Role of Herpes Simplex Type I Glycoproteins in Entry and Cell-cell Fusion

Herpes simplex virus type I (HSV-1) is a neurotropic virus that infects primarily mucocutaneous epithelial cells and nervous tissue. Membrane fusion is an important aspect of the HSV-1 lifecycle, that occurs during viral entry (virus-cell fusion), viral spread (cell-to-cell fusion), as well as, during virion morphogenesis (assembly and egress). These membrane fusion steps involve complex interactions between multiple viral glycoproteins and cellular receptors. HSV-1 glycoprotein B (gB) is necessary but not sufficient for membrane fusion events. Despite the fact, that the majority of known hypermorphic mutations which cause extensive virus-induced cell fusion occur within glycoprotein K (gK); yet the role of gK in gB-mediated cell fusion is not well-understood. We found that a mutation within the carboxyl terminal of gB that resulted in extensive cell fusion is lost in presence of a mutation in gK amino terminus(deletion of amino acids 31-68). These results suggest that gK may regulate gB mediated virus-induced cell fusion. Co-immunoprecipitation experiments revealed, that a peptide specifying the amino terminus of gK physically interacted with others members of the fusion complex i.e. gB, gH but not gD. Moreover, UL20p, known to interact with gK, interacted with gB as revealed by immunoprecipitatin reaction. Virus entry was also modulated by gK, since gK mutants lacking the entire gK gene or a deletion in the amino terminus (amino acids 31-68) failed to enter Chinese hamster ovary cells (CHO) expressing the gB receptor paired immunoglobulin-like type 2 receptor alpha (PILRα). While these gK mutants efficiently entered into CHO cells expressing the gD specific receptors, nectin-1 and HVEM. Co-immunoprecipitation experiments revealed that PILRα formed a multi-protein complex with gB and gK. Thus gK functions in entry in presence of gB specific receptors but not gD specific receptors. Overall, results obtained in this study, show that gK and UL20 are part of the fusion complex (gB,gD,gH/gL) that functions during virus entry and cell spread, and regulates interactions of gB with gB-specific cellular receptors