GSK3β phosphorvlates serine 33 of p53. wtp53-GST (W), p53-GSTS33A (33) or p53-GSTS37A (37) were preincubated in buffer (-) or DNA-PK (+) for 5 h in the presence of excess ATP. Following heat inactivation of the DNA-PK, aliquots of the phosphorylated p53-GST fusion proteins were incubated for 30 min without (-) or with (+) GSK-3β. Position of p53-GST and GSK3β is indicated. The sequence of p53 around serines 15 and 37 is shown below the figure.