The multifunctional herpes simplex virus IE63 protein interacts with heterogeneous ribonucleoprotein K and with casein kinase 2

Herpes simplex virus type 1 (HSV-1), the prototype alpha-herpesvirus, cause s several prominent diseases. The HSV-1 immediate early (IE) protein IE63 ( ICP27) is the only regulatory gene with a homologue in every mammalian and avian herpesvirus sequenced so far, IE63 is a multifunctional protein affec ting transcriptional and post-transcriptional processes, and it can shuttle from the nucleus to the cytoplasm, To identify interacting cellular protei ns, a HeLa cDNA library was screened in the yeast two-hybrid system using I E63 as bait. Several interacting proteins were identified including heterog eneous nuclear ribonucleoprotein K (hnRNP K), a multifunctional protein lik e IE63, and the beta subunit of casein kinase 2 (CK2), a protein kinase, an d interacting regions were mapped. Confirmation of interactions was provide d by fusion protein binding assays, co-immunoprecipitation from infected ce lls, and CK2 activity assays. hnRNP K co-immunoprecipitated from infected c ells with anti-IE63 serum was a more rapidly migrating subfraction than hnR NP K immunoprecipitated by anti-hnRNP K serum. Using anti-IE63 serum, both IE63 and hnRNP K were phosphorylated in vitro by CK2, while in immunoprecip itates using anti-hnRNP K serum, IE63 but not hnRNP K was phosphorylated by CK2. These data provide important new insights into how this key viral reg ulatory protein exerts its functions.