The unique N terminus of herpes simplex virus type 1 ribonucleotide reductase large subunit is phosphorylated by casein kinase 2, which may have a homologue in Escherichia coli

Studies were performed to determine if the unique N-terminal domain of the pi subunit from herpes simplex virus (HSV) type 1 ribonucleotide reductase is a substrate for casein kinase 2 (CK2). Transphosphorylation assays demonstrated that R1 was highly phosphorylated by this enzyme with multiple phosphorylation sites mapped to the N terminus between residues 1 and 245. Immunoprecipitation pull-down assays using R1-specific antisera failed to demonstrate a stable interaction between pi and CK2 but residual amounts of CK2 present after immunoprecipitation efficiently transphosphorylated R1. Activity assays with a peptide substrate identified CK2 in R1 immunoprecipitated from infected-cell extracts but did not detect activity in R1 proteins immunoprecipitated from bacterial extracts. However, Western blotting identified potential E. coli homologues of the CK2 alpha and beta subunits. These results support conclusions that the N-terminal domain of HSV R1 is not a protein kinase and that all previous results can be explained by contaminating kinases, principally CK2.