UL54

UL54 is a gene designation used in the herpesvirus genome, most notably in herpes simplex virus type 1 (HSV-1). In HSV-1, the UL54 open reading frame encodes ICP27, a multifunctional regulatory protein that is essential for efficient viral replication. The protein primarily functions at the level of RNA and gene expression, coordinating the expression of viral transcripts throughout infection.

ICP27 localizes to both the nucleus and cytoplasm and influences several steps in RNA metabolism. It promotes the processing and export of viral mRNAs by interacting with cellular mRNA export machinery, and it can modulate host pre-mRNA splicing in ways that favor viral over host gene expression. ICP27 also affects the transition from immediate-early to late viral gene expression, contributing to the timely production of structural components needed for virion assembly. Its activity is regulated by phosphorylation by viral and cellular kinases, and it exerts its effects through RNA-binding and protein–protein interactions rather than enzymatic catalysis.

Disruption or mutation of UL54/ICP27 markedly impairs viral replication, underscoring its central role in the viral life cycle. Because of its critical function, ICP27 has been a focus of antiviral research and studies into host–virus interactions. Homologs of UL54 exist in related herpesviruses, where they often retain regulatory roles in gene expression, though sequence and functional details can vary between viruses.