Localization and characterization of Sendai virus nonstructural C and C′ proteins by antibodies against synthetic peptides

Abstract

Antibodies were raised in rabbits against two synthetic peptides, each 30 residues in length, one corresponding to the predicted common carboxyl termini of the nonstructural C and C' proteins of Sendai virus and the other to the unique amino terminus of the larger C protein. Each peptide was inoculated as a covalent complex with tetanus toxoid or in uncomplexed form. Only antibodies to the free carboxyl-terminal peptide precipitated both C and C' proteins made by in vitro translation of viral mRNA and reacted with the C protein from infected cells. These results confirm that the C and C' proteins are carboxyl-coterminal. Contrasting with the reported colocalization of intracellular measles virus C proteins with nucleocapsid inclusions, immunofluorescence studies revealed that Sendai virus C proteins were uniformly distributed in the cytoplasm whereas the viral P protein was present in inclusions that were mainly perinuclear. Since almost all P protein molecules are associated with viral nucleocapsids, these observations suggested that Sendai virus C protein molecules may be both nucleocapsid-associated and free in the cytoplasm. This interpretation was supported when the C and C' proteins were found in both nucleocapsid and free protein fractions of cell lysates. Anti-C antibodies did not inhibit viral RNA synthesis when added to an extract of infected cells. This result was consistent with the conclusion that the C proteins have no direct role in viral transcription, since virions lack C proteins but are transcriptionally active. Therefore, the functions of the C proteins remain undefined.