Structural and Mechanistic Insights into the Tropism of Epstein-Barr Virus

Abstract

Epstein-Barr virus (EBV) is the prototypical γ-herpesvirus and an obligate human pathogen that infects mainly epithelial cells and B cells, which can result in malignancies. EBV infects these target cells by fusing with the viral and cellular lipid bilayer membranes using multiple viral factors and host receptor(s) thus exhibiting a unique complexity in its entry machinery. To enter epithelial cells, EBV requires minimally the conserved core fusion machinery comprised of the glycoproteins gH/gL acting as the receptor-binding complex and gB as the fusogen. EBV can enter B cells using gp42, which binds tightly to gH/gL and interacts with host HLA class II, activating fusion. Previously, we published the individual crystal structures of EBV entry factors, such as gH/gL and gp42, the EBV/host receptor complex, gp42/HLA-DR1, and the fusion protein EBV gB in a postfusion conformation, which allowed us to identify structural determinants and regions critical for receptor-binding and membrane fusion. Recently, we reported different low resolution models of the EBV B cell entry triggering complex (gHgL/gp42/HLA class II) in "open" and "closed" states based on negative-stain single particle electron microscopy, which provide further mechanistic insights. This review summarizes the current knowledge of these key players in EBV entry and how their structures impact receptor-binding and the triggering of gB-mediated fusion.

Keywords: Epstein-Barr virus; entry; fusion; herpesvirus; tropism.

Fig. 1.

Model of EBV entry into B lymphocytes. First, gp42 binds to its B lymphocyte receptor HLA class II, which cause a widening of the hydrophobic pocket within gp42. The conformational change within gp42 might be transmitted to gH/gL, which allows the interaction with the fusogen gB. In this model, D-I of gH/gL includes the residues Q54/K94 (yellow spheres), which are supposed to be involved in gB interactions and are directed towards a model of prefusion gB. The interaction of the tripartite complex gH/gL/gp42 with gB triggers the conformational transition to postfusion gB, which drives fusion with B cell membranes. The structural view of EBV gH/gL shows the disulfide bonds (orange spheres), gp42-binding region (blue spheres) and the KGD-motif (red spheres). TM is transmembrane region, α is α-helix and β is β-strand. The protein secondary structure prediction for the CTDs of EBV gB and gH were designed using JPred4 (Drozdetskiy et al., 2015). The structural views of prefusion and postfusion gB (PDB ID: 3FVC) (Backovic et al., 2009) as well as the complex composed of gH/gL (3PHF) (Matsuura et al., 2010) and gp42-HLA-DR1 (1KG0) (Mullen et al., 2002; Sathiyamoorthy et al., 2014) were generated using The PyMOL Molecular Graphics System, Version 1.3 Schrödinger, LLC.

Fig. 2.

Model of EBV entry into epithelial cells. First, gH binds to its epithelial cell receptor integrin ανβ6 causing a conformational change within the large groove of gH/gL, which might allow the interaction with the fusogen gB. This interaction triggers the conformational transition to postfusion gB, which drives fusion with the plasma membrane. The K/RGD-motif was used to model integrin binding to gH/gL based on the structure of ανβ6/TGF-β3 (4UM9). The structural views of integrin ανβ6 (4UM9) (Dong et al., 2014), prefusion and postfusion gB (3FVC) (Backovic et al., 2009) were generated using The PyMOL Molecular Graphics System, Version 1.3 Schrödinger, LLC.