gH has a large ectodomain and a single C-terminal transmembrane anchor while gL lacks a transmembrane region. The two proteins form a stable 1:1 complex. The crystal structure of gH/gL ectodomain from HSV-2 [8
] revealed an unusual boot-shaped complex (), reminiscent of the 15-20-nm curved glycoprotein spikes observed by cryo-electron tomography [11
]. Given the conservation of gH/gL in sequence and function, it was a surprise when the subsequently determined structure of EBV gH/gL [9
] showed a cylindrical molecule (). Yet despite different spatial arrangement, individual domains within HSV-2 gH align well with their EBV counterparts (). To highlight fundamental similarities between HSV and EBV gH molecules and to reconcile somewhat different domain assignments proposed for the two structures, we divided gH into H1A, H1B, H2A, H2B, and H3 domains, from N to C terminus (). The distinct shapes of two gH/gL complexes, boot vs
cylinder, arise from differences in relative domain orientations, especially at the H1B/gL and H2A/H2B interfaces, demonstrating a certain structural plasticity.
Fig. 1 Structural similarities between HSV and EBV gH/gL. (a) Structures of HSV-2 gH/gL (PDB: 3M1C)  and EBV gH/gL (PDB: 3PHF)  are colored by domain, and all domains are labeled. (b) Aligned individual domains are shown side by side and labeled. Color (more ...)
gL is “sandwiched” between domains H1A and H1B and interacts extensively with both, illustrating why gH and gL are always found as a complex. H1A forms a continuous 5-stranded β-sheet with gL and is probably unstructured in the absence of gL. H1A is the target of several anti-HSV neutralizing (or inhibiting spread) antibodies [12
] and some mutations that disrupt HSV fusion [14
] or affect VZV tropism [15
]. No functional role has yet been assigned to the much smaller H1A of EBV gH/gL. The relatively unstructured N terminus of domain H1B has many contacts with gL as well as a long lasso-like loop that interacts with H2A and H2B. The C terminus of domain H1B forms a 5-stranded β-sheet, a “picket-fence” [9
] that wraps around domain H2A. Both EBV and HSV gH/gL contain non-conserved integrin-binding motifs within domain H1B.
Sequence conservation in gH increases from N to C terminus, so domains H2A, H2B, and H3 share a higher degree of structural similarity. The presence of a three-helix bundle in H2A evokes a structural similarity with other helical bundles, e.g., syntaxin, while the helical arrangement in the crescent-shaped H2B is reminiscent of HEAT repeats [10
The C-terminal domain H3 has a β-sandwich fold. It is the most highly conserved domain and likely critical for fusion. H3 in HSV, EBV, and VZV gH is very sensitive to point mutations and insertions as most abolish fusion [15
] or alter tropism in EBV [18
]. A long extended polypeptide, termed a “flap” [10
], wraps around one side of the β-sandwich (). The hydrophobic surface patch underneath the flap was postulated to interact with membranes, were the flap to relocate [10
]. In support of this idea, replacement of some residues with more hydrophobic ones enhances fusion or rescues fusion reduced by other nearby mutations [15
] (). The relocation of the flap and membrane interaction remains to be demonstrated.
Fig. 2 Functional sites in HSV-2 and EBV gH/gL. HSV-1 gH/gL (a) and EBV gH/gL (b) are shown using wireframe representation and paler colors than in . An extended polypeptide in H3, the “flap”, is shown as a thick tube. Disulfide bonds that (more ...)