The Amino Terminus of Herpes Simplex Virus Type 1 Glycoprotein K (gK) Modulates gB-Mediated Virus-Induced Cell Fusion and Virion Egress

Herpes simplex virus type 1 (HSV-1)-induced cell fusion is mediated by viral glycoproteins and other membrane proteins expressed on infected cell surfaces. Certain mutations in the carboxyl terminus of HSV-1 glycoprotein B (gB) and in the amino terminus of gK cause extensive virus-induced cell fusion. Although gB is known to be a fusogenic glycoprotein, the mechanism by which gK is involved in virus-induced cell fusion remains elusive. To delineate the amino-terminal domains of gK involved in virus-induced cell fusion, the recombinant viruses gK Delta 31-47, gK Delta 31-68, and gK Delta 31-117, expressing gK carrying in-frame deletions spanning the amino terminus of gK immediately after the gK signal sequence (amino acids [aa] 1 to 30), were constructed. Mutant viruses gK Delta 31-47 and gK Delta 31-117 exhibited a gK-null (Delta gK) phenotype characterized by the formation of very small viral plaques and up to a 2-log reduction in the production of infectious virus in comparison to that for the parental HSV-1(F) wild-type virus. The gK Delta 31-68 mutant virus formed substantially larger plaques and produced 1-log-higher titers than the gK Delta 31-47 and gK Delta 31-117 mutant virions at low multiplicities of infection. Deletion of 28 aa from the carboxyl terminus of gB (gB Delta 28syn) caused extensive virus-induced cell fusion. However, the gB Delta 28syn mutation was unable to cause virus-induced cell fusion in the presence of the gK Delta 31-68 mutation. Transient expression of a peptide composed of the amino-terminal 82 aa of gK (gKa) produced a glycosylated peptide that was efficiently expressed on cell surfaces only after infection with the HSV-1( F), gK Delta 31-68, Delta gK, or UL20-null virus. The gKa peptide complemented the gK Delta 31-47 and gK Delta 31-68 mutant viruses for infectious-virus production and for gK Delta 31-68/gB Delta 28syn-mediated cell fusion. These data show that the amino terminus of gK modulates gB-mediated virus-induced cell fusion and virion egress.