Molecular genetics and functions of herpes simplex virus type 1 (HSV-1) glycoprotein K (gK) in the morphogenesis of infectious virion particles

Herpes simplex virus type 1 (HSV-1) specifies at least 11 glycoproteins that are expressed in infected cells and in virions. HSV-1 enters into cells via fusion of the viral envelope with cellular membranes and can spread to adjacent cells via fusion of cellular membranes. Glycoproteins gB, gD, gH, and gL are known to be essential for virus entry, virus spread, and virus induced cell-to-cell fusion. The majority of spontaneous single amino acid changes that cause extensive cell fusion map within the UL53 gene encoding glycoprotein K (gK); however, the role of gK in virus entry and virus egress had not been defined. Specific antibodies against portions of gK expressed in E. coli were raised in mice and detected gK as a 40-kDa protein in purified virions. To investigate the effect of gK in virus entry via the known HSV-1 HveAh and HveCh receptors, two recombinant viruses were constructed, KOS/EGFP (wild-type) and DeltagK/EGFP (gK null) expressing the enhanced green fluorescent protein (EGFP) constitutively. The simian homolog of HveA (HveAs) was cloned, sequenced, and CHO/HveAs transformed cell-lines were isolated. The DeltagK/EGFP virus failed to enter into CHO/HveAs, while DeltagK/EGFP prepared in complementing Vero (VK302) cells entered efficiently. In contrast, DeltagK/EGFP entered efficiently into CHO/HveAh and CHO/HveCh cells. A series of hybrid receptors were made expressing different portions of HveAh and HveAs and were used to map a short region of HveAh that conferred receptor-mediated entry to an HveAs receptor background. The role of gK in virion morphogenesis and egress was addressed by constructing a panel of recombinant viruses expressing gK carboxyl-terminal truncations and single amino acid substitutions. Two cellular vesicular transport motifs, a cysteine-rich motif and a tyrosine-based motif were found to be essential for infectious virus production, intracellular virion transport, and virus egress. Therefore, gK may interact with vesicular transport pathways during virion morphogenesis and egress. The overall results from this research suggest that HSV-1 gK is a multifunctional protein involved in receptor utilization during virus entry and transport of virion particles to extracellular spaces through cellular vesicular transport pathways during virus egress.