Differentiation of multiple domains in the herpes simplex virus 1 protease encoded by the UL26 gene.

Abstract

Previous studies have shown that the herpes simplex virus 1 gene U,26 encodes a 635-amino acid protease that cleaves approximately 20 amino acids from the carboxyl terminus of itself and ofa 329-amino acid product ofthe UL,26.5 gene. The results of studies with a variety of protease inhibitors showed that the UL26 protease was inhibited by serine protease inhibitors but not by inhibitors of cysteine protease, aspartic acid protease, or metalloprotease. Mutations resulting in amino acid substitutions, deletions, or insertion of stop codons in the gene or of 20-amino acid stretches into the protease have delineated the dispensable domains I and IV at the amino and carboxyl domains of the gene product. The essential carboxylproximal domain (Ill) can be separated from the essential amino-proximal domain (H) by at least 20 amino acids. The amino-proximal domain is the most conserved region among varicella-zoster virus and human cytomegalovirus homologues of UL26. Of the conserved aspartic acid, histidine, or serine amino acids in this domain, only histidine-61 and -148 could not be replaced without impairment of the proteolytic activity.Earlier we reported that the herpes simplex virus 1 (HSV-1) open reading frame (ORF) UL26 encodes a protease (1). The sequence of UL26 (2) predicts that the protease contains 635 amino acids. The substrate of the protease is the protease itself and a more abundant protein encoded by the ORF UL26.5. The promoter of the UL26.5 ORF is imbedded in the 5' terminal domain of the coding sequences of UL26 ORF. Its coding domain is identical to the coding domain of the terminal 329 amino acids of UL26 ORF, and its product was previously designated as the infected-cell protein no. 35 (ICP35) (1,3). The protease and its more abundant substrate are both cleaved by the protease approximately 20 amino acids from its carboxyl terminus. We report here that (i) the activity of the protease is consistent with that of an enzyme with serine at the active site, a seine protease; (ii) the protease protein contains several domains that are not required for its catalytic activity; and (iii) the active site is near

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