title: Human Cytomegalovirus gH/gL/gO Promotes the Fusion Step of Entry into All Cell Types, whereas gH/gL/UL128-131 Broadens Virus Tropism through a Distinct Mechanism.
abstract: Interaction between gH/gL and the fusion protein gB is likely a conserved feature of the entry mechanism for all herpesviruses. Human cytomegalovirus (HCMV) gH/gL can be bound by gO or by the set of proteins UL128, UL130, and UL131, forming gH/gL/gO and gH/gL/UL128-131. The mechanisms by which these complexes facilitate entry are poorly understood. Mutants lacking UL128-131 replicate well on fibroblasts but fail to enter epithelial/endothelial cells, and this has led to the general assumption that gH/gL/UL128-131 promotes gB-mediated fusion on epithelial/endothelial cells whereas gH/gL/gO provides this function on fibroblasts. This was challenged by observations that gO-null mutants were defective on all of these cell types, suggesting that entry into epithelial/endothelial cells requires both of the gH/gL complexes, but the severe replication defect of the gO mutants precluded detailed analysis. We previously reported that the ratio of gH/gL/gO and gH/gL/UL128-131 in the virion envelope varied dramatically among HCMV strains. Here, we show that strains not only differ in the ratio, but also vary in the total amount of gH/gL in the virion. Cell-type-specific particle-to-PFU ratios of HCMV strains that contained different amounts of gH/gL/gO and gH/gL/UL128-131 were determined. Infection of both fibroblasts and epithelial cells was generally correlated with the abundance of gH/gL/gO, but not with that of gH/gL/UL128-131. The low infectivity of virions rich in gH/gL/UL128-131 but low in gH/gL/gO could be overcome by treatment with the chemical fusogen polyethylene glycol (PEG), strongly arguing that gH/gL/gO provides the conserved herpesvirus gH/gL entry function of promoting gB-mediated fusion for entry into all cell types, whereas gH/gL/UL128-131 acts through a distinct mechanism to allow infection of select cell types.
  
  
  OBJECTIVE
  The functions of HCMV gH/gL complexes in entry are unclear. Unlike the well-studied Epstein-Barr virus (EBV), where gH/gL and gH/gL/gp42 complexes both seem capable of promoting gB fusion during entry into different cell types, our studies here suggest that for HCMV, gH/gL/gO promotes gB fusion on all cell types, whereas gH/gL/UL128-131 broadens virus tropism through a distinct, as yet unknown mechanism. To our knowledge, this is the first suggestion of a herpesvirus gH/gL that does not act by promoting gB fusion, which might make HCMV a useful model to study the fundamental mechanisms by which herpesvirus gH/gL regulates gB fusion. Moreover, gH/gL/UL128-131 is a candidate vaccine target. Our findings help to explain the cell-type-dependent virus neutralization exhibited by anti-gH/gL/UL128-131 antibodies and underscore the importance of gH/gL/gO as another important part of vaccine or therapeutic strategies.
authors.affiliation: Division of Biological Sciences, University of Montana, Missoula, Montana, USA Cellular, Molecular and Microbial Biology Program, University of Montana, Missoula, Montana, USA.
authors.forename: Momei
authors.initials: M
authors.lastname: Zhou
authors.affiliation: Division of Biological Sciences, University of Montana, Missoula, Montana, USA Cellular, Molecular and Microbial Biology Program, University of Montana, Missoula, Montana, USA.
authors.forename: Jean-Marc
authors.initials: JM
authors.lastname: Lanchy
authors.affiliation: Division of Biological Sciences, University of Montana, Missoula, Montana, USA Cellular, Molecular and Microbial Biology Program, University of Montana, Missoula, Montana, USA Biochemistry and Biophysics Program, University of Montana, Missoula, Montana, USA Center for Biomolecular Structure and Dynamics, University of Montana, Missoula, Montana, USA brent.ryckman@mso.umt.edu.
authors.forename: Brent J
authors.initials: BJ
authors.lastname: Ryckman
mesh_terms: D002460:Cell Line; D003587:Cytomegalovirus / Q000472:pathogenicity*; D003586:Cytomegalovirus Infections / Q000473:pathology; D004847:Epithelial Cells / Q000821:virology; D005347:Fibroblasts / Q000821:virology; D006801:Humans; D008562:Membrane Glycoproteins / Q000235:genetics / Q000378:metabolism*; D009154:Mutation / Q000235:genetics; D011092:Polyethylene Glycols / Q000494:pharmacology; D014759:Viral Envelope Proteins / Q000235:genetics / Q000378:metabolism*; D056189:Viral Tropism; D053586:Virus Internalization; D014779:Virus Replication / Q000235:genetics
chemical_list: D008562:Membrane Glycoproteins; C077406:UL115 protein, Human herpesvirus 5; C532797:UL128 protein, human cytomegalovirus; C502890:UL130 protein, human cytomegalovirus; D014759:Viral Envelope Proteins; C071440:glycoprotein H, Cytomegalovirus; C439068:glycoprotein O, cytomegalovirus; D011092:Polyethylene Glycols