TI - p53 PHOSphorylation mutants also induce VRK1 downregulation . AB - PHOSphorylation in the N-terminal region of p53 can modify the interactions of p53 with other proteins and this can , in turn , modify the transcriptional activity of p53 in response to different types of cellular stimulation [21] , [22] . However , PHOSphorylation in this region of p53 appears to be dispensable for the activation of some gene transcription by p53 [23] , [24] , although it may affect the specificity of interactions with other proteins . Therefore the potential effect of different PHOSphorylation mutants of p53 , either in its amino ( DeltaN has mutated the following residues : S6A , S9A , S15A , T18A , S20A , S33A and S37A ) or carboxy terminus ( DeltaC has mutated the following residues : S315A , S371A , S376A , S378A , S392A ) [25] , were tested to determine if they have any effect on the p53 induction of VRK1 protein downregulation . Several p53 mutants affecting individual or all PHOSphorylatable residues were also tested . All the p53 phosphorylaTION mutants , including p53T18D that mimics PHOSphorylation by VRK1 and prevents interaction with hdm2 [12] , induced a VRK1 downregulation like the wild-type p53 ( Fig 3 ) . In this experiment , a p53 mutant that is transcriptionally inactive by mutation in the DNA binding domain , p53R280K , was included as a positive control for loss of effect , and as expected [12] it did not induce a VRK1 downregulation ( Fig 3 ) .