HSV gene functions: what have we learned that could be generally applicable to its near and distant cousins?

Herpes simplex virus 1 (HSV-1) encodes at least 84 polypeptides to perform two functions: to enable viral replication and to create the environment in which the entry of the virus into host cells, synthesis of virion components, assembly and egress are optimized. Whereas the former are indispensable for viral replication, the latter, numbering 47, can be deleted without a major effect on viral replication in cells in culture. Of particular interest are gene products whose function is either to modify cellular proteins (set 1) or to block entirely their function (set 2). An example of set 1 is the infected cell protein No. 0 (ICP0), a promiscuous transactivator of genes introduced into cells by infection or transfection. In its nuclear phase this protein binds to cyclin D3, extends its life by many hours, and sequesters it in nuclear structures known as ND10. In its cytoplasmic phase, ICP0 binds the translation elongation factor EF-1 delta. Another viral protein, the UL13 protein kinase, hyper-phosphorylates EF-1 delta. ICP0 and the protein kinase stimulate protein synthesis and cause the cell to induce the synthesis of pre-S phase cellular proteins the virus needs for its replication. The gamma 134.5 protein, a prototype of set 2, also has multiple functions. One, mapped at its carboxyl terminus, blocks the effects of double-stranded RNA-dependent protein kinase R (PKR) that is activated by all wild-type and mutant viruses examined to date. PKR phosphorylates eIF-2 alpha and shuts off protein synthesis. gamma 134.5 protein binds protein phosphatase 1 and redirects it to dephosphorylate eIF-2 alpha. Although PKR is activated in wild-type-infected cells, protein synthesis is unaffected. HSV-1 encodes in addition at least two proteins, ORF O and ORF P that are repressed during productive infection. The ORF P protein localizes in spliceosomes and blocks the synthesis of viral proteins derived from spliced mRNA. The ORF O protein binds ICP4, the major regulatory protein, and prevents it from binding to DNA. The role of ORF O and ORF P proteins in the establishment of latency is uncertain. A significant discovery that has emerged from these studies is that viral proteins can perform several functions that may be totally unrelated.