The varicella zoster virus glycoprotein B (gB) plays a role in virus binding to cell surface heparan sulfate proteoglycans

139

124

Glycoprotein B (gB) homologs are conserved throughout the family Herpesviridae and appear to serve essential, universal functions, as well as specific functions unique to a particular herpesvirus. Genetic analysis is a powerful tool to analyze protein function, and while it has been possible to generate virus mutants, complementation of essential virus knockouts has been problematic. Human cytomegalovirus (HCMV) gB (UL55) plays an essential role in the replication cycle of the virus. To define the function(s) of gB in HCMV infection, the BAC system was used to generate a recombinant virus in which the UL55 gene was replaced with galK (pAD/Cre⌬UL55). UL55 deletions in the viral genome have been made before, demonstrating that UL55 is an essential gene. However, without being able to successfully complement the genetic defect, a phenotypic analysis of the mutant virus was impossible. We generated fibroblasts expressing HCMV gB that complement pAD/Cre⌬UL55 and produce infectious virions lacking the UL55 gene but containing wild-type gB on the virion surface (⌬UL55-gB HCMV). This is the first successful complementation of an HCMV mutant with a glycoprotein deleted. To characterize ⌬UL55 infection in the absence of gB, noncomplementing cells were infected with ⌬UL55-gB virus. All stages of gene expression were detected, and significant amounts of DNase-resistant viral DNA genomes, representing whole intact virions, were released into the infected cell supernatant. Gradient purification of these virions revealed they lacked gB but contained other viral structural proteins. The gB-null virions were able to attach to the cell surface similarly to wild-type gB-containing virions but were defective in virus entry and cell-to-cell spread. Glycoprotein B-null virions do, however, contain infectious DNA, as IE gene expression can be detected in fibroblasts following treatment of attached gB-null virions with a membrane fusion agent, polyethylene glycol. Taken together, our results indicate that gB is required for virus entry and cell-to-cell spread of the virus. However, HCMV gB is not absolutely required for virus attachment or assembly and egress from infected cells.

Section: Discussionmentioning

confidence: 60%

Human Cytomegalovirus Glycoprotein B Is Required for Virus Entry and Cell-to-Cell Spread but Not for Virion Attachment, Assembly, or Egress

Isaacson

Compton

2009

139

124

“…Similarly, varicella zoster virus gB interacts with HSPG, a process that takes part in the initial attachment of the virus to the cell. However, like HSV gB, varicella zoster virus gB can still attach to HSPG deficient cells (26). Also, cell surface HSPGs are not essential for infection by pseudorabies virus (28).…”

Section: Discussionmentioning

confidence: 99%

Herpes Simplex Virus Glycoprotein B Binds to Cell Surfaces Independently of Heparan Sulfate and Blocks Virus Entry

Bender

Whitbeck

Lou

et al. 2005

116

Herpes simplex virus (HSV) is responsible for mucocutaneous lesions, commonly known as cold sores, and genital lesions or herpes genitalis. After primary infection virus resides latent in sensory and autonomic neurons from where it reactivates periodically (66). Among the important steps in infection, attachment and entry have attracted attention as targets for therapeutic treatments and vaccines. Binding and entry are complex biochemical processes involving 5 of the 12 known surface glycoproteins of the virus (59).Current models postulate an initial association of glycoprotein C (gC) and/or gB with cell surface heparan sulfate proteoglycan (HSPG) (56). Following binding, entry requires the interaction of gD with one of its receptors that include herpesvirus entry mediator (HVEM), nectin-1 and nectin-2, and specific sites in heparan sulfate generated by certain 3-O-sulfotransferases (18,38,55). The envelope-fusion mechanism is not solved yet, but this event requires gB, the heterodimer gH/gL, gD, and a gD entry receptor (7,39,48,63).HSV mutants lacking the gC gene exhibit reduced attachment, but the particles are still infectious (22). For that reason, gC is considered a nonessential glycoprotein for entry. Moreover, cells deficient in proteoglycan synthesis are still permissive to HSV (2). On the contrary, a gB null virus cannot penetrate target cells (9). A polylysine (pK) sequence located in the N terminus of gB is responsible for interaction of gB with HSPG. A virus mutant from which the pK coding sequence of gB is deleted is still infectious, although virus binding is reduced (32). Taken together, these data suggest that binding of HSV to HSPG through gC and/or gB is an important yet not essential event for entry. Since gB is essential for entry, this glycoprotein must carry out a function(s) other than binding to HSPG.Various criteria were used to show that gD binds receptors, and these same criteria should apply to demonstrate the existence of a gB receptor(s). HSV entry should be blocked by (i) antibodies to gB, (ii) soluble forms of gB, (iii) antibodies to receptor, and (iv) soluble forms of receptor. A number of monoclonal antibodies (MAbs) for gB have neutralizing activity (24,47,54). However, a soluble form of HSV-2 gB failed to block entry of HSV-1 and HSV-2 (27, 67). In the present study, we found that a soluble truncated form of HSV-1 gB bound saturably to the surface of different cell types, including HSPGdeficient cells Gro2C and Sog9 (2). The key finding was that soluble gB, like gD, efficiently inhibited HSV-1 entry into Gro2C cells in a dose-dependent manner. The results provide evidence for the existence of an entry receptor specific for gB.

“…An entry process similar to that described for HSV-1 has been proposed for VZV, commencing with binding of virions to heparan sulfate via gB (25,70), followed by interaction of viral glycoproteins with an entry receptor (7,70). The interaction of mannose-6-phosphate (M6P) groups found on at least four VZV glycoproteins (gB, gE, gH, and gI) with the cation-independent mannose-6-phosphate receptor (CI-MPR) is believed to facilitate virus entry (7,17,70).…”

mentioning

confidence: 87%

Postentry Events Are Responsible for Restriction of Productive Varicella-Zoster Virus Infection in Chinese Hamster Ovary Cells

Finnen¹,

Mizokami²,

Banfield

et al. 2006

Varicella-zoster virus (VZV) is a human alphaherpesvirus that causes chicken pox (varicella) upon primary infection and shingles (zoster) upon reactivation of the virus from latently infected sensory neurons. In contrast to the broad host range of most alphaherpesviruses in cultured cells, productive VZV infection in vitro is restricted almost exclusively to cells derived from humans and other primates (64). Reported exceptions are guinea pig embryonic fibroblasts (62, 64), rabbit kidney cells (64,68), and cotton rat fibroblasts (54). For many viruses, restriction in cultured cells is determined by specific cellular receptors that must be engaged for virus binding and entry. After successfully entering a cell, viruses can encounter other blocks to productive infection (20,36). In studies using rats, cotton rats, or mice as in vivo models of VZV latency, viral DNA (1,6,27,[51][52][53][54]69), viral transcripts (6,27,51,54,69), and immediate early viral proteins (18,27,51) were detected in infected cells, primarily neurons, indicating that VZV is capable of entering rodent cells and initiating early events in the infectious cycle. Cultured rat neurons and the mouse neuroblastoma cell line neuro-2A also permitted VZV entry, but productive infection was blocked in both cases (4, 37). Thus, the restriction of productive VZV infection in cultured nonprimate cells may be governed primarily by postentry events.Much of our understanding of alphaherpesvirus entry comes from studies of herpes simplex virus type 1 (HSV-1) (60, 61). HSV-1 envelope glycoproteins gB and/or gC mediate binding of virus to cell surface glycosaminoglycans, predominantly heparan sulfate (59, 63). Following virus binding, gD engages one of several entry receptors (19,40,58,60). Fusion of viral and cellular membranes then ensues, either directly at the cell surface (15, 16) or following endocytosis (38,42,43) by a mechanism that requires the participation of gB, gD, a heterodimer of gH and gL, and a gD entry receptor (5,44,47,65). The emerging picture of entry from HSV-1 studies is one that involves multiple entry receptors and multiple entry pathways. This versatility likely contributes to the wide host range observed for HSV-1 in vitro and the success of this virus as a pathogen.Much less is known about the binding and entry of VZV, largely due to the difficulty in obtaining high titers of cell-free (CF) virus. An entry process similar to that described for HSV-1 has been proposed for VZV, commencing with binding of virions to heparan sulfate via gB (25, 70), followed by interaction of viral glycoproteins with an entry receptor (7, 70). The interaction of mannose-6-phosphate (M6P) groups found on at least four VZV glycoproteins (gB, gE, gH, and gI) with the cation-independent mannose-6-phosphate receptor (CI-MPR) is believed to facilitate virus entry (7,17,70). While the exact consortium of VZV proteins needed for virus-cell fusion has not been defined, results from cell-cell fusion assays (9,11,12,46), blocking assays with antiglycoprotein antibo...