INFLUENZA-C VIRUS CM2 INTEGRAL MEMBRANE GLYCOPROTEIN IS PRODUCED FROMA POLYPEPTIDE PRECURSOR BY CLEAVAGE OF AN INTERNAL SIGNAL SEQUENCE

The influenza C virus CM2 protein is a small glycosylated integral mem brane protein (115 residues) that spans the membrane once and contains a cleavable signal sequence at its N terminus. The coding region for CM2 (CM2 ORF) is located at the C terminus of the 342-amino acid (aa) ORF of a colinear mRNA transcript derived from influenza C virus RNA s egment 6. Splicing of the colinear transcript introduces a translation al stop codon into the ORF and the spliced mRNA encodes the viral matr ix protein (CM1) (242 aa), The mechanism of CM2 translation was invest igated by using in vitro and in vivo translation of RNA transcripts. I t was found that the colinear mRNA derived from influenza C virus RNA segment 6 serves as the mRNA for CM2, Furthermore, CM2 translation doe s not depend on any of the three in-frame methionine residues located at the beginning of CM2 ORF. Rather, CM2 is a proteolytic cleavage pro duct of the p42 protein product encoded by the colinear mRNA: a cleava ge event that involves the recognition and cleavage of an internal sig nal peptide presumably by signal peptidase resident in the endoplasmic reticulum, Alteration of the predicted signal peptidase cleavage site by mutagenesis blocked generation of CM2. The other polypeptide speci es resulting from the cleavage of p42, designated p31, contains the CM 1 coding region and an additional C-terminal 17 aa (formerly the CM2 s ignal peptide). Protein p31, in comparison to CM1, displays characteri stics of an integral membrane protein.