Herpes simplex virus (HSV)2 entry into the cell occurs by fusion, and requires a multipartite apparatus made of a glycoprotein quartet: gD, gB, and the heterodimer gH/gL; for reviews, see Refs. 1-3). When ectopically expressed, the four glycoproteins mediate cell-cell fusion (4). gD serves as the receptor-binding glycoprotein and interacts with three alternative receptors, nectin1, herpesvirus entry mediator (HVEM) and modified heparan sulfate (5-8). It also represents the key player in the triggering of fusion, i.e. in inducing fusion execution by gB and gH/gL (9 -12). These are among the most highly conserved proteins across the Herpesviridae family and constitute the fusion core apparatus. Their respective roles in fusion execution are unclear at present. Thus, gH carries elements typical of fusion glycoproteins, i.e. hydrophobic regions able to interact with target cells or artificial membranes and two heptad repeats potentially able to form a coiled coil (13-18). Peptides mimicking the hydrophobic regions of gH promote fusion of artificial membranes (19,20). With respect to gB, the crystal structure shows a trimer with a central coiled-coil and an overall structure similar to that of the fusion glycoprotein G of vesicular stomatitis virus in its postfusion conformation (21,22). Recently, a receptor for gB was described (23). Of note, although for virus-to-cell entry and for cell-cell fusion, fusion requires the simultaneous presence of gB and gH/gL, fusion of perinuclear virions with the outer nuclear membranes appears to necessitate either gH/gL or gB (24). Furthermore, it has been reported that HSV fusion may be preceded by a hemifusion intermediate mediated by gD and gH/gL (25).A major focus of current research is definition of proteinprotein interactions. With respect to HSV entry/fusion, to understand how the four glycoproteins cross-talk to each other, it is pivotal to detect which complexes are formed by the glycoproteins in their prefusion and fusion-active conformations, and, ultimately, the chain of interactions that signal the gD encounter with its cellular receptor and culminate in activation of gB and gH/gL. The proposed model of gD-activated entry/ fusion (2, 9 -12, 26, 27) envisions that (i) gD ectodomain is organized in two functionally and topologically distinct regions, the N-terminal one, spanning amino acids (aa) 1 to ϳ240/260, carrying the receptor binding sites, and the C-terminal one (aa 240/260 -310), carrying the profusion domain required for fusion but not for receptor binding; (ii) the unliganded gD adopts an auto-inhibited conformation, whereby the C-terminal domain folds around the N-terminal one and occupies or hinders the receptor-binding sites; (iii) gD undergoes a closed-to-open switch in conformation, whereby the C-terminal domain is dislodged from its binding site and