₈₃S, SeV M-C₁₀₆S, and SeV M-C₂₉₅S were successfully recovered from cDNA, while recombinant SeVs possessing other mutations were not. SeV M-C₈₃S and SeV M-C₁₀₆S had smaller virus particles than did the wild-type SeV, whereas SeV M-C₂₉₅S had larger and heterogeneously sized particles. Furthermore, SeV M-C₁₀₆S had a significant amount of empty particles lacking nucleocapsids. These results indicate that a single-point mutation at a cysteine residue of the M protein affects virus morphology and nucleocapsid incorporation, showing direct involvement of the M protein in SeV assembly. Cysteine-dependent conformation of the M protein was not due to disulfide bond formation, since the cysteines were shown to be free throughout the viral life cycle.