Modulation of p53 activity by IkappaBalpha: evidence suggesting a common phylogeny between NF-kappaB and p53...

In this work we present evidence that the p53 tumor suppressor protein and NF-kappaB transcription factors could be related through common descent from a family of ancestral transcription factors regulating cellular proliferation and apoptosis. P53 is a homotetrameric transcription factor known to interact with the ankyrin protein 53BP2 (a fragment of the ASPP2 protein). NF-kappaB is also regulated by ankyrin proteins, the prototype of which is the IkappaB family. The DNA binding sequences of the two transcription factors are similar, sharing 8 out of 10 nucleotides. Interactions between the two proteins, both direct and indirect, have been noted previously and the two proteins play central roles in the control of proliferation and apoptosis.

Using previously published structure data, we noted a significant degree of structural alignment between p53 and NF-kappaB p65. We also determined that IkappaBalpha and p53 bind in vitro through a specific interaction in part involving the DNA binding region of p53, or a region proximal to it, and the amino terminus of IkappaBalpha independently or cooperatively with the ankyrin 3 domain of IkappaBalpha In cotransfection experiments, kappaBalpha could significantly inhibit the transcriptional activity of p53. Inhibition of p53-mediated transcription was increased by deletion of the ankyrin 2, 4, or 5 domains of IkappaBalpha Co-precipitation experiments using the stably transfected ankyrin 5 deletion mutant of kappaBalpha and endogenous wild-type p53 further support the hypothesis that p53 and IkappaBalpha can physically interact in vivo.

The aggregate results obtained using bacterially produced IkappaBalpha and p53 as well as reticulocyte lysate produced proteins suggest a correlation between in vitro co-precipitation in at least one of the systems and in vivo p53 inhibitory activity. These observations argue for a mechanism involving direct binding of IkappaBalpha to p53 in the inhibition of p53 transcriptional activity, analogous to the inhibition of NF-kappaB by kappaBalpha and p53 by 53BP2/ASPP2. These data furthermore suggest a role for ankyrin proteins in the regulation of p53 activity. Taken together, the NFkappaB and p53 proteins share similarities in structure, DNA binding sites and binding and regulation by ankyrin proteins in support of our hypothesis that the two proteins share common descent from an ancestral transcriptional factor.