The single-stranded DNA end binding site of p53 coincides with the C-terminal regulatory region

Defining TP53 pioneering capabilities with competitive nucleosome binding assays

Accurate gene expression requires the targeting of transcription factors (TFs) to regulatory sequences often occluded within nucleosomes. The ability to target a TF binding site (TFBS) within a nucleosome has been the defining characteristic for a special class of TFs known as pioneer factors. Recent studies suggest TP53 functions as a pioneer factor that can target its TFBS within nucleosomes, but it remains unclear how TP53 binds to nucleosomal DNA. To comprehensively examine TP53 nucleosome binding, we competitively bound TP53 to multiple in vitro–formed nucleosomes containing a high- or low-affinity TP53 TFBS located at differing translational and rotational positions within the nucleosome. Stable TP53–nucleosome complexes were isolated and quantified using next-generation sequencing. Our results demonstrate TP53 binding is limited to nucleosome edges with significant binding inhibition occurring within 50 bp of the nucleosome dyad. Binding site affinity only affects TP53 binding for TFBSs located at the same nucleosomal positions; otherwise, nucleosome position takes precedence. Furthermore, TP53 has strong nonspecific nucleosome binding facilitating its interaction with chromatin. Our in vitro findings were confirmed by examining TP53-induced binding in a cell line model, showing induced binding at nucleosome edges flanked by a nucleosome-free region. Overall, our results suggest that the pioneering capabilities of TP53 are driven by nonspecific nucleosome binding with specific binding at nucleosome edges.

Wild-type p53 binds to MYC promoter G-quadruplex

G-quadruplexes are four-stranded nucleic acid structures that are implicated in the regulation of transcription, translation and replication. Genome regions enriched in putative G-quadruplex motifs include telomeres and gene promoters. Tumour suppressor p53 plays a critical role in regulatory pathways leading to cell cycle arrest, DNA repair and apoptosis. In addition to transcriptional regulation mediated via sequence-specific DNA binding, p53 can selectively bind various non-B DNA structures. In the present study, wild-type p53 (wtp53) binding to G-quadruplex formed by MYC promoter nuclease hypersensitive element (NHE) III₁ region was investigated. Wtp53 binding to MYC G-quadruplex is comparable to interaction with specific p53 consensus sequence (p53CON). Apart from the full-length wtp53, its isolated C-terminal region (aa 320-393) as well, is capable of high-affinity MYC G-quadruplex binding, suggesting its critical role in this type of interaction. Moreover, wtp53 binds to MYC promoter region containing putative G-quadruplex motif in two wtp53-expressing cell lines. The results suggest that wtp53 binding to G-quadruplexes can take part in transcriptional regulation of its target genes.

DNA Structure and Polymerase Fidelity: A New Role for A-DNA

Abstract Although the recent structural studies on polymerases have brought new insights on polymerase fidelity, the role of DNA sequence and structure is not well understood. Here, the analysis of the crystal structures of hotspots for polymerase slippage shows that, in the B- form, these sequences share common structural alterations which may explain the high rate of replication errors. In (CA)(n) tracts, a "Janus-like" structure with shifted base pairs in the major groove but an apparent normal geometry in the minor groove constitutes a molecular decoy which can mislead the polymerases. A model of the rat polymerase β bound to this structure suggests that an altered conformation of the nascent template-primer duplex can interfere with correct nucleotide incorporation by affecting the geometry of the active site and breaking the rules of base pairing while at the same time escaping enzymatic mechanisms of error discrimination scanning for the correct geometry of the minor groove. In contrast, by showing that the A-form greatly attenuates the sequence-dependent structural alterations in hotspots, this study reveals that the A-conformation of the nascent template-primer duplex at the vicinity of the polymerase active site will contribute to fidelity. The A-form may play the role of a structural buffer which preserves the correct geometry of the active site for all sequences. The comparison of the conformation of the nascent template-primer duplex in five available crystal structures of DNA polymerase-DNA complexes shows indeed that polymerase β the least accurate enzyme, is unique in binding to a B-DNA duplex even close to its active site. This model leads to several predictions which are discussed in the light of published experimental data.

Sequence-specific and DNA structure-dependent interactions of Escherichia coli MutS and human p53 with DNA

Many proteins involved in DNA repair systems interact with DNA that has structure altered from the typical B-form helix. Using magnetic beads to immobilize DNAs containing various types of structures, we evaluated the in vitro binding activities of two well-characterized DNA repair proteins, Escherichia coli MutS and human p53. E. coli MutS bound to double-stranded DNAs, with higher affinity for a G/T mismatch compared to a G/A mismatch and highest affinity for larger non-B-DNA structures. E. coli MutS bound best to DNA between pH 6 and 9. Experiments discriminated between modes of p53-DNA binding, and increasing ionic strength reduced p53 binding to nonspecific double-stranded DNA, but had minor effects on binding to consensus response sequences or single-stranded DNA. Compared to nonspecific DNA sequences, p53 bound with a higher affinity to mismatches and base insertions, while binding to various hairpin structures was similar to that observed to its consensus DNA sequence. For hairpins containing CTG repeats, the extent of p53 binding was proportional to the size of the repeat. In summary, using the flexibility of the magnetic bead separation assay we demonstrate that pH and ionic strength influence the binding of two DNA repair proteins to a variety of DNA structures.

p53 suppresses BRCA2-stimulated ATPase and strand exchange functions of human RAD51

Although homologous recombination (HR) is an important pathway for DNA repair, it can also be a cause for deleterious genomic rearrangements leading to carcinogenesis. Therefore, cells have evolved elaborate mechanisms to regulate HR, positively as well as negatively. Among many molecular components that regulate HR are tumour suppressors p53, a negative regulator and breast cancer early-onset (BRCA)2, a positive regulator. Both the players not only interact with each other but also directly interact with human RAD51 (hRAD51), the key recombinase in HR. Here, for the first time we studied HR regulation by the combined action of p53 and BRCA2, in vitro. While BRC4 peptide inhibits ATP hydrolysis by hRAD51, BRCA2(BRC1-8) stimulates DNA-independent and double-stranded DNA-dependent ATPase several fold and only marginally single-stranded DNA-dependent ATPase. Pull down assays demonstrated the occurrence of complex comprising of all three proteins and DNA, where p53 tends to compete out hRAD51 and BRCA2(BRC1-8), leading to not only the decline in ATP hydrolysis but also the strand exchange function of hRAD51 that was stimulated by BRCA2(BRC1-8). Our findings suggest a rigorous p53-mediated regulation on hRAD51 functions in HR even in the presence of BRCA2.

Reciprocally regulated chromatin association of Cockayne syndrome protein B and p53 protein

The Cockayne syndrome complementation group B (CSB) protein is an ATP-dependent chromatin remodeler with an essential function in transcription-coupled DNA repair, and mutations in the CSB gene are associated with Cockayne syndrome. The p53 tumor suppressor has been known to interact with CSB, and both proteins have been implicated in overlapping biological processes, such as DNA repair and aging. The significance of the interaction between CSB and p53 has remained unclear, however. Here, we show that the chromatin association of CSB and p53 is inversely related. Using in vitro binding and chromatin immunoprecipitation approaches, we demonstrate that CSB facilitates the sequence-independent association of p53 with chromatin when p53 concentrations are low and that this is achieved by the interaction of CSB with the C-terminal region of p53. Remarkably, p53 prevents CSB from binding to nucleosomes when p53 concentrations are elevated. Examining the enzymatic properties of CSB revealed that p53 excludes CSB from nucleosomes by occluding a nucleosome interaction surface on CSB. Together, our results suggest that the reciprocal regulation of chromatin access by CSB and p53 could be part of a mechanism by which these two proteins coordinate their activities to regulate DNA repair, cell survival, and aging.

Tracing the protectors path from the germ line to the genome

One of the basic principles that nature uses in evolution is to recycle successful concepts and create new functions by modifying existing units. This conservatism in evolution has resulted in an astonishingly high sequence identity of genes, even between evolutionarily distant species such as the nematode Caenorhabditis elegans and Homo sapiens. The recycling of successful concepts in conjunction with gene duplication events has also led to the existence of highly homologous proteins within the genome of many species. Often, these homologous proteins show similar, yet distinct functions that, in combination with their individual tissue distribution, define their specific physiological role. One prominent example is the p53 protein family, which consists of p53, p63, and p73. Recent advances in understanding the specific biological functions of these members have shed some light onto the evolution of this crucial protein family, from a germ line-specific quality-control factor to a somatic tumor suppressor. Furthermore, structures of the oligomerization domains of the mammalian paralogs, p53 and p73, and invertebrate orthologs, CEP-1 and DMP53, have delineated evolutionary changes and revealed that the oligomerization domain of p53 lacks additional stabilizing structural elements present in all other p53 family members. This suggests that p53 is the most recent evolutionary member of this protein family and predicts a mechanism for p53 activation.

Restoration of wild-type p53 function in human tumors: strategies for efficient cancer therapy

The p53 tumor suppressor gene is mutated in around 50% of all human tumors. Most mutations inactivate p53's specific DNA binding, resulting in failure to activate transcription of p53 target genes. As a consequence, mutant p53 is unable to trigger a p53-dependent biological response, that is cell cycle arrest and apoptosis. Many tumors express high levels of nonfunctional mutant p53. Several strategies for restoration of wild-type p53 function in tumors have been designed. Wild-type p53 reconstitution by adenovirus-mediated gene transfer has shown antitumor efficacy in clinical trials. Screening of chemical libraries has allowed identification of small molecules that reactivate mutant p53 and trigger mutant p53-dependent apoptosis. These novel strategies raise hopes for more efficient cancer therapy.

Tumor-specific induction of apoptosis by a p53-reactivating compound

The tumor suppressor function of p53 is disabled in the majority of tumors, either by a point mutation of the p53 gene, or via MDM2-dependent proteasomal degradation. We have screened a chemical library using a cell-based assay and identified a low molecular weight compound named MITA which induced wild-type p53-dependent cell death in a variety of different types of human tumor cells, such as lung, colon and breast carcinoma cells, as well as in osteosarcoma and fibrosarcoma-derived cells. MITA inhibited p53-MDM2 interaction in vitro and in cells, which in turn prevented MDM2-mediated ubiquitination of p53 and resulted in a prolonged half-life and accumulation of p53 in tumor cells. Notably, p53 induction by MITA resulted in upregulated expression of p53 target genes MDM2, Bax, Gadd45 and PUMA, on protein and mRNA level. Importantly, neither p53 nor these target genes were induced in normal human fibroblasts (HDFs), which correlated with the absence of growth suppression in fibroblasts after treatment with MITA. However, upon activation of oncogenes in fibroblasts an induction and activation of p53 was observed, suggesting that activation of p53 by MITA occurs predominantly in tumor cells.

Characterization of the cell death induced by cadmium in HaCaT and C6 cell lines

Cell death resulting from cadmium (Cd) intoxication has been confirmed to induce both necrosis and apoptosis. The ratio between both types of cell death is dose- and cell-type-dependent. This study used the human keratinocytes HaCaT expressing a mutated p53 and the rat glial cells C6 expressing a wild p53 as models to characterize Cd-induced apoptosis, using sub-lethal and lethal doses. At these concentrations, features of apoptosis were observed 24 h after C6 cell treatment: apoptotic DNA fragmentation and caspase-9 activation, whereas Cd did not induce caspase-3. In HaCaT, Cd did not induce apoptotic DNA fragmentation or caspase-9 and -3 activation. The results also showed that the inhibition of p53 led to a resistance of the C6 cells to 20 microm Cd, decreased the apoptosis and increased the metallothioneins in these cells. p53 restoration increased the sensitivity of HaCaT cells to Cd but did not affect the MT expression. The results suggest that Cd induced apoptosis in C6 cells but a non-apoptotic cellular death in HaCaT cells.

DNA modification with cisplatin affects sequence-specific DNA binding of p53 and p73 proteins in a target site-dependent manner

Proteins p53 and p73 act as transcription factors in cell cycle control, regulation of cell development and/or in apoptotic pathways. Both proteins bind to response elements (p53 DNA-binding sites), typically consisting of two copies of a motif RRRCWWGYYY. It has been demonstrated previously that DNA modification with the antitumor drug cisplatin inhibits p53 binding to a synthetic p53 DNA-binding site. Here we demonstrate that the effects of global DNA modification with cisplatin on binding of the p53 or p73 proteins to various p53 DNA-binding sites differed significantly, depending on the nucleotide sequence of the given target site. The relative sensitivities of protein-DNA binding to cisplatin DNA treatment correlated with the occurrence of sequence motifs forming stable bifunctional adducts with the drug (namely, GG and AG doublets) within the target sites. Binding of both proteins to mutated p53 DNA-binding sites from which these motifs had been eliminated was only negligibly affected by cisplatin treatment, suggesting that formation of the cisplatin adducts within the target sites was primarily responsible for inhibition of the p53 or p73 sequence-specific DNA binding. Distinct effects of cisplatin DNA modification on the recognition of different response elements by the p53 family proteins may have impacts on regulation pathways in cisplatin-treated cells.

Reactivation of mutant p53 and induction of apoptosis in human tumor cells by maleimide analogs

Reactivation of mutant p53 is likely to provide important benefits for treatment of chemotherapy- and radiotherapy-resistant tumors. We demonstrate here that the maleimide-derived molecule MIRA-1 can reactivate DNA binding and preserve the active conformation of mutant p53 protein in vitro and restore transcriptional transactivation to mutant p53 in living cells. MIRA-1 induced mutant p53-dependent cell death in different human tumor cells carrying tetracycline-regulated mutant p53. The structural analog MIRA-3 showed antitumor activity in vivo against human mutant p53-carrying tumor xenografts in SCID mice. The MIRA scaffold is a novel lead for the development of anticancer drugs specifically targeting mutant p53.

Integrin alphav-mediated inactivation of p53 controls a MEK1-dependent melanoma cell survival pathway in three-dimensional collagen

Integrin αv is required for melanoma cell survival and tumor growth in various models. To elucidate integrin αv-mediated melanoma cell survival mechanisms, we used a three-dimensional (3D) collagen gel model mimicking the pathophysiological microenvironment of malignant melanoma in the dermis. We found that integrin αv inactivated p53 and that suppression of p53 activity by dominant negative p53 or p53-small interfering RNA obviated the need for integrin αv for melanoma cell survival in 3D-collagen and for tumor growth in vivo. This indicates that integrin αv-mediated inactivation of p53 functionally controls melanoma cell survival. Furthermore, we found that melanoma cell integrin αv was required for MAPK kinase (MEK) 1 and extracellular signal-regulated kinase (ERK)1/2 activity in 3D-collagen, whereas inhibition of MEK1 activity induced apoptosis. Surprisingly, MEK1 and ERK1/2 activities were restored in integrin αv-negative melanoma cells by suppression of p53, whereas concomitant block of MEK1 induced apoptosis. This suggests that integrin αv controls melanoma cell survival in 3D-collagen through a pathway involving p53 regulation of MEK1 signaling.

Sequestering of p53 into DNA-protein filaments revealed by electron microscopy

Using electron microscopy, we analyzed the interaction of bacterially expressed full-length p53, p53(1-393), and its C-terminal fragment, p53(320-393), with long (approximately 3000 bp) dsDNA in linear and supercoiled (|DeltaLk| approximately 4-6) forms containing or lacking the p53 recognition sequence (p53CON). The main structural feature of the complexes formed by either protein was a DNA-protein filament, in which two DNA duplexes are linked (synapsed) via bound protein tetramers. The efficiency of the synapse, reflected in its length and the fraction of molecules exhibiting DNA-protein filaments, was significantly modulated by the molecular form of the protein and the topological state of the DNA. With linear DNA, the synapse yield promoted by the C-terminus fragment was very low, but the full-length protein was effective in linking noncontiguous duplexes, leading to the formation of intramolecular loops constrained at their bases by short regions of synapsed DNA duplexes. When the linear DNA contained p53CON, regions of preferential sequence, i.e., encompassing p53CON and probably p53CON-like sequences, were predominantly synapsed, indicating a sequence specificity of the p53 core domain. With scDNA, the synapse yield was significantly higher compared to the linear counterparts and was weakly dependent on the sign of superhelicity and presence or absence of p53CON. However, the full-length protein was more effective in promoting DNA synapses compared to the C-terminal fragment. The overall structure of the DNA-protein filaments was apparently similar for either protein form, although the apparent width differed slightly (approximately 7-9 nm and approximately 10-12 nm for p53(320-393) and p53(1-393), respectively). No distortion of the DNA helices involved in the synapse was found. We conclude that the structural similarity of DNA-protein filaments observed for both proteins is attributable mainly to the C-terminus, and that the yield is dictated by the specific and possibly nonspecific interactions of the core domain in combination with DNA topology. Possible implications for the sequestering of p53 in DNA-protein filaments are discussed.

Small molecule RITA binds to p53, blocks p53-HDM-2 interaction and activates p53 function in tumors

In tumors that retain wild-type p53, its tumor-suppressor function is often impaired as a result of the deregulation of HDM-2, which binds to p53 and targets it for proteasomal degradation. We have screened a chemical library and identified a small molecule named RITA (reactivation of p53 and induction of tumor cell apoptosis), which bound to p53 and induced its accumulation in tumor cells. RITA prevented p53-HDM-2 interaction in vitro and in vivo and affected p53 interaction with several negative regulators. RITA induced expression of p53 target genes and massive apoptosis in various tumor cells lines expressing wild-type p53. RITA suppressed the growth of human fibroblasts and lymphoblasts only upon oncogene expression and showed substantial p53-dependent antitumor effect in vivo. RITA may serve as a lead compound for the development of an anticancer drug that targets tumors with wild-type p53.

Investigations of the supercoil-selective DNA binding of wild type p53 suggest a novel mechanism for controlling p53 function

The tumor suppressor protein, p53, selectively binds to supercoiled (sc) DNA lacking the specific p53 consensus binding sequence (p53CON). Using p53 deletion mutants, we have previously shown that the p53 C-terminal DNA-binding site (CTDBS) is critical for this binding. Here we studied supercoil-selective binding of bacterially expressed full-length p53 using modulation of activity of the p53 DNA-binding domains by oxidation of cysteine residues (to preclude binding within the p53 core domain) and/or by antibodies mapping to epitopes at the protein C-terminus (to block binding within the CTDBS). In the absence of antibody, reduced p53 preferentially bound scDNA lacking p53CON in the presence of 3 kb linear plasmid DNAs or 20 mer oligonucleotides, both containing and lacking the p53CON. Blocking the CTDBS with antibody caused reduced p53 to bind equally to sc and linear or relaxed circular DNA lacking p53CON, but with a high preference for the p53CON. The same immune complex of oxidized p53 failed to bind DNA, while oxidized p53 in the absence of antibody restored selective scDNA binding. Antibodies mapping outside the CTDBS did not prevent p53 supercoil-selective (SCS) binding. These data indicate that the CTDBS is primarily responsible for p53 SCS binding. In the absence of the SCS binding, p53 binds sc or linear (relaxed) DNA via the p53 core domain and exhibits strong sequence-specific binding. Our results support a hypothesis that alterations to DNA topology may be a component of the complex cellular regulatory mechanisms that control the switch between latent and active p53 following cellular stress.

Structured DNA promotes phosphorylation of p53 by DNA-dependent protein kinase at serine 9 and threonine 18

Phosphorylation at multiple sites within the N-terminus of p53 promotes its dissociation from hdm2/mdm2 and stimulates its transcriptional regulatory potential. The large phosphoinositide 3-kinase-like kinases ataxia telangiectasia mutated gene product and the ataxia telangectasia and RAD-3-related kinase promote phosphorylation of human p53 at Ser15 and Ser20, and are required for the activation of p53 following DNA damage. DNA-dependent protein kinase (DNA-PK) is another large phosphoinositide 3-kinase-like kinase with the potential to phosphorylate p53 at Ser15, and has been proposed to enhance phosphorylation of these sites in vivo. Moreover, recent studies support a role for DNA-PK in the regulation of p53-mediated apoptosis. We have shown previously that colocalization of p53 and DNA-PK to structured single-stranded DNA dramatically enhances the potential for p53 phosphorylation by DNA-PK. We report here the identification of p53 phosphorylation at two novel sites for DNA-PK, Thr18 and Ser9. Colocalization of p53 and DNA-PK on structured DNA was required for efficient phosphorylation of p53 at multiple sites, while specific recognition of Ser9 and Thr18 appeared to be dependent upon additional determinants of p53 beyond the N-terminal 65 amino acids. Our results suggest a role for DNA-PK in the modulation of p53 activity resultant from the convergence of p53 and DNA-PK on structured DNA.

High-affinity binding of tumor-suppressor protein p53 and HMGB1 to hemicatenated DNA loops

We have recently observed that chromatin architectural protein HMGB1 (previously reported to be involved in numerous biological processes such as DNA replication, recombination, repair, tumor growth, and metastasis) could bind with extremely high affinity (K(d) < 1 pM) to a novel DNA structure that forms a DNA loop maintained at its base by a hemicatenane (hcDNA). The loop of hcDNA contains a track of repetitive sequences derived from CA-microsatellites. Here, we report using a gel-retardation assay that tumor-suppressor protein p53 can also bind to hcDNA. p53 is a crucial molecule protecting cells from malignant transformation by regulating cell-cycle progression, apoptosis, and DNA repair by activation or repression of transcription of its target genes by binding to specific p53 DNA-binding sites and/or certain types of DNA lesions or alternative DNA structures. The affinity of p53 for hcDNA (containing sequences with no resemblance to the p53 DNA consensus sequence) is >40-fold higher (K(d) approximately 0.5 nM) than that for its natural specific binding sites within its target genes (Mdm2 promoter). Binding of p53 to hcDNA remains detectable in the presence of up to approximately 4 orders of magnitude of mass excess of competitor linear DNA, suggesting a high specificity of the interaction. p53 displays a higher affinity for hcDNA than for DNA minicircles (lacking functional p53-specific binding sequence) with a size similar to that of the loop within the hcDNA, indicating that the extreme affinity of p53 for hcDNA is likely due to the binding of the protein to the hemicatenane. Although binding of p53 to hcDNA occurs in the absence of the nonspecific DNA-binding extreme carboxy-terminal regulatory domain (30-C, residues 363-393), the isolated 30-C domain (but not the sequence-specific p53 "core domain", residues 94-312) can also bind hcDNA. Only the full-length p53 can form stable ternary complexes with hcDNA and HMGB1. The possible biological relevance of p53 and HMGB1 binding to hemicatenanes is discussed.

YY1 binding to a subset of p53 DNA-target sites regulates p53-dependent transcription

The tumor suppressor protein p53 regulates gene transcription through binding to specific DNA-target sites. We here demonstrate that a subset of these sites is targeted by another DNA-binding factor. Binding specificity, reactivity with specific antibodies, and experiments with purified protein identified the factor as the multifunctional transcription regulator YY1. The YY1 core binding sequence ACAT is present in the center of p53-half-binding sites in the p21 and GADD45 genes regulating growth arrest and DNA repair, respectively, but is absent in those of the Bax gene critical for apoptosis. In transfection experiments YY1 inhibits p53-activated transcription from the p53-binding site that contains the ACAT sequence. YY1 and p53 are colocalized around the nucleoli and in discrete nuclear domains in PC12 cells undergoing apoptosis. YY1 might attenuate p53-dependent transcription from a subset of p53-target genes and this may be relevant for directing cells either to growth arrest or apoptosis upon p53 activation.

Activation of the DNA-binding ability of latent p53 protein by protein kinase C is abolished by protein kinase CK2

p53 is one of the most important regulators of cell proliferation and differentiation and of programmed cell death, triggering growth arrest and/or apoptosis in response to different cellular stress signals. The sequence-specific DNA-binding function of p53 protein can be activated by several different stimuli that modulate the C-terminal domain of this protein. The predominant mechanism of activation of p53 sequence-specific DNA binding is phosphorylation at specific sites. For example, phosphorylation of p53 by PKC (protein kinase C) occurs in undamaged cells, resulting in masking of the epitope recognized by monoclonal antibody PAb421, and presumably promotes steady-state levels of p53 activity in cycling cells. In contrast, phosphorylation by cdk2 (cyclin-dependent kinase 2)/cyclin A and by the protein kinase CK2 are both enhanced in DNA-damaged cells. We determined whether one mechanism to account for this mutually exclusive phosphorylation may be that each phosphorylation event prevents modification by the other kinase. We used non-radioactive electrophoretic mobility shift assays to show that C-terminal phosphorylation of p53 protein by cdk2/cyclin A on Ser315 or by PKC on Ser378 can efficiently stimulate p53 binding to DNA in vitro, as well as binding of the monoclonal antibody Bp53-10, which recognizes residues 371-380 in the C-terminus of p53. Phosphorylation of p53 by CK2 on Ser392 induces its DNA-binding activity to a much lower extent than phosphorylation by cdk2/cyclin A or PKC. In addition, phosphorylation by CK2 strongly inhibits PKC-induced activation of p53 DNA binding, while the activation of p53 by cdk2/cyclin A is not affected by CK2. The presence of CK2-mediated phosphorylation promotes PKC binding to its docking site within the p53 oligomerization domain, but decreases phosphorylation by PKC, suggesting that competition between CK2 and PKC does not rely on the inhibition of PKC-p53 complex formation. These results indicate the crucial role of p53 C-terminal phosphorylation in the regulation of its DNA-binding activity, but also suggest that antagonistic relationships exist between different stress signalling pathways.

Denaturation of dsDNA by p53: fluorescence correlation spectroscopy study

p53 activates transcription through interaction with specific DNA sequences in gene promoters. It also regulates DNA replication, recombination, and repair apparently through interactions with DNA intermediates of these reactions. Biochemical activities relevant for these functions of p53 include binding to the ends and internal segments of single-stranded DNA molecules, catalysis of DNA renaturation, and strand exchange. We report a novel activity of p53, its ability to denature double-stranded DNA molecules aggregated by basic peptides. Stable complexes of coiled single-stranded DNA molecules with basic peptides are formed in this reaction. Thus, complementary to the ability to catalyze DNA renaturation, p53 denatures double-stranded DNA when the latter reaction is thermodynamically favorable. This p53 activity, along with its ability to interact physically with DNA helicases, may be relevant for resolving double-stranded DNA intermediates and inhibition of DNA recombination, which is critical for guarding of the genome.

Recognition of DNA modified by antitumor cisplatin by "latent" and "active" protein p53

Tumor suppressor protein p53 possesses two DNA-binding sites. One that is located within its core domain is responsible for sequence-specific DNA binding of the protein, non-specific binding to internal segments of single- or double-stranded DNA, and to certain kinds of non-B DNA structures. The other that is contained in the C-terminus of the protein binds to damaged DNA. Binding of active, latent, and in vitro-activated p53 protein to DNA fragments modified by antitumor cisplatin was studied using electrophoretic mobility shift assay in agarose gels and immunoblotting analysis. We found that both latent and active p53 forms bound to random sequences of DNA globally modified by cisplatin with a higher affinity than to unmodified DNA. Interestingly, the latent form exhibited a more pronounced selectivity for platinated DNA than the active p53. Consistently with this observation, the preference of the latent form for platinated DNA decreased as a consequence of the activation of latent p53 by phosphorylation at the protein kinase C site within its C-terminus or by binding of the monoclonal antibody Bp53-10.1. Competition experiments involving a 20-bp consensus sequence of p53 suggested that the p53 core domain was a primary binding site of the active p53 when it bound to DNA fragments lacking consensus sequence, but modified by cisplatin. In addition, the latent protein was found to selectively interact with DNA modified by cisplatin probably via its C-terminus.

Identification of functional p53-binding motifs in the mouse wig-1 promoter

We previously identified wig-1 as a p53-induced mouse gene that encodes a nuclear zinc finger protein with unknown function. To investigate whether wig-1 is a direct target of p53-dependent transactivation, a DNA fragment corresponding to the promoter region was cloned and sequenced. Three regions containing consensus p53-binding sites were identified. Two p53-binding motifs formed DNA-protein complexes with p53 and were able to drive p53-dependent transcription in a luciferase reporter assay. Our results demonstrate that wig-1 is a direct target of p53-mediated transcriptional transactivation.

Characterization of the p53-rescue drug CP-31398 in vitro and in living cells

The Pfizer compound CP-31398 has been reported to stabilize the core domain of the tumour suppressor p53 in vitro and be an effective anti-cancer drug by virtue of rescuing destabilized mutants of p53. We did not detect any interaction between the p53 core domain and CP-31398 in vitro by a wide range of quantitative biophysical techniques over a wide range of conditions. CP-31398 did not stabilize p53 in our experiments. However, we found that CP-31398 intercalated with DNA and also altered and destabilized the DNA-p53 core domain complex. We analysed by NMR TROSY the interaction of the domain with a DNA oligomer and identified the changes in the complex on the binding of CP-31398. CP-31398 also decreased sequence-specific DNA binding of wild-type p53 and His-273 mutant p53. CP-31398 had a non-specific toxic effect independent of mutant p53 expression in several cell lines carrying Tet-regulated mutant p53. CP-31398 caused a small increase in MDM-2 expression and a more pronounced p53-independent increase in Bax expression. CP-31398 did, however, induce the PAb1620 epitope (characteristic of native p53) in cells expressing His-175 mutant p53. This was prevented by cycloheximide, suggesting that any stabilizing action of CP-31398 would have to be on newly synthesized p53. One of the unstable mutants that was reported to have been rescued by CP-31398, R249S, does not bind DNA when folded at lower temperatures.

Restoration of the tumor suppressor function to mutant p53 by a low-molecular-weight compound

The tumor suppressor p53 inhibits tumor growth primarily through its ability to induce apoptosis. Mutations in p53 occur in at least 50% of human tumors. We hypothesized that reactivation of mutant p53 in such tumors should trigger massive apoptosis and eliminate the tumor cells. To test this, we screened a library of low-molecular-weight compounds in order to identify compounds that can restore wild-type function to mutant p53. We found one compound capable of inducing apoptosis in human tumor cells through restoration of the transcriptional transactivation function to mutant p53. This molecule, named PRIMA-1, restored sequence-specific DNA binding and the active conformation to mutant p53 proteins in vitro and in living cells. PRIMA-1 rescued both DNA contact and structural p53 mutants. In vivo studies in mice revealed an antitumor effect with no apparent toxicity. This molecule may serve as a lead compound for the development of anticancer drugs targeting mutant p53.

Phosphodiester oligonucleotides inhibit mitosis and trigger apoptosis by a non-antisense, p53-mediated mechanism

Oligodeoxyribonucleotides (ODNs) are currently employed to switch-off genes selectively routinely in the laboratory practice. The drawback of ODN application is that they have been often reported to elicit non-antisense effects by different mechanisms. Recently, it has been shown that double-stranded DNA oligonucleotides (30-mers) with protruding ends activate p53 in a cell-free system. In a previous work, we described that simple addition to the culture medium of heterogeneous DNA combined with cationic lipids culminated in inhibition of mitosis and induction of apoptosis. Here, we report that the same effects are achieved by lipotransfecting cultured cells with phosphorodiester ODNs (30-mers). Such effects of ODN were mediated by a non-antisense mechanism that required the wild-type form of the p53 oncosuppressor protein and was dependent on ODN concentration. Mitosis inhibition and apoptosis induction appeared to be determined by the 3' and 5' free ends of ODNs, which activated p53 independently from their sequence. Most probably, this mechanism is analogous to that evoked by genotoxic agent-induced DNA damage or by lipotransfecting cells with heterogeneous DNA.

The effects of wild-type p53 tumor suppressor activity and mutant p53 gain-of-function on cell growth

The tumor suppressor p53 plays a central role in the protection against DNA damage and other forms of physiological stress primarily by inducing cell cycle arrest or apoptosis. Mutation of p53, which is the most frequent genetic alteration detected in human cancers, inactivates these growth regulatory functions and causes a loss of tumor suppressor activity. In some cases, mutation also confers tumor-promoting functions, such as the transcriptional activation of genes involved in cell proliferation, cell survival and angiogenesis. Consequently, cells expressing some forms of mutant p53 show enhanced tumorigenic potential with increased resistance to chemotherapy and radiation. Our current understanding of these activities is summarized in this review. By dissecting out mechanistic differences between wild-type and mutant p53 activities, it may be possible to develop therapeutics that restore tumor suppressor function to mutant p53 or that selectively inactivate mutant p53 tumor-promoting functions.

Regulation of p53 sequence-specific DNA-binding by covalent poly(ADP-ribosyl)ation

We have characterized the covalent poly(ADP-ribosyl)ation of p53 using an in vitro reconstituted system. We used recombinant wild type p53, recombinant poly(ADP-ribose) polymerase-1 (PARP-1) (EC ), and betaNAD(+). Our results show that the covalent poly(ADP-ribosyl)ation of p53 is a time-dependent protein-poly(ADP-ribosyl)ation reaction and that the addition of this tumor suppressor protein to a PARP-1 automodification mixture stimulates total protein-poly(ADP-ribosyl)ation 3- to 4-fold. Electrophoretic analysis of the products synthesized indicated that short oligomers predominate early during hetero-poly(ADP-ribosyl)ation, whereas longer ADP-ribose chains are synthesized at later times of incubation. A more drastic effect in the complexity of the ADP-ribose chains generated was observed when the betaNAD(+) concentration was varied. As expected, increasing the betaNAD(+) concentration from low nanomolar to high micromolar levels resulted in the slower electrophoretic migration of the p53-(ADP-ribose)(n) adducts. Increasing the concentration of p53 protein from low nanomolar (40 nm) to low micromolar (1.0 microm) yielded higher amounts of poly(ADP-ribosyl)ated p53 as well. Thus, the reaction was acceptor protein concentration-dependent. The hetero-poly(ADP-ribosyl)ation of p53 also showed that high concentrations of p53 specifically stimulated the automodification reaction of PARP-1. The covalent modification of p53 resulted in the inhibition of the binding ability of this transcription factor to its DNA consensus sequence as judged by electrophoretic mobility shift assays. In fact, controls carried out with calf thymus DNA, betaNAD(+), PARP-1, and automodified PARP-1 confirmed our conclusion that the covalent poly(ADP-ribosyl)ation of p53 results in the transcriptional inactivation of this tumor suppressor protein.

Functional impact of concomitant versus alternative defects in the Chk2-p53 tumour suppressor pathway

Recent evidence identified a genetic and functional link between Chk2 kinase and p53 as a candidate genome integrity checkpoint and a tumour suppressor pathway. Here we report that in human cells, Chk2 and p53 form protein-protein complexes whose abundance increased upon DNA damage, and whose formation was abrogated through cancer associated mutations in the FHA domain of Chk2, or mutations in the tetramerization domain of p53. Whereas among Li-Fraumeni syndrome families mutations of Chk2 or p53 occur in a mutually exclusive manner, we document that the colon cancer cell line HCT-15 concomitantly lacks functions of both Chk2 and p53, the latter demonstrated by a non-invasive reporter assay monitoring p53-dependent transactivation in live cells. Despite the preserved ability of common cancer-derived mutant p53 proteins to bind and potentially 'titrate' activated Chk2, the integrity of the S phase checkpoint response to ionizing radiation remained largely intact and dependent on Chk2 in cells with wild-type, mutant, or no p53. These results provide new mechanistic insights into the Chk2-p53 interplay, suggest how mutations in Chk2 may abrogate its tumour suppressor function, and indicate that compared with individual defects in either Chk2 or p53, concomitant mutations in both of these cell cycle checkpoint regulators may provide some additional selective advantage to tumour cells.

Activation and autoregulation of DNA-PK from structured single-stranded DNA and coding end hairpins

DNA-dependent protein kinase (DNA-PK) acts through an essential relationship with DNA to participate in the regulation of multiple cellular processes. Yet the role of DNA as a cofactor in kinase activity remains to be completely elucidated. For example, although DNA-PK activity appears to be required for the resolution of hairpin coding ends in variable diversity joining recombination, kinase activity remains to be demonstrated from hairpin ends or other DNA structures. In the present study we report that DNA-PK is strongly activated from hairpin ends and structured single-stranded DNA, but that the phosphorylation of many heterologous substrates is blocked efficiently by inactivation of the kinase through autophosphorylation. However, substrates that bound efficiently to single-stranded DNA such as p53 and replication protein A were efficiently phosphorylated by DNA-PK from structured DNA. DNA-PK also was found to be active toward heterologous substrates from hairpin ends on double-stranded DNA under conditions where autophosphorylation was minimized. These results suggest that the role of DNA-PK in resolving coding end hairpins is likely to be enzymatic rather than structural, expand understanding of how DNA-PK binding to structured DNA relates to enzyme activity, and suggest a mechanism for autoregulatory control of its kinase activity in the cell.

Tumor suppressor p53 protein is a new target for the metastasis-associated Mts1/S100A4 protein: functional consequences of their interaction

A physical and functional interaction between the Ca(2+)-binding protein Mts1 (S100A4) and the tumor suppressor p53 protein is shown here for the first time. We demonstrate that Mts1 binds to the extreme end of the C-terminal regulatory domain of p53 by several in vitro and in vivo approaches: co-immunoprecipitation, affinity chromatography, and far Western blot analysis. The Mts1 protein in vitro inhibits phosphorylation of the full-length p53 and its C-terminal peptide by protein kinase C but not by casein kinase II. The Mts1 binding to p53 interferes with the DNA binding activity of p53 in vitro and reporter gene transactivation in vivo, and this has a regulatory function. A differential modulation of the p53 target gene (p21/WAF, bax, thrombospondin-1, and mdm-2) transcription was observed upon Mts1 induction in tet-inducible cell lines expressing wild type p53. Mts1 cooperates with wild type p53 in apoptosis induction. Our data imply that the ability of Mts1 to enhance p53-dependent apoptosis might accelerate the loss of wild type p53 function in tumors. In this way, Mts1 can contribute to the development of a more aggressive phenotype during tumor progression.

p53 contains a DNA break-binding motif similar to the functional part of BRCT-related region of Rb

The BRCT regions are two repeating structures at BRCA1 carboxyl-terminus and are ubiquitous in some proteins involved in DNA repair and cell cycle checkpoints. It was shown that BRCTs of TopBP1, BRCA1, and BRCT-Ws of Rb bound DNA ends and breaks. We indicate here that the C-terminus of p53 tumor suppressor contains a DNA binding motif (residues 327-333 in human), whose features are similar to those of the part of BRCT-W in Rb with DNA binding activity. The short motif was required for the gel retardation activity of DNA fragments, since residues 311-333 showed the activity while residues 311-326 showed no activity. Significant numbers of total p53 and its fragments with the motif formed multimerizing complexes and associated with DNA ends and breaks. These results suggest the common presence of DNA binding motifs that can recognize DNA ends or damages in major tumor suppressors, Rb, BRCA1 and p53. The oncogenic activity of p53 C-terminus (residues 311-393) required both the DNA damage recognition motif and the repair enzyme-associating domain.

p53 Latency. C-terminal domain prevents binding of p53 core to target but not to nonspecific DNA sequences

The p53 transcription factor is either latent or activated through multi-site phosphorylation and acetylation of the negative regulatory region in its C-terminal domain (CTD). How CTD modifications activate p53 binding to target DNA sequences via its core domain is still unknown. It has been proposed that nonmodified CTD interacts either with the core domain or with DNA preventing binding of the core domain to DNA and that the fragments of the CTD regulatory region activate p53 by interfering with these interactions. We here characterized the sequence and target specificity of p53 activation by CTD fragments, interaction of activating peptides with p53 and target DNA, and interactions of "latent" p53 with DNA by a band shift assay and by fluorescence correlation spectroscopy. In addition to CTD fragments, several long basic peptides activated p53 and also transcription factor YY1. These peptides and CTD aggregated target DNA but apparently did not interact with p53. The potency to aggregate DNA correlated with the ability to activate p53, suggesting that p53 binds to target sequences upon interactions with tightly packed DNA in aggregates. Latent full-length p53 dissociated DNA aggregates via its core and CTD, and this effect was potentiated by GTP. Latent p53 also formed complexes via both its core and CTD with long nontarget DNA molecules. Such p53-DNA interactions may occur if latent p53 binding to DNA via CTD prevents the interaction of the core domain with target DNA sites but not with nonspecific DNA sequences.

Interaction with p53 enhances binding of cisplatin-modified DNA by high mobility group 1 protein

A nonhistone chromosomal protein, high mobility group (HMG) 1, is ubiquitous in higher eukaryotic cells and binds preferentially to cisplatin-modified DNA. HMG1 also functions as a coactivator of p53, a tumor suppressor protein. We investigated physical interactions between HMG1 and p53 and the influence of p53 on the ability of HMG1 to recognize damaged DNA. Using immunochemical coprecipitation, we observed binding of HMG1 and p53. Interaction between HMG1 and p53 required the HMG A box of HMG1 and amino acids 363-376 of p53. Cisplatin-modified DNA binding by HMG1 was significantly enhanced by p53. An HMG1-specific antibody that recognized the A box of this protein also stimulated cisplatin-modified DNA binding. These data suggest that an interaction with either p53 or antibody may induce conformational change in the HMG1 A box that optimizes DNA binding by HMG1. Interaction of p53 with HMG1 after DNA damage may promote activation of specific HMG1 binding to damaged DNA in vivo and provide a molecular link between DNA damage and p53-mediated DNA repair.

p53 C-terminal interaction with DNA ends and gaps has opposing effect on specific DNA binding by the core

In addition to binding DNA in a sequence-specific manner, the p53 tumour suppressor protein can interact with damaged DNA. In order to understand which structural features in DNA the C-teminal domain recognises we have studied the interaction of p53 protein with different types of DNA oligonucleotides imitating damaged DNA. Here we show that one unpaired nucleotide within double-stranded (ds)DNA is sufficient for recognition by the p53 C-terminus, either as a protruding end or as an internal gap in dsDNA. C-terminal interaction with DNA ends facilitated core domain binding to DNA, whereas interaction with gaps prevented core domain-DNA complexing, implying that p53 might adopt distinct conformations upon binding to different DNA lesions. These observations suggest that both single-strand and double-strand breaks can serve as a target for p53 C-terminal recognition in vivo and indicate that p53 might recruit different repair factors to the sites of damaged DNA depending on the type of lesion.

Induction of apoptosis and mitosis inhibition by degraded DNA lipotransfection mimicking genotoxic drug effects

Genotoxic damage induces cell cycle arrest and/or apoptosis by activation of p53 oncosuppressor protein. A number of anticancer drugs are genotoxic and their damaging effect upon cells is mediated by this mechanism. Microinjection of defined DNA species directly into nucleus has been reported previously to activate p53 and inhibit cell cycle. Here, we demonstrate that simple addition of heterogeneous degraded DNA to cultured cells (Rat-1 fibroblasts) in combination with lipotransfecting agent DOTAP leads to apoptosis induction and mitosis inhibition by a molecular mechanism which mimics that of the cellular response to genotoxic anticancer agents. Indeed, both cellular effects induced by lipotransfected degraded DNA (essentially, heterogeneous small DNA fragments) are associated to p53 activation and modulated by two apoptosis-related genes, such as bcl-2 and c-myc, which also modulate the apoptotic threshold to anticancer agents. Here we raise the hypothesis of exogenous DNA segment lipotransfection as possible new tool for anticancer therapy.

p53 splice acceptor site mutation and increased HsRAD51 protein expression in Bloom's syndrome GM1492 fibroblasts

GM1492 human diploid skin fibroblasts derived from a patient with Bloom's syndrome (BS), lack detectable p53 mRNA and protein as shown by Northern and Western blotting, and express an increased RecA-like activity. Here we demonstrate that the p53 gene is grossly intact in GM1492 cells according to Southern blotting. DNA sequencing did not reveal any mutations in the promoter region of p53. A highly sensitive RT-PCR produced a p53 cDNA fragment that was shorter than expected. DNA sequence analysis of p53 cDNA showed that exon 6 was missing, explaining the shorter PCR product. Furthermore, sequencing of genomic DNA revealed a base substitution at the nucleotide preceding the AG splice acceptor site of intron 5. The omission of exon 6 creates a frameshift at the junction of exons 5 and 7, and a premature stop codon in exon 7. The aberrant transcript is predicted to encode a truncated p53 protein containing 189 amino acid residues. Moreover, Western blotting demonstrated elevated HsRAD51 protein levels in GM1492 cells. The lack of sufficient levels of wild-type p53 and increased levels of HsRad51 protein may contribute to the elevated RecA-like activity in the GM1492 fibroblasts.

DNA bending due to specific p53 and p53 core domain-DNA interactions visualized by electron microscopy

We have used transmission electron microscopy to analyze the specificity and the extent of DNA bending upon binding of full-length wild-type human tumor suppressor protein p53 (p53) and the p53 core domain (p53CD) encoding amino acid residues 94-312, to linear double-stranded DNA bearing the consensus sequence 5'-AGACATGCCTAGACATGCCT-3' (p53CON). Both proteins interacted with high specificity and efficiency with the recognition sequence in the presence of 50 mM KCl at low temperature ( approximately 4 degrees C) while the p53CD also exhibits a strong and specific interaction at physiological temperature. Specific complex formation did not result in an apparent reduction of the DNA contour length. The interaction of p53 and the p53CD with p53CON induced a noticeable salt-dependent bending of the DNA axis. According to quantitative analysis with folded Gaussian distributions, the bending induced by p53 varied from approximately 40 degrees to 48 degrees upon decreasing of the KCl concentration from 50 mM to approximately 1 mM in the mounting buffer used for adsorption of the complexes to the carbon film surface. The p53CD bent DNA by 35-37 degrees for all salt concentrations used in the mounting buffer. The bending angle of the p53/DNA complex under low salt conditions showed a somewhat broader distribution (sigma approximately 39 degrees ) than at high salt concentration (sigma approximately 31 degrees ) or for p53CD (sigma approximately 24-27 degrees ). Together, these results demonstrate that the p53CD has a dominant role in complex formation and that the complexes formed both by p53 and p53CD under moderate salt conditions are similar. However, the dependence of the bending parameters on ambient conditions suggest that the segments flanking the p53CD contribute to complex formation as well. The problems associated with the analysis of bending angles in electron microscopy experiments are discussed.

Reactivation of mutant p53 through interaction of a C-terminal peptide with the core domain

A synthetic 22-mer peptide (peptide 46) derived from the p53 C-terminal domain can restore the growth suppressor function of mutant p53 proteins in human tumor cells (G. Selivanova et al., Nat. Med. 3:632-638, 1997). Here we demonstrate that peptide 46 binds mutant p53. Peptide 46 binding sites were found within both the core and C-terminal domains of p53. Lys residues within the peptide were critical for both p53 activation and core domain binding. The sequence-specific DNA binding of isolated tumor-derived mutant p53 core domains was restored by a C-terminal polypeptide. Our results indicate that C-terminal peptide binding to the core domain activates p53 through displacement of the negative regulatory C-terminal domain. Furthermore, stabilization of the core domain structure and/or establishment of novel DNA contacts may contribute to the reactivation of mutant p53. These findings should facilitate the design of p53-reactivating drugs for cancer therapy.

DNA damage-inducible phosphorylation of p53 at N-terminal sites including a novel site, Ser20, requires tetramerization

Upon DNA damage, p53 has been shown to be modified at a number of N-terminal phosphorylation sites including Ser15 and -33. Here we show that phosphorylation is induced as well at a novel site, Ser20. Phosphorylation at Ser15, -20 and -33 can occur within minutes of DNA damage. Interestingly, while the DNA-binding activities of p53 appear to be dispensable, efficient phosphorylation at these three sites requires the tetramerization domain of p53. Substitution of an artificial tetramerization domain for this region also permits phosphorylation at the N-terminus, suggesting that oligomerization is important for DNA damage-induced signalling to p53.

p53 and the CNS: tumors and developmental abnormalities

This article reviews the recent molecular and clinical studies that characterize the role of p53 in pathologies of the central nervous system, p53 has many important biological functions, notably, maintenance of DNA stability and regulation of apoptosis. These features are essential to avoid cellular transformation and ensure normal brain development. Lack of p53 function in the brain results in tumor formation in the astrocytic and lymphoid lineages and in severe neurodevelopmental diseases, such as exencephaly.

New p53-based anti-cancer therapeutic strategies

The p53 gene is frequently mutated in human tumours and therefore an important target for therapeutic intervention. Several p53-based strategies for treatment of cancer are currently under development. p53 gene therapy has resulted in tumour regression in patients with lung cancer. A mutant adenovirus can obliterate tumour cells carrying mutant p53 or lacking p53, but is unable to replicate in normal cells. Furthermore, current studies suggest that reactivation of mutant p53 proteins in tumours using small p53-activating molecules may initiate p53-dependent apoptosis and thus eliminate the tumour.

Activation of c-myc gene expression by tumor-derived p53 mutants requires a discrete C-terminal domain

Mutation of the p53 tumor suppressor gene is the most common genetic alteration in human cancer, and tumors that express mutant p53 may be more aggressive and have a worse prognosis than p53-null cancers. Mutant p53 enhances tumorigenicity in the absence of a transdominant negative mechanism, and this tumor-promoting activity correlates with its ability to transactivate reporter genes in transient transfection assays. However, the mechanism by which mutant p53 functions in transactivation and its endogenous cellular targets that promote tumorigenicity are unknown. Here we report that (i) mutant p53 can regulate the expression of the endogenous c-myc gene and is a potent activator of the c-myc promoter; (ii) the region of mutant p53 responsiveness in the c-myc gene has been mapped to the 3' end of exon 1; (iii) the mutant p53 response region is position and orientation dependent and therefore does not function as an enhancer; and (iv) transactivation by mutant p53 requires the C terminus, which is not essential for wild-type p53 transactivation. These data suggest that it may be possible to selectively inhibit mutant p53 gain of function and consequently reduce the tumorigenic potential of cancer cells. A possible mechanism for transactivation of the c-myc gene by mutant p53 is proposed.

Poly(ADP-ribose) binds to specific domains of p53 and alters its DNA binding functions

DNA strand breaks are potential interaction sites for the nuclear enzyme poly(ADP-ribose) polymerase (PARP; E.C. 2.4.2.30) and the tumor suppressor protein p53. Both proteins bind and respond to DNA breaks and both play a role in DNA damage signaling. A temporary colocalization and complex formation between these proteins has been demonstrated in mammalian cells. Here we show that free and poly(ADP-ribose) polymerase-bound ADP-ribose polymers target three domains in p53 protein for strong noncovalent interactions. The polymer binding sites could be mapped to two amino acid sequences in the sequence-specific core DNA binding domain of p53 (amino acid positions 153-178 and 231-253) and another one in the oligomerization domain (amino acids 326-348). In mobility shift experiments, poly(ADP-ribose) effectively prevented and reversed p53 binding to the palindromic p53 consensus sequence. Additionally, poly(ADP-ribose) also interfered with the DNA single strand end binding of p53. The results suggest that ADP-ribose polymers could play a role in regulating the DNA binding properties of p53.

Regulation of the p53 protein by protein kinase C alpha and protein kinase C zeta

The C-terminal of p53 (amino-acids 368-383) represses the DNA binding activity of p53. In vitro, phosphorylation of this region by Protein Kinase C (PKC) is associated with increased DNA binding activity. However, whether PKC can directly modulate p53 function in vivo is not known. Here, we demonstrate that cotransfection of p53 with either PKC alpha or PKC zeta increases p53's transcriptional activity. Mutagenesis of p53 indicates that serine 371 is the major site for phosphorylation by PKC alpha in vitro. Mutation of serine 371 caused a small decline in p53 activation by PKC alpha and PKC zeta. However, the alternatively spliced murine p53, which lacks the PKC phosphorylation sites, still demonstrated increased transcriptional activation when cotransfected with either PKC alpha or PKC zeta. The results indicate that phosphorylation of p53 by PKC in vitro does not correlate with the ability of PKC to upregulate p53's transcriptional activity in vivo.

Reciprocal interference between the sequence-specific core and nonspecific C-terminal DNA binding domains of p53: implications for regulation

The tumor suppressor p53 has two DNA binding domains: a central sequence-specific domain and a C-terminal sequence-independent domain. Here, we show that binding of large but not small DNAs by the C terminus of p53 negatively regulates sequence-specific DNA binding by the central domain. Four previously described mechanisms for activation of specific DNA binding operate by blocking negative regulation. Deletion of the C terminus of p53 activates specific DNA binding only in the presence of large DNA. Three activator molecules (a small nucleic acid, a monoclonal antibody against the p53 C terminus, and a C-terminal peptide of p53) stimulate sequence-specific DNA binding only in the presence of both large DNA and p53 with an intact C terminus. Our findings argue that interactions of the C terminus of p53 with genomic DNA in vivo would prevent p53 binding to specific promoters and that cellular mechanisms to block C-terminal DNA binding would be required.