STRUCTURE OF THE SPIKE GLYCOPROTEIN OF INFLUENZA C VIRUS

Posttranslational cleavage of the precursor glycoprotein gp88 of influenza C virus results in two subunit glycoproteins, gp65 and gp30, each of which exist in two molecular forms. Influenza C glycoproteins were isolated from purified influenza C virions by selective solubilization with Triton X-100 or octylglucoside. Only preparations obtained with octylglucoside showed hemagglutinating activity. Tryptic peptide analysis of the three species of viral glycoproteins, gp88, gp65 and gp30, revealed that gp30 and gp65 are distinct; when the peaks resolved for the two subunit glycoproteins are superimposed, the pattern corresponding to that of gp88 is obtained. The two molecular forms of gp65 have an identical polypeptide backbone as shown by tryptic peptide analysis. The N-termini of gp88 as well as gp65 were resistant to sequential Edman degradation. The gp30 terminal sequence contains a preponderance of hydrophobic residues and shows homology with the corresponding sequence of the HA^ subunit of influenza A viruses, except for an additional N-terminal glycine residue. The glycoprotein spikes on the surface of influenza C virions were found in regular hexagonal arrays, which appear to involve lateral interaction between the glycoprotein molecules, since the spikes sometimes maintain their arrangement in a network upon release from the viral membrane by limited proteolytic digestion, or upon spontaneous disruption of the viral membrane. In infected cells, closely packed surface projections were observed on crescent shaped outfoldings of the plasma membrane, where virus maturation is occurring. The fact that, in most cases, no nucleocapsids can be seen in such regions suggests that nucleocapsids may not be required to initiate outfolding of the plasma membrane in the budding process of influenza C virions.