Cytomegalovirus UL131-128 products promote gB conformational transition and gB-gH interaction during entry into endothelial cells.
Herpesviruses use gB and gH-gL glycoproteins to execute fusion. Other virus-specific glycoproteins are required for receptor binding and fusion activation. The human cytomegalovirus (HCMV) UL131-128 proteins are essential for the infection of leukocytes, endothelial cells (ECs), and many epithelial cell lines. Here we show that UL131-128 play a role in a chain of events involving gB and gH during HCMV entry into ECs. An HCMV strain bearing the wild-type (wt) UL131-128 locus exhibited a gB transition from a protease-resistant to protease-sensitive form, a conformational change that was suppressed by a thiourea inhibitor of fusion (WY1768); in contrast, gH was susceptible to proteolysis throughout entry. Moreover, gB and gH transiently interacted, and a lipid mixing assay showed that the wt strain had carried out fusion by 60 min postinfection. However, these events were greatly altered when UL131-128-defective strains were used for infection or when there was an excess of soluble pUL128 during wt infection: the gB conformational change became WY1768 resistant, the gB-gH complex was no longer observed, and fusion was prevented. Both gB and gH in this case showed late protease resistance, related to their endocytic uptake. Our data point to the involvement of UL131-128 proteins in driving gB through a WY1768-sensitive fold transition, thus promoting a short-lived gB-gH complex and fusion; they also suggest that HCMV fuses with the EC plasma membrane and that endocytosis ensues only when the virus cannot trigger UL131-128-dependent steps.
University of Milano School of Medicine, Department of Medicine, Surgery and Dentistry, Polo San Paolo, 20142 Milano, Italy.
Marco
M
Patrone
Massimiliano
M
Secchi
Eleonora
E
Bonaparte
Gabriele
G
Milanesi
Andrea
A
Gallina
D003587:Cytomegalovirus / Q000472:pathogenicity*; D004705:Endocytosis; D042783:Endothelial Cells / Q000821:virology*; D006801:Humans; D008562:Membrane Glycoproteins / Q000502:physiology*; D011485:Protein Binding; D011487:Protein Conformation; D014759:Viral Envelope Proteins / Q000737:chemistry* / Q000378:metabolism* / Q000502:physiology*; D014764:Viral Proteins / Q000737:chemistry / Q000502:physiology
D008562:Membrane Glycoproteins; C502890:UL130 protein, human cytomegalovirus; D014759:Viral Envelope Proteins; D014764:Viral Proteins; C040495:glycoprotein B, Simplexvirus; C071440:glycoprotein H, Cytomegalovirus