Isolation and structural analysis of influenza C virion glycoproteins

Influenza C virions possess a single glycoprotein that is cleaved into two disulfide-linked subunits, gp65 and gp30. When analyzed under nonreducing conditions, the uncleaved (gpI) and cleaved (gpI) glycoproteins differ significantly in apparent molecular weight; however, we observed no difference in their tryptic peptide patterns. We have isolated the glycoproteins by selective solubilization with Triton X-100 or octylglucoside; only preparations obtained with the latter detergent showed hemagglutinating activity. In purified glycoprotein samples, gp65 was routinely observed as a doublet on SDS-polyacrylamide gels. Analysis of tryptic peptides by ion-exchange chromatography demonstrated that the two gp65 bands have indistinguishable polypeptide backbones; they appear to differ, however, in carbohydrate content. The uncleaved glycoprotein as well as gp65 were resistant to Edman degradation indicating the presence of blocked amino termini, whereas gp30 was observed to have the N-terminal tripeptide sequence NH₂-Ile-Phe-Gly. This sequence is homologous to a sequence at the N termini of influenza A and B HA₂ glycoproteins, except for the presence of an additional terminal glycine residue in these viruses. The influenza C glycoproteins form a regular hexagonal lattice on the viral envelope. This arrangement is sometimes maintained in disrupted virus preparations and in glycoprotein subunits released from the envelope by limited proteolysis, indicating that direct interactions between the glycoprotein molecules are responsible, at least in part, for the observed arrangement. Observations of clustered surface projections on plasma membranes of infected cells, and of released virus particles apparently devoid of internal nucleoproteins, are consistent with the suggestion that lateral interactions between the influenza C glycoproteins may be important in virus assembly.