Respiratory syncytial virus G glycoprotein expressed using the Semliki Forest virus replicon is biologically active

The respiratory syncytial virus (RSV) G glycoprotein mediates attachment of RSV to cells via an unknown receptor. To study G glycoprotein function we have cloned two variants of the RSV G gene into a Semliki Forest virus (SFV) expression vector, a full length (rG) and soluble (srG) G glycoprotein variant. By immunofluorescence microscopy, rG was found to be predominantly membrane associated, while srG was mostly cytoplasmic. The rG (80-85 kDa) and srG (75-80 kDa) constructs produced heavily glycosylated proteins, however they were slightly smaller than the G glycoprotein expressed in RSV infected HEp-2 cells (85-90 kDa). The biological activity of purified srG was tested by its ability to bind to RSV permissive cells. Purified srG bound to HEp-2 cells and the amount bound increased linearly with the quantity added. Binding was not saturable with the small quantities of protein available. Binding of srG to HEp-2 cells was inhibited (67-68%) by MAb 30 and neutralising anti-G MAb 29. Nonpermissive SF9 insect cells bound 20-50 times less srG than HEp-2 cells. SFV expressed recombinant RSV G glycoprotein should be useful for studying interactions between the RSV G glycoprotein and cells.