Regulation of Herpes Simplex Virus gB-Induced Cell-Cell Fusion by Mutant Forms of gH/gL in the Absence of gD and Cellular Receptors

Herpesvirus entry requires the viral glycoprotein triad of gB and gH/gL to carry out fusion between the virion envelope and a cellular membrane in order to release the nucleocapsid into the target cell. Herpes simplex virus (HSV) also requires glycoprotein gD to initiate the fusion cascade by binding a cell receptor such as nectin 1 or herpesvirus entry mediator (HVEM). While the structure of gB is that of a class III fusion protein, gH/gL has no features that resemble other viral fusion proteins. Instead, it is suggested that gH/gL acts as a regulator of gB. The crystal structure of HSV-2 gH/gL was obtained with a functional protein that had a deletion of 28 residues at the gH N terminus (gHΔ48/gL). Unexplainably, monoclonal antibodies (MAbs) with virus-neutralizing activity map to these residues. To reconcile these two disparate observations, we studied the ability of gHΔ48/gL to regulate fusion. Here, we show that the protein induces low (constitutive) levels of fusion by gB in the absence of gD and/or receptor. However, when gD and receptor are present, this mutant functions as well as does wild-type (wt) gH/gL for fusion. We propose that gHΔ48/gL has an intermediate structure on the pathway leading to full regulatory activation. We suggest that a key step in the pathway of fusion is the conversion of gH/gL to an activated state by receptor-bound gD; this activated gH/gL resembles gHΔ48/gL.