Stimulation of p53-mediated Transcriptional Activation by the p53-binding Proteins, 53BP1 and 53BP2

Abstract

p53 is a tumor suppressor protein that controls cell proliferation by regulating the expression of growth control genes. In a previous study, we identified two proteins, 53BP1 and 53BP2, that are able to bind to wild type but not to mutant p53 via the DNA-binding domain of p53. We isolated cDNAs expressing a full-length human 53BP1 clone, which predicts a protein of 1972 residues that can be detected in the H358 human lung carcinoma cell line. The 53BP1 and 53BP2 genes were mapped to chromosomes 15q15-21 and 1q41-42, respectively. Immunofluorescence studies showed three types of staining patterns for 53BP1 as follows: both cytoplasmic and nuclear, homogeneous nuclear, and a nuclear dot pattern. In contrast, 53BP2 localized exclusively to the cytoplasm, and this pattern did not change upon coexpression of wild type p53. Although our previous study revealed that p53 is not able to bind simultaneously to either 53BP1 or 53BP2 and to DNA carrying a consensus binding site, both 53BP1 and 53BP2 enhanced p53-mediated transcriptional activation and induced the expression of a p53-dependent protein, suggesting that these proteins might function in signal transduction pathways to promote p53 activity.The p53 protein is the product of a tumor-suppressor gene (1, 2), with mutations in this protein being the most common genetic change in human cancer (3, 4). The observations that introduction of the wild type (wt) 1 p53 gene into cells leads to growth arrest (5-8) or apoptosis (9, 10) and that DNA damage leads to increases in the level of p53 (11,12) suggest that the protein acts at a checkpoint to regulate cell cycle arrest in the G 1 (13), G 2 /M (14), and G 0 phases (15). The cell cycle arrest in G 1 phase is mediated, at least in part, by the trans-activation function of p53 (16,17), which can induce the expression of p21 (WAF1/CIP1) (18 -21), a cyclin-dependent kinase inhibitor. However, the signal from p53 to the Gas1 gene product, which can result in a G 0 arrest, does not require the trans-activation function of p53 (15). Furthermore, in some cell types, a mutant p53 that lacks this function can induce apoptosis (22).We have used the yeast two-hybrid system to identify two cellular proteins that bind to wt but not to mutant p53, designated p53-binding protein 1 and 2 (53BP1 and 53BP2) (23). Both 53BP1 and 53BP2 bind to the central domain of p53 which is required for site-specific DNA binding. Although neither 53BP1 and 53BP2 has extensive homology to other known proteins, recent sequence analysis revealed that the C terminus of 53BP1, which is sufficient for binding to p53, has homology both to the C terminus of BRCA1, a tumor suppressor specific for breast and ovarian cancer, and to Rad9, a yeast cell cycle checkpoint protein (24). This BRCT (BRCA1 C terminus) domain is found in other proteins involved in a checkpoint that responds to DNA damage (25,26), suggesting that it may mediate protein-protein interactions involved in this process.53BP2 has four ankyrin repeats and a single Src homology-3 domain in its C term...

NF-κB subunit p65 binds to 53BP2 and inhibits cell death induced by 53BP2

“…One of the possibilities is that 53BP2 may induce apoptosis through its interaction with p53 or Bcl-2 considering the facts that Bcl-2 prevented p53 from binding to 53BP2 (Naumovski and Cleary, 1996) and inhibited the p53-induced apoptosis Strasser et al, 1994). In supporting our observation that 53BP2 induced cell death, it was recently reported that overexpression of 53BP2 could stimulate the p53-mediated transcriptional activation and could enhance the trans-activation of p21 gene (Iwabuchi et al, 1998). It is possible, yet not proven, that the 53BP2-induced cell death may through p53-dependent pathway.…”

ASPP1, a common activator of TP53, is inactivated by aberrant methylation of its promoter in acute lymphoblastic leukemia

“…These studies identified two proteins 53BP2 and Bbp that are now recognized as truncated forms of ASPP2 (C-terminal 529 amino acids and the truncation of 123 amino acid at the N-terminal, respectively). The fact that the initial studies were performed with the truncated forms further added some confusion to their physiological role as the results were sometimes contradictory (Gorina and Pavletich, 1996;Iwabuchi et al, 1998;Lopez et al, 2000;Mori et al, 2000;Ao et al, 2001). However, a more detailed and clear description of their structure and function has only recently been performed along with isolation of the third member of the family, an inhibitor of p53 function iASPP (Samuels- Lev et al, 2001;Bergamaschi et al, 2003).…”

The RNF8/RNF168 ubiquitin ligase cascade facilitates class switch recombination

“…53BP1 interacts with p53 through its C-terminal tandem-BRCT motif in a phosphorylation-independent manner. This interaction is thought to stabilize p53 and stimulate p53-mediated transcriptional activation (40). It is possible that p53, on release from MDM2-mediated inhibition, is degraded in 53BP1-deficient cells, and thus apoptosis is not induced, or that 53BP1 functions as a cofactor of p53 to induce apoptosis.…”