The Epstein-Barr virus (EBV) BZLF2 gene product associates with the gH and gL homologs of EBV and carries an epitope critical to infection of B cells but not of epithelial cells

Abstract

Glycoprotein gp85, the product of the BXLF2 open reading frame (ORF), is the gH homolog of Epstein-Barr virus (EBV) and has been implicated in penetration of virus into B cells. Like its counterparts in other herpesviruses, it associates with a gL homolog, gp25, which is the product of the BKRF2 ORF. Unlike the gH homologs of other herpesviruses, however, gp85 also complexes with two additional glycoproteins of 42 and 38 kDa. Glycoproteins gp42 and gp38 were determined to be alternatively processed forms of the BZLF2 gene product. Coexpression of EBV gH and gL facilitated transport of gH to the cell surface and resulted in formation of a stable complex of gH and gL. It also restored expression of an epitope recognized by monoclonal antibody E1D1, which immunoprecipitates the native gH complex but not recombinant gH expressed in isolation. Coexpression of gH, gL, and the BZLF2 ORF restored expression of an epitope recognized by a second monoclonal antibody, F-2-1, which immunoprecipitates the native gH-gL-gp42/38 complex but not the complex of recombinant gH and gL alone. The epitope recognized by antibody F-2-1 was mapped to the BZLF2 gene product itself. Antibody F-2-1 inhibited the ability of EBV to infect B lymphocytes but had no effect on the ability of the virus to infect the epithelial cell line SVK-CR2. In contrast, antibody E1D1 had no effect on infection of the B-cell line but inhibited infection of the epithelial cell line. These results indicate that penetration of the two cell types by EBV involves differential use of the gH-gL-gp42/38 complex and suggest the hypothesis that the BZLF2 gene product has evolved as a unique adaptation to infection of B lymphocytes by EBV.

The Amino Terminus of Epstein-Barr Virus Glycoprotein gH Is Important for Fusion with Epithelial and B Cells

“…2A). The E1D1 Ab recognizes the native gH/gL complex and blocks epithelial cell entry and fusion, while it has no effect on entry of EBV into B cells and B cell fusion (20,24). The cell surface expression of the single L65A mutant was about 50% of that of the wild type, while the expression of both L55/65A and L65/74A mutants was reduced by about 25%.…”

“…gH mutants retain their ability to associate with gp42. For the gH/gL complex to mediate EBV entry into B cells, association with gp42 is also required (20,47). Therefore, we examined whether leucine mutations in the N terminus of gH had any effect on the binding of the gH/gL complex to gp42.…”

“…Subsequent to this binding, viral glycoprotein gp42 binds to the B-cell surface protein HLA class II, which triggers fusion mediated by a concerted action of three glycoproteins (gB, gH, and gL) (19,43). The glycoprotein requirement for EBV entry differs between epithelial and B cells; gB, gH, and gL are essential for infection of both epithelial and B cells, while gp42 is required only for B-cell entry (12,20,24,47). Even though gB, gH, and gL are conserved throughout the herpesvirus family, the structure and the exact mechanism of action of these three glycoproteins in the fusion process are still unclear.EBV gH and gL are present on the viral envelope as a heterodimer complex, gH/gL.…”

“…In EBV, gH and gL form two different complexes, a bipartite complex that contains only gH and gL and a tripartite complex that also includes gp42 (2,15,48). These two complexes have a mutually exclusive ability to mediate infection of epithelial cells and B cells, respectively (2,20).…”

The Transmembrane Domain and Cytoplasmic Tail of Herpes Simplex Virus Type 1 Glycoprotein H Play a Role in Membrane Fusion

“…gB contains a GAG-binding domain, but cell surface GAGs are not necessary for cell fusion induced by the expression of HSV-1 glycoproteins (4,34). The gHL homologue of Epstein-Barr virus associates with a further glycoprotein, g42, which interacts with major histocompatibility complex class II on B cells (22,23), and, in combination with the gB homologue, this complex mediates cell-cell fusion (17). A putative receptor for human cytomegalovirus gH has been identified (1,2), though this molecule has yet to be characterized as a cell surface component.…”