Characterization of Sendai virus M protein mutants that can partially interfere with virus particle production.

Substitution of Val(113) in Sendai virus (SeV) M protein generates non-functional polypeptides, characterized by their exclusion from virus particles and by their ability to interfere with virus particle production. These phenotypic traits correlate with a single-band PAGE migration profile, in contrast to wild-type M (M(wt )), which separates into two species, one of which is a phosphorylated form. The single-band migration is likely to result from a conformational change, as evidenced by the lack of maturation of a native epitope and by a particular tryptic digestion profile, and not from the phosphorylation of all M molecules, an assumption consistent with the PAGE migration feature. One of the M mutants (HA-M(30 ), an M protein carrying Thr(112)Met and Val(113) Glu substitutions tagged with an influenza virus haemagglutinin epitope) was characterized further in the context of SeV infection, i.e. under conditions of co-expression with M(wt). HA-M (30) is shown (i) to bind mainly to membrane fractions, (ii) not to co-precipitate M(wt), as HA-M(wt) does, (iii) to interfere with the binding of nucleocapsids to membranes and (iv) to accumulate in perinuclear regions, in contrast to HA-M(wt ), which is also found at the cell periphery. Such mutants constitute potential tools for the identification of critical steps in paramyxovirus assembly and budding.