Structural and biochemical studies of HCMV gH/gL/gO and Pentamer reveal mutually exclusive cell entry complexes

Significance Human cytomegalovirus (HCMV) is a major cause of morbidity and mortality in transplant patients and in fetuses after congenital infection. gH/gL/gO and Pentamer are targets for neutralizing antibodies. We show that gO and UL128/UL130/UL131A bind to the same site on gH/gL through formation of a disulfide bond with gL-Cys144. The alternative use of this binding site by either gO or the ULs may provide a mechanism for cell tropism modulation. Our analysis reveals that gH/gL antigenic sites are conserved among gH/gL, gH/gL/gO, and Pentamer, whereas gH/gL/gO- and Pentamer-specific neutralizing antibody-binding sites are located in the gH/gL N terminus protrusion that contains the gO and the UL subunits. These data support the development of vaccines and antibody therapeutics against HCMV. Human cytomegalovirus (HCMV) is a major cause of morbidity and mortality in transplant patients and the leading viral cause of birth defects after congenital infection. The glycoprotein complexes gH/gL/gO and gH/gL/UL128/UL130/UL131A (Pentamer) are key targets of the human humoral response against HCMV and are required for HCMV entry into fibroblasts and endothelial/epithelial cells, respectively. We expressed and characterized soluble forms of gH/gL, gH/gL/gO, and Pentamer. Mass spectrometry and mutagenesis analysis revealed that gL-Cys144 forms disulfide bonds with gO-Cys351 in gH/gL/gO and with UL128-Cys162 in the Pentamer. Notably, Pentamer harboring the UL128-Cys162Ser/gL-Cys144Ser mutations had impaired syncytia formation and reduced interference of HCMV entry into epithelial cells. Electron microscopy analysis showed that HCMV gH/gL resembles HSV gH/gL and that gO and UL128/UL130/UL131A bind to the same site at the gH/gL N terminus. These data are consistent with gH/gL/gO and Pentamer forming mutually exclusive cell entry complexes and reveal the overall location of gH/gL-, gH/gL/gO-, and Pentamer-specific neutralizing antibody binding sites. Our results provide, to our knowledge, the first structural view of gH/gL/gO and Pentamer supporting the development of vaccines and antibody therapeutics against HCMV.