Influenza C virus matrix protein (M1) is encoded by a spliced mRNA derived from RNA segment 6. Unspliced mRNA from this RNA segment, which has not been previously identified, can potentially encode a polypeptide that contains an additional 132 amino acids on the carboxy terminus of the M1 protein. Here the nucleotide sequences of RNA segment 6 of four influenza C strains, isolated in Japan between 1964 and 1988, were compared with the previously determined sequence of C/Ann Arbor/1/50. The results indicated that the deduced amino acid sequence of the carboxyterminal 132 amino acid domain is conserved fairly well although it is more divergent than the M1 protein sequence. Examination of RNA segment 6-specific mRNAs also showed that unspliced mRNA is present, although in small quantities (≈ 13% of spliced mRNA), in influenza C virus-infected cells. To search for a polypeptide encoded by the unspliced mRNA, the extra carboxy-terminal domain was expressed in as the glutathione -transferase fusion protein, and rabbit immune serum was raised against the purified fusion protein. Immunoprecipitation experiments with this antiserum revealed that a previously unrecognized protein of apparent ≈ 18000, designated CM2, is synthesized in influenza C virus-infected cells.


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