The novel protein kinase of the RR1 subunit of herpes simplex virus has autophosphorylation and transphosphorylation activity that differs in its ATP requirements for HSV-1 and HSV-2.

The large subunit of herpes simplex virus (HSV) ribonucleotide reductase (RR1) designated ICP6 and ICP10 for HSV-1 and HSV-2, respectively, has a novel protein kinase (PK) enzymatic activity. ICP10 is localized on the cell surface, a localization that depends on an intact transmembrane (TM) segment. We used immunocomplex PK assays to examine the PK activity of ICP10 in stably transfected eukaryotic cells. Activity was distinct from that of casein kinase II (CKII) in that it did not require monovalent ions and was not inhibited by zinc sulfate. PK activity was eliminated by deletion of the conserved PK catalytic motifs or of the TM segment and it was significantly impaired by mutation of the invariant Lys (Lys176). Loss of PK activity by Lys176 mutation resulted in the failure to bind ATP. A truncated ICP10 PK expressed in bacteria (pp29 1a1) retained auto- and transphosphorylating activity (for calmodulin) after purification to apparent homogeneity. PK activity was also absent in cells infected with a recombinant virus (ICP10 delta PK) deleted in the ICP10 PK catalytic motifs. In cells infected with HSV-1 or HSV-2, RR1 had auto- and transphosphorylating activity for the small subunit of HSV ribonucleotide reductase (RR2) and immunoglobulin G (IgG). Comparing the PK activity of ICP6 and ICP10 we found that ICP6 requires five-fold higher concentrations of [gamma-32P]ATP than ICP10 and both enzymes are Mn2+ dependent, which is also different from CKII that is primarily Mg2+-dependent. Similar results were obtained for various HSV strains and in different cell lines. The data are consistent with the conclusion that the RR1 PK activity is intrinsic.