TI - Results p300 and CBP protect VRK1 from its p53-induced downregulation . AB - The downregulation of VRK1 by p53 has been shown to be dependent on p53 -induced transcription of an unknown protein that controls VRK1 proteolytic degradation [12] . Therefore it was decided to identify the contribution of p53 transcriptional cofactors to this process . p300 and CBP are p53 coactivators that participate in its transcriptional activation by acetylation of Lys373 and Lys382 residues [14] , [19] . It has been previously reported that VRK1 was able to increase p53 acetylation after its specific PHOSphorylation on Thr-18 [5] . First , it was tested if p300 could have any effect on the transcriptionally dependent downregulation of VRK1 induced by p53 . For this aim H1299 cells were cotransfected with plasmids pCEFL-HA-VRK1 , pCB6+p53 and increasing amounts of pCMV-p300 (Fig 1A) . Surprisingly , as the p300 protein level increased , it was accompanied by a parallel increase in the protection of VRK1 degradation induced by p53 (Fig 1A) . To confirm that the effect requires the participation of p53 , the same experiment was performed in the absence of p53 (Fig 1B) . In this situation p300 over-expression by itself has no effect on VRK1 , or perhaps even induces a minor increase in its levels . These results suggested an implication of the p300 coactivator in preventing VRK1 downregulation by p53 . An unknown connection between these proteins may occur or it might be possible that the gene controlled by p53 that regulates VRK1 does not require p300 as coactivator and that unique over-expression of p300 is sufficient to direct the transcriptional activity of p53 to other targets and to abrogate the negative effect of p53 on VRK1 levels . The expression of actin was not affected by either p53 or any of the cofactors . P300 and CBP are two related proteins with an 80 per cent homology suggesting many of their effects are probably similar . Therefore it is likely that CBP could also protect VRK1 from downregulation induced by p53 ; or alternatively detection of a differential response would contribute to determine the specificity of the effect . To distinguish between these two possibilities , a similar set of experiments was performed . Increasing amounts of CBP were able to protect VRK1 from downregulation induced by p53 (Fig 1C) , and CBP by itself in the absence of p53 had no effect on VRK1 levels (Fig 1D) . As a negative control the lack of effect on another protein that it is not susceptible to this downregulation mechanism was determined . Cells were transfected with a plasmid expressing human TSG101 [20] . The levels of this transfected protein , as well as that of the endogenous actin , are not downregulated by p53 (Fig 1E) .