Interactions between a herpes simplex virus regulatory protein and cellular mRNA processing pathways

The herpes simplex virus type 1 (HSV-1) immediate-early regulatory protein ICP27 performs essential functions during viral lytic infection. Studies with viral mutants have demonstrated that ICP27 affects the shutoff of host protein synthesis, HSV-1 DNA replication, and the expression of viral early and late genes. Mounting evidence has been presented to demonstrate that ICP27 functions predominantly at the posttranscriptional level by affecting mRNA processing. That is, ICP27 alters poly(A) site usage, impairs host cell splicing, and facilitates the export of viral intronless mRNAs. These diverse effects occur by the interaction of ICP27 with viral and host proteins and by binding RNA. To define the precise mechanisms by which ICP27 affects RNA processing pathways, it is necessary to identify all of the molecular interactions of ICP27 in vivo and to determine the functional significance of these interactions. In vivo approaches will be emphasized here. Protein-protein interactions have been analyzed by coimmunoprecipitation studies, followed by immunoblotting to confirm the identity of coprecipitating proteins. Indirect immunofluorescence staining has been performed on cells treated with RNA polymerase II inhibitors to determine the intracellular distribution of ICP27 related to its RNA export function. Finally, in vivo UV irradiation has been used to covalently cross-link ICP27 to mRNAs in direct contact. This was followed with procedures to isolate and analyze the protein-RNA complexes. These studies have revealed several splicing complex proteins with which ICP27 interacts and have identified a number of intronless RNA transcripts to which ICP27 binds in the nucleus and cytoplasm in its role in RNA transport.