| Respiratory syncytial virus (RSV) expresses three glycoproteins on its virion surface: the attachment, G, protein; the fusion (F) protein; and the small hydrophobic (SH) protein. Surprisingly, a viable mutant RSV expressing the F protein as its only glycoprotein was isolated by others suggesting that the F protein alone can perform the initial functions in the viral life cycle. However, retention of the G and SH genes during evolution indicates that they must contribute in some way. We have generated three recombinant green fluorescent protein-expressing RSV with the F (rgRSV-F), G and F (rgRSV-GF), or SH and F (rgRSV-SF) glycoprotein genes, and compared their functions to the parent, rgRSV-SGF. The two viruses lacking the G protein displayed smaller plaques, lower cell fusion, and lower infectious virus release. The reduction in infectious virus was due to inefficient virion assembly/release and lower binding activity. These results indicate that the G protein enhances virion assembly, and binding, therefore infectivity. The two viruses lacking G protein were similar in binding and infectivity. But the virus expressing both the F and SH proteins displayed somewhat smaller plaques, lower fusion activity, and slower viral entry than the virus expressing F alone, suggesting that the SH protein has a negative effect on virus fusion in cell culture. RSV requires cell surface glycosaminoglycans (GAGs) to efficiently initiate infection. To identify which viral glycoprotein, G or F, binds to GAGs, binding and infectivity of rgRSV-SGF and rgRSV-F were compared on parental and GAG-deficient CHO cells. Binding and infection activities of rgRSV-SGF in the deficient cells were 20% that of parental cells, while these activities for rgRSV-F were 50% to 60%, indicating that rgRSV-F is less dependent on GAGs for infectivity. This suggestion was confirmed by competitive inhibition with soluble heparin, a model GAG: rgRSV-F was five times more resistant to heparin than rgRSV-SGF. These results are consistent with the possibility that both the G and F proteins bind to GAGs, but that virus containing F as its only glycoprotein depends more heavily on another molecule(s), perhaps an F protein receptor. |
