Coiled-Coil Domains in Glycoproteins B and H Are Involved in Human Cytomegalovirus Membrane Fusion

Abstract

Human cytomegalovirus (CMV) utilizes a complex route of entry into cells that involves multiple interactions between viral envelope proteins and cellular receptors. Three conserved viral glycoproteins, gB, gH, and gL, are required for CMV-mediated membrane fusion, but little is known of how these proteins cooperate during entry (E. R. Kinzler and T. Compton, submitted for publication). The goal of this study was to begin defining the molecular mechanisms that underlie membrane fusion mediated by herpesviruses. We identified heptad repeat sequences predicted to form alpha-helical coiled coils in two glycoproteins required for fusion, gB and gH. Peptides derived from gB and gH containing the heptad repeat sequences inhibited virus entry when introduced coincident with virus inoculation onto cells or when mixed with virus prior to inoculation. Neither peptide affected binding of CMV to fibroblasts, suggesting that the peptides inhibit membrane fusion. Both gB and gH coiled-coil peptides blocked entry of several laboratory-adapted and clinical strains of human CMV, but neither peptide affected entry of murine CMV or herpes simplex virus type 1 (HSV-1). Although murine CMV and HSV-1 gB and gH have heptad repeat regions, the ability of human CMV gB and gH peptides to inhibit virus entry correlates with the specific residues that comprise the heptad repeat region. The ability of gB and gH coiled-coil peptides to inhibit virus entry independently of cell contact suggests that the coiled-coil regions of gB and gH function differently from those of class I, single-component fusion proteins. Taken together, these data support a critical role for alpha-helical coiled coils in gB and gH in the entry pathway of CMV.