HIGH-MOBILITY GROUP PROTEIN-1 (HMG-1) IS A UNIQUE ACTIVATOR OF P53

The binding of p53 protein to DNA is stimulated by its interaction wit h covalent as well as noncovalent modifiers. We report the identificat ion of a factor from HeLa nuclear extracts that activates p53 DNA bind ing. This factor was purified to homogeneity and identified as the hig h mobility group protein, HMG-1. HMG-1 belongs to a family of highly c onserved chromatin-associated nucleoproteins that bend DNA and facilit ate the binding of various transcription factors to their cognate DNA sequences. We demonstrate that recombinant His-tagged HMG-1 enhances p 53 DNA binding in vitro and also that HMG-1 and p53 can interact direc tly in vitro. Unexpectedly, HMG-1 also stimulates DNA binding by p53 D elta 30, a carboxy terminally deleted form of the protein that is cons idered to be constitutively active, suggesting that HMG-1 stimulates p 53 by a mechanism that is distinct from other known activators of p53. Finally, using transient transfection assays we show that HMG-1 can i ncrease p53 and p53 Delta 30-mediated transactivation in vivo. HMG-1 p romotes the assembly of higher order p53 nucleoprotein structures, and these data, along with the fact that HMG-1 is capable of bending DNA, suggest that HMG-1 may activate p53 DNA binding by a novel mechanism involving a structural change in the target DNA.