Origin of Expression of the Herpes Simplex Virus Type 1 Protein U S 1.5

Abstract

ICP22, an immediate-early protein of herpes simplex virus type 1 (HSV-1), is required for viral replication in nonpermissive cell types and for expression of a class of late viral proteins which includes glycoprotein C. An understanding of the mechanism of ICP22 function has been complicated by the coexpression of the full-length protein with an in-frame, C-terminus-specific protein, U _S 1.5. In this report, we confirm that the U _S 1.5 protein is a bona fide translation product since it is detected during infections with three laboratory strains and two low-passage clinical isolates of HSV-1. To clarify the expression patterns of the ICP22 and U _S 1.5 proteins, we examined their synthesis from plasmids in transient expression assays. Because previous studies had identified two different U _S 1.5 translational start sites, we attempted to determine which is correct by studying the effects of a series of deletion, nonsense, and methionine substitutions on U _S 1.5 expression. First, amino acids 90 to 420 encoded by the ICP22 open reading frame (ORF) migrated at the mobility of U _S 1.5 in sodium dodecyl sulfate-polyacrylamide gels. Second, introduction of a stop codon downstream of M90 ablated expression of both ICP22 and U _S 1.5. Finally, mutation of M90 to alanine (M90A) allowed expression of full-length ICP22 while dramatically reducing expression of U _S 1.5. Levels of U _S 1.5 but not ICP22 protein expression were also reduced in cells infected with an M90A mutant virus. Thus, we conclude that expression of IC22 and that of U _S 1.5 can occur independently of each other and that U _S 1.5 translation initiates at M90 of the ICP22 ORF.