RECEPTOR-BINDING PROPERTIES OF A SOLUBLE FORM OF HUMAN CYTOMEGALOVIRUS GLYCOPROTEIN-B

The human cytomegalovirus (HCMV) glycoprotein B (gB) (also known as gp UL55) homolog is an important mediator of virus entry and cell-to cell dissemination of infection, To examine the potential ligand-binding p roperties of gB, a soluble form of gB (gB-S) was radiolabeled, purifie d, and tested in cell-binding experiments. Binding of gB-S to human fi broblast cells was found to occur in a dose-dependent, saturable, and specific manner, Scatchard analysis demonstrated a biphasic plot with the following estimated dissociation constants (K-d): K-d1, 4.96 x 10( -6) M; K-d2, 3.07 x 10(-7) hi, Cell surface heparan sulfate proteoglyc ans (HSPCs) were deter mined to serve as one class of receptors able t o facilitate gB-S binding, Both HSPG-deficient Chinese hamster ovary ( CHO) cells and fibroblast cells with enzymatically removed HSPGs had 4 0% reductions in gB-S binding, whereas removal of chondroitin sulfate had no effect. However, a significant proportion of gB-S was able to a ssociate with the cell surface in the absence of HSPGs via an undefine d nonheparin component. Binding affinity analysis of gB-S binding to w ild-type CHO-K1 cells demonstrated biphasic binding kinetics (K-d1, 9. 85 x 10(-6) M; K-d2, 4.03 x 10(-8) M), whereas gB-S binding to HSPG-de ficient CHO-677 cells exhibited single-component binding kinetics (K-d , 7.46 x 10(-6) M). Together, these data suggest that gB-S associates with two classes of cellular receptors, The interaction of gB with its receptors is physiologically relevant, as evidenced by an inhibitory effect on HCMV entry when cells were pretreated with purified gB-S. Th is inhibition was determined to be manifested at the level of virus at tachment. We conclude that SE is a ligand for HCMV that mediates an in teraction with a cellular receptor(s) during HCMV infection.