Analysis of synthetic peptides from heptad-repeat domains of herpes simplex virus type 1 glycoproteins H and B

T he entry of free herpes simplex virus 1 (HSV-1) into susceptible cells requires the coordinated activities of 4 viral envelope glycoproteins (1-4). After virion adsorption to cell surface heparan sulfates, mediated mainly by a fifth envelope glycoprotein, glycoprotein C (gC) (5, 6), gD binds to one of its specific receptors, herpesvirus entry mediator (HVEM or HveA), nectin-1 (HveC), or 3-O-sulfated heparan sulfate (7-9). Receptor binding results in a conformational change in gD, which in turn activates the fusion mechanism mediated by gB and the gH/gL heterodimer; fusion merges the viral envelope with the cell surface or endosomal membrane, resulting in capsid release into the cytoplasm (10-15).HSV entry by direct cell-to-cell (lateral) spread involves a similar fusion step and requires the same 4 entry glycoproteins along with a receptor for gD (1-4, 16-18). However, evidence exists to indicate that the two entry processes are not mechanistically identical. For example, gD mutants that allow gD receptor-independent cell-to-cell spread, but not receptor-independent entry, have been isolated (19). Likewise, gD is required for entry of the related alphaherpesvirus pseudorabies virus (PRV) but not for PRV lateral spread (20-23). Two additional glycoproteins, gE and gI, are important for HSV-1 spread but are dispensable for entry of free virus (reviewed in reference 24). However, it is unknown whether the roles of gB and gH/gL are different in entry and spread.The crystal structure of the HSV-1 gB ectodomain shows unexpected homology to the postfusion form of glycoprotein G from vesicular stomatitis virus, a well-characterized fusion protein (25), suggesting direct participation of gB in the fusion reaction. Although previous studies reported that gH displays hallmarks of fusion proteins (26-29), the crystal structure of the HSV-2 gH ectodomain bound to gL bears no similarity to any known viral fusogen (30). Nonetheless, both gB and gH may possess fusogenic capabilities, as each alone can promote membrane fusion during nuclear egress (31). Previous evidence indicated that gH/gL could carry out hemifusion, an intermediate state in which the outer membrane leaflets of the target and viral membranes mix, whereas both gB and gH/gL were required for complete fusion (32), but these findings were recently called into question (33). Recent studies suggested that gH/gL activates the dormant fusogenic activity of gB in response to signaling by receptor-bound gD (30,34,35), indicating that gH/gL performs an intermediate regulatory function in the steps leading to membrane fusion. However, since these studies were performed in virus-free cell fusion systems, it is unclear whether they apply equally to virus entry and spread. Moreover, it remains uncertain whether gH/gL also has a role as cofusogen in either event.By taking advantage of the ability of HSV to adapt to constraints on receptor usage for entry, we previously identified gainof-function derivatives of a gD mutant virus that was impaired for its ability to use necti...

mentioning

confidence: 94%

Novel Mutations in gB and gH Circumvent the Requirement for Known gD Receptors in Herpes Simplex Virus 1 Entry and Cell-to-Cell Spread

Uchida

Chan

Shrivastava

et al. 2013

“…This complex is a major target of virus-neutralizing antibodies in several herpesviruses (17,43), emphasizing its role in virus entry. It has been proposed that gH has features characteristic of class I viral fusogens (14)(15)(16). However, the recently published structures of HSV-2 and EBV gH/gL (9,39) suggested that these proteins do not likely act as cofusogen but rather regulate fusion by gB.…”

mentioning

confidence: 99%

Bovine Herpesvirus Type 4 Glycoprotein L Is Nonessential for Infectivity but Triggers Virion Endocytosis during Entry

Lété

Machiels

Stevenson

et al. 2012

The core entry machinery of mammalian herpesviruses comprises glycoprotein B (gB), gH, and gL. gH and gL form a heterodimer with a central role in viral membrane fusion. When archetypal alpha-or betaherpesviruses lack gL, gH misfolds and progeny virions are noninfectious. However, the gL of the rhadinovirus murid herpesvirus 4 (MuHV-4) is nonessential for infection. In order to define more generally what role gL plays in rhadinovirus infections, we disrupted its coding sequence in bovine herpesvirus 4 (BoHV-4). BoHV-4 lacking gL showed altered gH glycosylation and incorporated somewhat less gH into virions but remained infectious. However, gL ؊ virions showed poor growth associated with an entry deficit. Moreover, a major part of their entry defect appeared to reflect impaired endocytosis, which occurs upstream of membrane fusion itself. Thus, the rhadinovirus gL may be more important for driving virion endocytosis than for incorporating gH into virions, and it is nonessential for membrane fusion.

“…It has been proposed that HSV-1 gH has features characteristic of class I viral fusogens, such as putative heptad repeats and fusion peptides (6,(9)(10)(11). Also, peptides matching the sequence of gH can interact with lipids and/or induce the fusion of lipid vesicles (4,5,8).…”

mentioning

confidence: 99%

Insertional Mutations in Herpes Simplex Virus Type 1 gL Identify Functional Domains for Association with gH and for Membrane Fusion

Fan

Lin

Spear

2009

Glycoprotein L (gL) is one of four glycoproteins required for the entry of herpes simplex virus (HSV) into cells and for virus-induced cell fusion. This glycoprotein oligomerizes with gH to form a membrane-bound heterodimer but can be secreted when expressed without gH. Twelve unique gL linker-insertion mutants were generated to identify regions critical for gH binding and gH/gL processing and regions essential for cell fusion and viral entry. All gL mutants were detected on the cell surface in the absence of gH, suggesting incomplete cleavage of the signal peptide or the presence of a cell surface receptor for secreted gL. Coexpression with gH enhanced the levels of cell surface gL detected by antibodies for all gL mutants except those that were defective in their interactions with gH. Two insertions into a conserved region of gL abrogated the binding of gL to gH and prevented gH expression on the cell surface. Three other insertions reduced the cell surface expression of gH and/or altered the properties of gH/gL heterodimers. Altered or absent interaction of gL with gH was correlated with reduced or absent cell fusion activity and impaired complementation of virion infectivity. These results identify a conserved domain of gL that is critical for its binding to gH and two noncontiguous regions of gL, one of which contains the conserved domain, that are critical for the gH/gL complex to perform its role in membrane fusion.