Characterization of a second protein (CM2) encoded by RNA segment 6 of influenza C virus

The biochemical properties of a second protein (CM2) encoded by RNA segment 6 of influenza C virus were investigated. Three forms of CM2 with different electrophoretic mobilities (CM2(0), CM2a, and CM2b) were detected in infected cells by immunoprecipitation with antiserum to the glutathione S-transferase (GST)-CM2 fusion protein. Treatment of infected cells with tunicamycin and digestion of immunoprecipitated proteins with endoglycosidase H or peptide-N-glycosidase F suggested that a mannose-rich oligosaccharide core is added to unglycosylated CM2(0) (Mr, approximately 16,000) to form CM2a (Mr, approximately 18,000) and that the processing of the carbohydrate chain from the high-mannose type to the complex type converts CM2a into CM2b, which is heterogeneous in electrophoretic mobility (Mr, approximately 22,000 to 30,000). Labeling of infected cells with [3H]palmitic acid showed that CM2 is fatty acylated. The fatty acid bond was sensitive to treatment with hydroxylamine and mercaptoethanol, which indicates a labile thioester-type linkage. The CM2 protein was also found to form disulfide-linked dimers and tetramers on sodium dodecyl sulfate-polyacrylamide gels under nonreducing conditions. Trypsin treatment of infected cell surfaces as well as of microsome vesicles from infected cells followed by immunoprecipitation with antiserum to the GST fusion protein containing the 56 C-terminal amino acid residues of CM2 suggested that this C-terminal domain is intracellular and exposed to the cytoplasms of microsomes. Furthermore, evidence that a small amount of CM2 is incorporated into progeny virus particles was obtained by Western blot analysis. These results, altogether, suggest that CM2 is an integral membrane protein with biochemical properties similar to those of influenza A virus M2 and influenza B virus NB proteins.