Characterization of a protein candidate for the ion channel of influenza C virus and its expression.

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The biochemical properties of a second protein (CM2) encoded by RNA segment 6 of influenza C virus were investigated.1.Three forms of CM2 with different electrophoretic mobilities (CM2_0, CM2a and CM2b) were detected in infected cells by immunoprecipitation with antiserum to GST-CM2 fusion protein. 2.A mannose-rich oligosaccharide core is added to unglycosylated CM2_0 (Mr-16,000) to form CM2a (Mr-18,000) and the processing of the carbohydrate chain from high mannose type to complex one converts CM2a into CM2b heterogeneous in electrophoretic mobility (Mr-22,000 to 30,000). 3.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. 4.The CM2 protein was also found to form disulfide-linked dimers and tetramers on SDS-PAGE under nonreducing conditions. 5.Trypsin treatment of infected cell surface as well as that 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 domain is intracellular and exposed on the cytoplasmic side of microsomes. 6.A small amount of CM2 is incorporated into progeny virus particles.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.