Association of the Sendai virus C protein with nucleocapsids.

The subcellular localization of the nonstructural protein C of Sendai virus was investigated by means of indirect immunofluorescence microscopy of Sendai virus-infected cells, using an antiserum specific for C protein. In infected cells, C protein was detected exclusively in the cytoplasm as granular fluorescence, which coincided very well with the distribution of nucleocapsid protein NP and phosphoprotein P, which were also detected with specific antisera. This suggested that these proteins are present together in inclusions, probably forming nucleocapsids. In contrast, when the NP and C proteins were individually expressed in COS cells by transfection with expression plasmids containing cDNA for these proteins, their distribution patterns in the cytoplasm were found to be quite different from each other. Protein-blot analyses of purified virions revealed the presence of a significant amount of the C protein in virions, which indicated that C protein is integrated into virions. Under conditions in which most of the envelope-associated proteins, such as HN, F, and M, were removed from the virions by a detergent, the C protein remained tightly associated with the nucleocapsids--about 40 molecules per nucleocapsid.