The Role of the Herpes Simplex Virus Type 1 UL25 Protein in DNA Packaging and Virion Assembly

Herpes simplex virus type 1 replicates its DNA and builds progeny nucleocapsids in the nucleus of the infected cell. Replicated viral DNA is a concatemer that is cleaved to unit-length genomes and packaged into capsid precursors, and successful DNA packaging is required for the mature capsid to exit the nucleus and become incorporated into virions. The DNA cleavage and packaging machinery is highly conserved across herpesvirus subfamilies; they represent novel targets for anti-herpesviral treatments. Seven genes have been identified that are essential for the packaging reaction. Of these, the UL25 gene product is unique because it is required for the completion rather than the initiation of DNA packaging. In the absence of functional UL25, the nuclei of infected cells accumulate empty capsids and free, close to genome-length viral DNA. This phenotype differs from null mutations of the other packaging genes, which block cleavage of the concatemer and expulsion of the capsid scaffold protein. While the functions of several members of the packaging machinery have been deduced by analogy with the dsDNA bacteriophages, the role of UL25 protein (pUL25) in this process is unclear. pUL25 is stably associated with the capsid vertices, where it forms a heterodimer with the packaging protein UL17. The goal of this project is to elucidate the pUL25 capsid-binding mechanism and its significance for DNA packaging and virion assembly. Mutational analysis of UL25 mapped the capsid-binding domain to the pUL25 N-terminus. pUL25 interactions with the capsid surface proteins were visualized with cryo-electron microscopy and 3D image reconstructions of capsids containing pUL25 fusion proteins. Finally, we demonstrated that UL25 mutants aberrantly cleave viral DNA before the second packaging signal. These data support a model in which pUL25 provides structural support to capsid vertices without interacting directly with DNA during the packaging reaction