Characterization of an essential HSV-1 protein encoded by the UL25 gene reported to be involved in virus penetration and capsid assembly.

The 2.3-kb BamHI-U DNA fragment (map units 0.319-0.335) of herpes simplex virus type 1 (HSV-1) genome contains the complete UL25 open reading frame (ORF). It specifies an essential viral protein reported previously to be involved in virus penetration and capsid assembly (C. Addison, F. J. Rixon, J. W. Palfreyman, M. O'Hara, and V. G. Preston, Virology 138, 246-259, 1984). To identify the protein encoded by the UL25 gene, the UL25 ORF was cloned in a eukaryotic expression vector (p91023) downstream of the adenovirus major late promoter to generate the expression plasmid p9-UL25. Synthesis of a 60-kDa protein was observed in COS-7 cells transfected with p9-UL25 plasmid DNA, but not in cells transfected with p91023 control plasmid DNA. To identify and characterize the UL25 protein from HSV-1-infected cells, we prepared a rabbit antiserum by using UL25-GST fusion protein expressed in Escherichia coli as immunogen. This rabbit antiserum readily immunoprecipitated the 60-kDa UL25 protein from HSV-1-infected cells. In HSV-1-infected cells, UL25 protein was expressed as a late (gamma) or a leaky late (gamma 1) viral protein. The rabbit antiserum raised against HSV-1 UL25 protein immunoprecipitated a UL25-homologue of identical size from HSV-2-infected cells. However, the reactivity of the antiserum with HSV-2 UL25-homologue was weaker than compared to the corresponding HSV-1 protein. Consistent with its classification as a virion component, the UL25 protein was found to be associated with purified HSV-1 virions.