Characteristics of Epstein-Barr virus envelope protein gp42

Abstract

Epstein-Barr virus (EBV) glycoprotein 42 (gp42) is a membrane protein essential for fusion and entry of EBV into host B-lymphocytes. Gp42 is a member of the protein-fold family C-type lectin or lectin-like domains (CLECT or CTLD) and specifically is classified as a natural-killer receptor (NKR)-like CLECT. Literature review and phylogenetic comparison show that EBV gp42 shares a common structure with other NKR-like CLECTs and possibly with many viral CTLDs, but does not appear to exhibit some common binding characteristics of many CTLDs, such as features required for calcium binding. The flexible N-terminal region adjacent to the CTLD fold is important for binding to other EBV glycoproteins and for a cleavage site that is necessary for infection of host cells. From structural studies of gp42 unbound and bound to receptor and extensive mutational analysis, a general model of how gp42 triggers membrane fusion utilizing both the flexible N-terminal region and the CTLD domain has emerged.

Fig. 1

Fig. 1a Linear graphical representation of the sequence and structural features of EBV gp42 Fig. 1b Corresponding x-ray crystallization-determined structure of gp42 in its unbound form beginning with amino acid 33 (Protein DataBank identification number 3FD4). Blue coils=alpha helices; red arrows=beta strands; yellow tubeworms=disulfide bridges; black arrowhead=disulfide bridge conserved in all canonical c-type lectin-like domains (CTLDs); gray arrowhead=disulfide bridge conserved in long-loop CTLDs; black arrow=disulfide bridge conserved in Ly49 natural killer (NK) CTLDs; gray arrow=disulfide bridge conserved in CD94 and NK2GD CTLD families; red arrow=disulfide bridge unique to EBV gp42. Figure created with Pymol

Fig. 2

Overlay of the HLA class II-bound and unbound structures of EBV gp42. Bound structure is light blue-gray, unbound is yellow, HLA class II is deep blue. Figure created with Pymol

Fig. 3

Rooted phylogenetic cladogram of viral CTLDs. Gamma herpesvirus CTLDs are grouped (green branch). Tree visualized with TreeDyn

Fig. 4

Fig. 4a Nearest structural neighbors of EBV gp42 as determined by number of residues aligned with VAST. EBV gp42 is highlighted in yellow. Areas demarcated by brackets show less-aligned loops. Blue=alpha helices; red arrows=beta strands. Structures visualized with Cn3D Fig. 4b Re-alignment of nearest structural neighbors using the Loop-Hausdorff method with VAST. Loops are now more compactly aligned. Colors as in Fig. 4a. Structures visualized with Cn3D

Fig. 5

Structure of EBVgp42 bound to HLA class II (PDB identification number 1KG0) with residues selected for mutation highlighted in color. Amino acids in the N-terminal domain are shown in silver-gray; HLA class II contacting residues are brown; hydrophobic pocket residues are pink-purple; residues in other structural features are green. Hydrogen bonds between selected gp42 and HLA contact residues are shown by chartreuse dotted lines. Structure visualized with Pymol