Glycoprotein K of Herpes Simplex Virus: A Transmembrane Protein Encoded by the UL53 Gene which Regulates Membrane Fusion

Glycoprotein K (gK) encoded by the UL53 gene is the ninth out of eleven HSV glycoproteins (gps). The precursor gK (pgK) is a transmembrane protein with four hydrophobic domains, which consists of 338 amino acids. The UL53 gene has two initiation codons: the upper overlaps with the UL52 ORF, while the lower is located 55 codons downstream and specifies a truncated precursor of the gK polypeptide. The UL53 gene and the upstream located UL52 gene have a common polyadenylation signal downstream from the UL53 stop codon so that the UL53 mRNA is completely nested within the UL52 transcript. The syn1 mutations in several KOSsyn mutants and in the MPsyn virus, which had been fine mapped to DNA coordinates 0.735–0.740, were later on located to the UL53 gene, especially to its portion which specifies the first 120 amino acids (aa) from the N-terminus (most frequently residue 40) and to a less precisely defined locus between aa 301–310 (close to the C-terminus). Point mutations in the N-terminal ectodomain of gK, which are related to syn formation, impair the putative ability of this region to down-regulate membrane fusion. The two N-glycosylated mannose core oligosaccharides are attached to the Asn residues of the gK polypeptide at positions 48 and 58, respectively. In infected cells, gK is localized mainly in the nuclear and endoplasmic reticulum (ER) membranes. It is not clear, whether gK becomes incorporated into the envelope of mature HSV particles. Studies with the insertion/deletion gK mutants showed the importance of gK for capsid envelopment, for the transportation and egress or virions from infected cells. It seems that gK has an essential role in virion egress, even though this glycoprotein acts in accord with gH and with another membrane protein encoded by the UL20 gene.