Kinetic Properties of p53 Phosphorylation by the Human Vaccinia-Related Kinase 1

The vaccinia-related kinase 1 (VRK1) protein is a nuclear Ser-Thr kinase that phosphorylates p53 in Thr18. We have determined the enzyme properties regarding its different substrates. VRK1 has a high affinity for ATP (Km 50 μM) and is thus saturated by the intracellular concentration of ATP in vivo. VRK1 uses preferentially magnesium, but is also functional with manganese and zinc. The VRK1 protein is autophosphorylated in multiple residues without effect on its activity. One autophosphorylated residue, T355, is within the VRK1 regulatory carboxy terminus. The kinase phosphorylates p53 with a Km of 1 μM and is well suited to respond to the variations of intracellular p53 concentration, which fluctuates as a response to different types of cellular stress.