gH/gL supercomplexes at early stages of herpesvirus entry

Abstract

Membrane fusion during herpesvirus entry into host cells is a complex process where multiple glycoproteins interact to relay the triggering signal from a receptor-binding protein to the conserved fusogen gB through the conserved heterodimer gH/gL. Crystal structures of individual glycoproteins are available, yet high-order 'supercomplexes' have been elusive. Recent structures of complexes between gH/gL from human cytomegalovirus or Epstein-Barr virus and the receptor-binding proteins that form at early stages of herpesviral entry highlighted mechanisms that control tropism and revealed dynamic intermediate complexes containing gH/gL that may directly participate in membrane deformation and juxtaposition. Determining how the triggering signal reaches the fusogen gB represents the next frontier in structural biology of herpesvirus entry.

Fig. 1. Fusogenic cascade in prototypes of three subfamilies of herpesviruses

This diagram is limited to proteins that are essential for membrane fusion during viral entry. The conserved gH/gL (blue) and gB (red) components are colored. Known or presumed stable binding events are shown as solid arrows. Triggering events that may involve only transient binding are shown as dashed arrows. Proteins and complexes, for which crystal structures are known, are shown in bold. Note that only postfusion structures are available for gB. Complexes recently reconstructed by EM, which are the focus of this review, are boxed. R_fib and R_epi are postulated CMV receptors in fibroblasts and epithelial cells, respectively.

Fig. 2. Structural similarities and differences between gH/gL complexes from HSV, EBV, and CMV

Crystal structures of gH/gL from HSV-2 gH/gL (PDB: 3M1C) [27] and EBV gH/gL (PDB: 3PHF) [32] are shown in surface representation and colored by domain as described earlier [31]. gL is shown in blue and labeled. Figure was made using Pymol (http://www.pymol.org). Schematic depiction of CMV gH/gL structure is based on EM reconstruction of gH/gL-containing complexes and the observation that its structure resembles HSV-2 gH/gL [33]. Approximate location of CMV gL is shown. Membrane and transmembrane anchors are shown schematically.

Fig. 3. CMV and EBV have mutually exclusive gH/gL-containing complexes

(A) CMV gH/gL/gO (trimer) and gH/gL/UL128-131 (trimer) determine viral tropism for fibroblasts and epithelial cells, respectively. The binding sites for gO and UL128 on gL overlap as each forms a disulfide with the same cysteine on gL, C144. Cysteines forming this disulfide are shown as stars and labeled. Schematic depiction of CMV gH/gL complexes is based on their EM reconstructions [33]. Approximate location of CMV gL is shown. The shape and the location of gO, UL128, UL130, and UL131 are approximate. Membrane and transmembrane anchors are shown schematically. (B) EBV gH/gL/gp42 and gH/gL determine viral tropism for B cells and epithelial cells, respectively. The binding sites for gp42 and integrins αVβ6-8 overlap. Schematic depiction of EBV gH/gL structure is based on its crystal structure [32]. Location and shape of gp42 is based on EM reconstruction of gp42/gH/gL/HLA complex [46]. Membrane and transmembrane anchors are shown schematically.

Fig. 4. A diagram illustrating the dynamic nature of the EBV B cell entry complex and its potential direct role in membrane fusion

(A) EM reconstructions show that a soluble variant of the gH/gL/gp42/HLA complex has a dynamic structure that exists in heterogeneous open or a fixed closed conformations [46]. Proteins are shown schematically, but their relative orientations are based on EM reconstructions. (B) A model of how conformational flexibility of the gH/gL/gp42/HLA complex could facilitate fusion by deforming and bringing the apposed membranes into proximity. Membrane and transmembrane anchors are shown schematically.