Herpes Simplex Virus Glycoprotein C Regulates Low-pH Entry

Abstract

Herpes simplex viruses (HSVs) cause significant morbidity and mortality in humans worldwide. Herpesviruses mediate entry by a multicomponent virus-encoded machinery. Herpesviruses enter cells by endosomal low-pH and pH-neutral mechanisms in a cell-specific manner. HSV mediates cell entry via the envelope glycoproteins gB and gD and the heterodimer gH/gL regardless of pH or endocytosis requirements. Specifics concerning HSV envelope proteins that function selectively in a given entry pathway have been elusive. Here, we demonstrate that gC regulates cell entry and infection by a low-pH pathway. Conformational changes in the core herpesviral fusogen gB are critical for membrane fusion. The presence of gC conferred a higher pH threshold for acid-induced antigenic changes in gB. Thus, gC may selectively facilitate low-pH entry by regulating conformational changes in the fusion protein gB. We propose that gC modulates the HSV fusion machinery during entry into pathophysiologically relevant cells, such as human epidermal keratinocytes. IMPORTANCE Herpesviruses are ubiquitous pathogens that cause lifelong latent infections and that are characterized by multiple entry pathways. We propose that herpes simplex virus (HSV) gC plays a selective role in modulating HSV entry, such as entry into epithelial cells, by a low-pH pathway. gC facilitates a conformational change of the main fusogen gB, a class III fusion protein. We propose a model whereby gC functions with gB, gD, and gH/gL to allow low-pH entry. In the absence of gC, HSV entry occurs at a lower pH, coincident with trafficking to a lower pH compartment where gB changes occur at more acidic pHs. This report identifies a new function for gC and provides novel insight into the complex mechanism of HSV entry and fusion.

Contributions of the four essential entry glycoproteins to HSV-1 tropism and the selection of entry routes

“…For example, deletion of glycoprotein K (gK) significantly reduced HSV-1 entry into Vero cells (67). Another HSV-1 glycoprotein, gC, aids viral attachment by binding heparan sulfate moieties of cell surface proteoglycans (68) and promotes efficient entry into cells that HSV-1 enters by an endocytic route (69). Thus, it is plausible that HSV-1 glycoproteins outside of the core set of four could modulate HSV-1 entry efficiency by tuning the cellular tropism of the virus.…”

“…For example, gK is thought to promote entry into neurons by fusion with the plasma membrane because deletion of the N terminus of gK switches entry to endocytosis (82). gC promotes efficient endocytic entry into CHO-HVEM and primary human keratinocytes (69). Other HSV-1 envelope proteins likely play yet unappreciated roles in specifying tropism and influencing the selection of entry pathways, potentially, in a cell-specific manner.…”

“…The N terminus of gK may thus regulate the fusion of the viral envelope with the plasma membrane (85). Another HSV-1 glycoprotein, gC, aids viral attachment by binding heparan sulfate moieties of cell surface proteoglycans (87) and promotes efficient entry into cells that HSV-1 enters by an endocytic route (88). Thus, envelope proteins outside of the core set of four could, indeed, modulate HSV-1 tropism by tuning entry efficiency.…”

“Non-Essential” Proteins of HSV-1 with Essential Roles In Vivo: A Comprehensive Review

“…Additionally, gC can regulate cell entry and infection by a low-pH pathway [ 748 ]. The presence of gC confers a higher pH threshold for acid-induced changes in gB, affecting fusion.…”

“…When they assessed for infectious virus after infection in a low-pH environment, gC-null HSV-1 was lagging in intracellular transport or release from intracellular vesicles formed after endocytic entry. After treating HSV-1 ΔgC versus WT HSV-1 virions with different pH solutions, it was shown that the presence of gC increases the pH at which fusogenic conformational change of gB occurs [ 748 ].…”