Herpes Simplex Virus gD Forms Distinct Complexes with Fusion Executors gB and gH/gL in Part through the C-terminal Profusion Domain

Herpes simplex virus entry into cells requires a multipartite fusion apparatus made of glycoprotein D (gD), gB, and heterodimer gH/gL. gD serves as a receptor-binding glycoprotein and trigger of fusion; its ectodomain is organized in an N-terminal domain carrying the receptor-binding sites and a C-terminal domain carrying the profusion domain, required for fusion but not receptor binding. gB and gH/gL execute fusion. To understand how the four glycoproteins cross-talk to each other, we searched for biochemical defined complexes in infected and transfected cells and in virions. Previously, interactions were detected in transfected whole cells by split green fluorescent protein complementation (Atanasiu, D., Whitbeck, J. C., Cairns, T. M., Reilly, B., Cohen, G. H., and Eisen-berg, R. J. (2007) Proc. Natl. Acad. Sci. U. S. A. 104, 18718-18723; Avitabile, E., Forghieri, C., and Campadelli-Fiume, G. (2007) J. Virol. 81, 11532-11537); it was not determined whether they led to biochemical complexes. Infected cells harbor a gD-gH complex (Perez-Romero, P., Perez, A., Capul, A., Montgomery, R., and Fuller, A. O. (2005) J. Virol. 79, 4540-4544). We report that gD formed complexes with gB in the absence of gH/gL and with gH/gL in the absence of gB. Complexes with similar composition were formed in infected and transfected cells. They were also present in virions prior to entry and did not increase at virus entry into the cell. A panel of gD mutants enabled the preliminary location of part of the binding site in gD to gB to the amino acids 240-260 portion and downstream with Thr(304)-Pro(305) as critical residues and of the binding site to gH/gL at the amino acids 260-310 portion with Pro(291)-Pro(292) as critical residues. The results indicate that gD carries composite-independent binding sites for gB and gH/gL, both of which are partly located in the profusion domain.