Biological significance of a small highly conserved region in the N terminus of the p53 tumour suppressor protein

The p53 tumour suppressor protein plays a central role in maintaining genom ic integrity in eukaryotic cells. The most significant biological function of p53 is to act as a sequence-specific DNA-binding transcription factor, w hich can induce the expression of a variety of target genes in response to diverse stress stimuli. The p53 protein contains six highly conserved regio ns, one of which, termed Box I is located in the N-terminal transactivation domain (amino acid residues 13 and 26). The second half of the Box I regio n is crucial for the interaction with the basal transcription machinery and is thus required for p53 s activity as a transcription factor. The same re gion also binds to Mdm2. Since p53 is targeted by Mdm2 for ubiquitin-mediat ed proteasome-dependent ' degradation, this region is also essential for th e regulation of p53 is stability in response to stress signals. Although th e first half of Box I is highly conserved, its biological function is not c learly defined. The aim of this study was to characterise this conserved re gion and investigate its role in the biological functions of p53. We have g enerated short deletions and point mutations within this region and analyse d their effect on p53 function and regulation. Biochemical analyses demonst rate that deletion of residues 13 to 16 significantly increases both the tr anscriptional transactivation and G(2) arrest-inducing activities of murine p53. Residues 13 to 16 appear to function as a regulatory element in p53, modulating p53-dependent transcriptional transactivation and cell-cycle arr est, possibly by affecting the structural stability of the core domain of t he protein. In support of this, the deletion was found to induce second-sit e reversion of the VaI135 temperature-sensitive mutant of murine p53. (C) 2 001 Academic Press.