Phosphorylation of influenza C virus CM2 protein

Labeling of influenza C virus-infected HMV-II cells with [P-32]orthophospha te showed that the CM2 protein is posttranslationally modified by phosphory lation. The unglycosylated form of CM2 synthesized in the presence of tunic amycin was found to be highly phosphorylated. This result, together with th e finding that digestion of CM2 with peptide-N-glycosidase F failed to remo ve the P-32 label from the glycosylated form of CM2, indicated that phospho rylation occurs in the polypeptide backbone and not in the oligosaccharide chain. Furthermore: phosphoamino acid analysis revealed that phosphorylatio n occurs exclusively on serine residues. Treatment of infected cells with b refeldin A resulted in a complete inhibition of phosphorylation, showing th at phosphorylation of CM2 occurs after its migration from the endoplasmic r eticulum to the Golgi apparatus. Phosphorylation of CM2 was also inhibited strongly, although not completely, by monensin treatment, suggesting that C M2 is phosphorylated predominantly after its movement from medial to trans Golgi cisternae. Finally, we found that the CM2 protein incorporated into t he progeny virion is phosphorylated, which indicates that there is no stric tly selective incorporation of the unphosphorylated form of CM2 into the vi rion. (C) 1998 Elsevier Science B.V. All rights reserved.