Herpes simplex virus (HSV) represents the most comprehensive example of virus-receptor interaction in the Herpesviridae family, and the prototype virus encoding multipartite entry genes. Whereas small enveloped viruses package the functions required for entry and fusion into one or two fusion glycoproteins, in HSV the same functions are distributed over several distinct glycoproteins, each with a specialized activity. In addition, HSV encodes a highly sophisticated system for promoting and blocking fusion between the viral envelope and cell membrane. Because the most obvious models of virus entry into the cell do not fit with the HSV complexity, and despite our detailed knowledge of the HSV receptors and of the crystal structure of glycoprotein D (gD), the receptor-binding glycoprotein, and of gB, HSV entry is still, in part, a puzzle (WuDunn and Spear, 1989; Cocchi et al., 1998b; Geraghty et al., 1998; Carfi et al., 2001).
The current model of HSV entry envisions that, first, the virus attaches to cell membranes by the interaction of gC, and possibly gB, to glycosaminoglycans (GAGs) (Herold et al., 1991). This binding likely creates multiple points of adhesion, is reversible, and the detached virus maintains its infectivity, indicating that fusion has yet to take place. Penetration requires gD, whose ectodomain contains two physically separate and functionally distinct regions, i.e., the region made of the N-terminus that carries the receptor-binding sites, and the C-terminus that carries the profusion domain (Ligas and Johnson, 1988; Cocchi et al., 2004).