Further characterization of the soluble form of the G glycoprotein of respiratory syncytial virus

Abstract

A soluble form of the G glycoprotein, the attachment protein, of respiratory syncytial virus is shed from infected HEp-2 cells. The Gs proteins of the Long and 18537 strains have apparent molecular sizes of 82 and 71 kilodaltons, respectively, 6 to 9 kilodaltons smaller than the virion-associated forms (Gv). The Gs protein of the Long strain was further characterized. Approximately one in six of all of the radiolabeled G molecules in these cultures at 24 h postinfection was present as the Gs protein. The Gs protein was clearly evident in culture fluids at 6 h postinfection, but the Gv protein could not be discerned until 12 h after infection, an observation that is consistent with the 12-h eclipse period for respiratory syncytial virus. Therefore, the Gs protein is shed, in part at least, from intact, infected cells and before the appearance of progeny virus. The appearance of a smaller Gs protein (74 kilodaltons) in fluids of infected calls which were incubated with tunicamycin shows that addition of N-linked oligosaccharides is not required for the genesis and shedding of the Gs protein. Sequencing of the amino terminus of purified Gs protein revealed two different termini, whose generations are consistent with cleavages of the full-length G protein between amino acids 65 and 66 and between residues 74 and 75. This result suggests that the Gs protein is present in two different forms which lack the proposed intracytoplasmic and transmembrane domains of the full-length G protein.