The Amino Terminus of Herpes Simplex Virus 1 Glycoprotein K Is Required for Virion Entry via the Paired Immunoglobulin-Like Type-2 Receptor Alpha

The herpes simplex virus 1 (HSV-1) glycoprotein K (gK)/UL20 protein complex is incorporated into virion envelopes and cellular membranes and functions during virus entry and cell-to-cell spread. To investigate the role of gK/UL20 in the context of a highly neurovirulent virus strain, the HSV-1(McKrae) genome was cloned into a bacterial artificial chromosome plasmid (McKbac) and utilized to construct the mutant virus McK(gK Delta 31-68), carrying a 37-amino-acid deletion within the gK amino terminus. The McKbac virus entered efficiently into Chinese hamster ovary (CHO) cells constitutively expressing HSV-1 human receptors, nectin-1, herpesvirus entry mediator (HVEM), or paired immunoglobulin-like type-2 receptor alpha (PILR alpha). In contrast, the McK(gK Delta 31-68) virus failed to enter into CHO-PILR alpha cells, while it entered CHO cells expressing HVEM and nectin-1 more efficiently than the McKbac virus. Both McKbac and McK(gK Delta 31-68) viruses entered all CHO cells expressing HSV-1 receptors via a pH-independent pathway. The HSV-1(F) gB Delta 28syn mutant virus, encoding a carboxyl-terminal truncated gB, causes extensive cell fusion. Previously, we showed that the gK Delta 31-68 amino acid deletion abrogated gB Delta 28syn virus-induced cell fusion, indicating that the amino terminus of gK is required for gB-mediated virus-induced cell fusion (V. N. Chouljenko, A. V. Iyer, S. Chowdhury, D. V. Chouljenko, and K. G. J. Kousoulas, Virology 83: 12301-12313, 2009). Surprisingly, the gK Delta 3168/gB Delta 28syn virus caused extensive fusion of CHO-nectin-1 cells but limited cell fusion of CHO-PILR alpha cells. Coimmunoprecipitation experiments revealed that both gK and PILR alpha bound gB in infected cells. Collectively, these results indicate that the amino terminus of gK is functionally and physically associated with the gB-PILR alpha protein complex and regulates membrane fusion of the viral envelope with cellular membranes during virus entry as well as virus-induced cell-to-cell fusion.