IDENTIFICATION OF A 374-AMINO-ACID PROTEIN ENCODED BY RNA SEGMENT-6 OF INFLUENZA-C VIRUS

Unspliced mRNA from RNA segment 6 of influenza C virus contains a sing le open reading frame that potentially encodes a polypeptide of 374 am ino acids. This polypeptide, which has not been identified as yet, is predicted to contain the complete amino acid sequence of the matrix pr otein, M1, as well as that of a small integral membrane protein, CM2, Here, we found that small amounts of two previously unrecognized prote ins with apparent molecular masses of 42 (P42) and 44 kDa (P44) were i mmunoprecipitated with immune serum against CM2, The electrophoretic m obilities of P42 and P44 varied depending on virus strain, indicating that they are virus-coded. Treatment of infected cells with tunicamyci n and digestion of immunoprecipitated proteins with various endoglycos idases revealed that P42 is modified by the addition of a high-mannose oligosaccharide chain to generate P44, A monoclonal antibody against M1, like anti-CM2 serum, was able to immunoprecipitate both the P42 an d P44 proteins. Furthermore, the tryptic peptide map of either P42 or P44 was indistinguishable from the map of the mixture of M1 and CM2, T hese results, taken together, suggest strongly that P42 and P44 corres pond to the 374 amino acid protein encoded by unspliced RNA segment 6 mRNA and its N-glycosylated form, respectively.