SENDAI VIRUS M-PROTEIN IS FOUND IN 2 DISTINCT ISOFORMS DEFINED BY MONOCLONAL-ANTIBODIES

The use of a monoclonal antibody defines a subset of Sendai virus M protein representing about 30% of total. This M protein acquires, during the hour following synthesis, an epitope not present on the bulk of M. This epitope maturation is observed in acutely as well as in persistently infected cells. It takes place in vivo in absence of other viral proteins, but it is not observed when the protein is synthesized in a reticulocyte lysate. Epitope maturation does not appear to result from phosphorylation, acylation or disulfide bond formation. If immunofluorescent staining seems to indicate a preferential association of this subset of M protein with nucleocapsids, this is not confirmed by immunogold staining or by nucleocapsid isolation. Incubation of cytoplasmic extracts or of purified M protein in conditions which do not favor M to M protein association results in a relative increase of M protein carrying the maturing epitope. It is concluded that M protein exists in two distinct isoforms.