Distinct residues of human p53 implicated in binding to DNA, simian virus 40 large T antigen, 53BP1, and 53BP2

Hidden electrostatic energy contributions define dynamic allosteric communications within p53 during molecular recognition

Molecular recognition is fundamental to transcription regulation. As a transcription factor, the tumor suppressor p53 has to recognize either specific DNA sequences or repressor protein partners. However, the molecular mechanism underlying the p53 conformational switch from the DNA-bound to repressor-bound states is not fully characterized. The highly charged nature of these interacting molecules prompted us to explore the nonbonded energy contributions behind molecular recognition of either a DNA or the repressor protein iASPP by p53 DNA binding domain (p53DBD), using molecular dynamics simulation followed by rigorous analyses of energy terms. Our results illuminate the allosteric pathway by which iASPP binding to p53 diminishes binding affinity between p53 and DNA. Even though the p53DBD uses a common framework of residues for recognizing both DNA and iASPP, a comparison of the electrostatics in the two p53DBD complexes revealed significant differences in residue-wise contributions to the electrostatic energy. We found that an electrostatic allosteric communication path exists in the presence of both substrates. It consists of evolutionarily conserved residues, from residue K120 of the binding loop L1 to a distal residue R213 of p53DBD. K120 is near the DNA in the p53DBD-DNA complex, whereas iASPP binding moves it away from its DNA binding position in the p53DBD-iASPP complex. The "energy hubs" (the residues show a higher degree of connectivity with other residues in the electrostatic networks) determined from the electrostatic network analysis established that this conformational change in K120 completely rewires the electrostatic network from K120 to R213, thereby impeding DNA binding. Furthermore, we found shifting populations of hydrogen bonds and salt bridges reduce pairwise electrostatic energies within p53DBD in its DNA-bound state.

Truncated ASPP2 Drives Initiation and Progression of Invasive Lobular Carcinoma via Distinct Mechanisms

Invasive lobular carcinoma (ILC) accounts for 8%-14% of all breast cancer cases. The main hallmark of ILCs is the functional loss of the cell-cell adhesion protein E-cadherin. Nonetheless, loss of E-cadherin alone does not predispose mice to mammary tumor development, indicating that additional perturbations are required for ILC formation. Previously, we identified an N-terminal truncation variant of ASPP2 (t-ASPP2) as a driver of ILC in mice with mammary-specific loss of E-cadherin. Here we showed that expression of t-ASPP2 induced actomyosin relaxation, enabling adhesion and survival of E-cadherin-deficient murine mammary epithelial cells on stiff matrices like fibrillar collagen. The induction of actomyosin relaxation by t-ASPP2 was dependent on its interaction with protein phosphatase 1, but not on t-ASPP2-induced YAP activation. Truncated ASPP2 collaborated with both E-cadherin loss and PI3K pathway activation via PTEN loss in ILC development. t-ASPP2-induced actomyosin relaxation was required for ILC initiation, but not progression. Conversely, YAP activation induced by t-ASPP2 contributed to tumor growth and progression while being dispensable for tumor initiation. Together, these findings highlight two distinct mechanisms through which t-ASPP2 promotes ILC initiation and progression. SIGNIFICANCE: Truncated ASPP2 cooperates with E-cadherin and PTEN loss to drive breast cancer initiation and progression via two distinct mechanisms. ASPP2-induced actomyosin relaxation drives tumor initiation, while ASPP2-mediated YAP activation enhances tumor progression.

Proximity biotinylation identifies a set of conformation-specific interactions between Merlin and cell junction proteins

Neurofibromatosis type 2 is an inherited, neoplastic disease associated with schwannomas, meningiomas, and ependymomas and that is caused by inactivation of the tumor suppressor gene NF2 The NF2 gene product, Merlin, has no intrinsic catalytic activity; its tumor suppressor function is mediated through the proteins with which it interacts. We used proximity biotinylation followed by mass spectrometry and direct binding assays to identify proteins that associated with wild-type and various mutant forms of Merlin in immortalized Schwann cells. We defined a set of 52 proteins in close proximity to wild-type Merlin. Most of the Merlin-proximal proteins were components of cell junctional signaling complexes, suggesting that additional potential interaction partners may exist in adherens junctions, tight junctions, and focal adhesions. With mutant forms of Merlin that cannot bind to phosphatidylinositol 4,5-bisphosphate (PIP₂) or that constitutively adopt a closed conformation, we confirmed a critical role for PIP₂ binding in Merlin function and identified a large cohort of proteins that specifically interacted with Merlin in the closed conformation. Among these proteins, we identified a previously unreported Merlin-binding protein, apoptosis-stimulated p53 protein 2 (ASPP2, also called Tp53bp2), that bound to closed-conformation Merlin predominately through the FERM domain. Our results demonstrate that Merlin is a component of cell junctional mechanosensing complexes and defines a specific set of proteins through which it acts.

53BP1 and USP28 mediate p53-dependent cell cycle arrest in response to centrosome loss and prolonged mitosis

Mitosis occurs efficiently, but when it is disturbed or delayed, p53-dependent cell death or senescence is often triggered after mitotic exit. To characterize this process, we conducted CRISPR-mediated loss-of-function screens using a cell-based assay in which mitosis is consistently disturbed by centrosome loss. We identified 53BP1 and USP28 as essential components acting upstream of p53, evoking p21-dependent cell cycle arrest in response not only to centrosome loss, but also to other distinct defects causing prolonged mitosis. Intriguingly, 53BP1 mediates p53 activation independently of its DNA repair activity, but requiring its interacting protein USP28 that can directly deubiquitinate p53 in vitro and ectopically stabilize p53 in vivo. Moreover, 53BP1 can transduce prolonged mitosis to cell cycle arrest independently of the spindle assembly checkpoint (SAC), suggesting that while SAC protects mitotic accuracy by slowing down mitosis, 53BP1 and USP28 function in parallel to select against disturbed or delayed mitosis, promoting mitotic efficiency.

DOI:http://dx.doi.org/10.7554/eLife.16270.001

Cyclic AMP-Rap1A signaling mediates cell surface translocation of microvascular smooth muscle α2C-adrenoceptors through the actin-binding protein filamin-2

The second messenger cyclic AMP (cAMP) plays a vital role in vascular physiology, including vasodilation of large blood vessels. We recently demonstrated cAMP activation of Epac-Rap1A and RhoA-Rho-associated kinase (ROCK)-F-actin signaling in arteriolar-derived smooth muscle cells increases expression and cell surface translocation of functional α2C-adrenoceptors (α2C-ARs) that mediate vasoconstriction in small blood vessels (arterioles). The Ras-related small GTPAse Rap1A increased expression of α2C-ARs and also increased translocation of perinuclear α2C-ARs to intracellular F-actin and to the plasma membrane. This study examined the mechanism of translocation to better understand the role of these newly discovered mediators of blood flow control, potentially activated in peripheral vascular disorders. We utilized a yeast two-hybrid screen with human microvascular smooth muscle cells (microVSM) cDNA library and the α2C-AR COOH terminus to identify a novel interaction with the actin cross-linker filamin-2. Yeast α-galactosidase assays, site-directed mutagenesis, and coimmunoprecipitation experiments in heterologous human embryonic kidney (HEK) 293 cells and in human microVSM demonstrated that α2C-ARs, but not α2A-AR subtype, interacted with filamin. In Rap1-stimulated human microVSM, α2C-ARs colocalized with filamin on intracellular filaments and at the plasma membrane. Small interfering RNA-mediated knockdown of filamin-2 inhibited Rap1-induced redistribution of α2C-ARs to the cell surface and inhibited receptor function. The studies suggest that cAMP-Rap1-Rho-ROCK signaling facilitates receptor translocation and function via phosphorylation of filamin-2 Ser(2113). Together, these studies extend our previous findings to show that functional rescue of α2C-ARs is mediated through Rap1-filamin signaling. Perturbation of this signaling pathway may lead to alterations in α2C-AR trafficking and physiological function.

Wild-type and mutant p53 mediate cisplatin resistance through interaction and inhibition of active caspase-9

The p53 gene has been implicated in many cancers due to its frequent mutations as well as mutations in other genes whose proteins directly affect p53's functions. In addition, high expression of p53 [wild-type (WT) or mutant] has been found in the cytoplasm of many tumor cells, and studies have associated these observations with more aggressive tumors and poor prognosis. Cytoplasmic mis-localization of p53 subsequently reduced its transcriptional activity and this loss-of-function (LOF) was used to explain the lack of response to chemotherapeutic agents. However, this hypothesis seemed inadequate in explaining the apparent selection for tumor cells with high levels of p53 protein, a phenomenon that suggests a gain-of-function (GOF) of these mis-localized p53 proteins. In this study, we explored whether the direct involvement of p53 in the apoptotic response is via regulation of the caspase pathway in the cytoplasm. We demonstrate that p53, when present at high levels in the cytoplasm, has an inhibitory effect on caspase-9. Concurrently, knockdown of endogenous p53 caused an increase in the activity of caspase-9. p53 was found to interact with the p35 fragment of caspase-9, and this interaction inhibits the caspase-9 activity. In a p53-null background, the high-level expression of both exogenous WT and mutant p53 increased the resistance of these cells to cisplatin, and the data showed a correlation between high p53 expression and caspase-9 inhibition. These results suggest the inhibition of caspase-9 as a potential mechanism in evading apoptosis in tumors with high-level p53 expression that is cytoplasmically localized.

BRCT domains: easy as one, two, three

BRCA1 C-terminal (BRCT) domains are integral signaling modules in the DNA damage response (DDR). Aside from their established roles as phospho-peptide binding modules, BRCT domains have been implicated in phosphorylation-independent protein interactions, DNA binding and poly(ADP-ribose) (PAR) binding. These numerous functions can be attributed to the diversity in BRCT domain structure and architecture, where domains can exist as isolated single domains or assemble into higher order homo- or hetero- domain complexes. In this review, we incorporate recent structural and biochemical studies to demonstrate how structural features allow single and tandem BRCT domains to attain a high degree of functional diversity.

Computational identification of a p38SAPK-regulated transcription factor network required for tumor cell quiescence

The stress-activated kinase p38 plays key roles in tumor suppression and induction of tumor cell dormancy. However, the mechanisms behind these functions remain poorly understood. Using computational tools, we identified a transcription factor (TF) network regulated by p38alpha/beta and required for human squamous carcinoma cell quiescence in vivo. We found that p38 transcriptionally regulates a core network of 46 genes that includes 16 TFs. Activation of p38 induced the expression of the TFs p53 and BHLHB3, while inhibiting c-Jun and FoxM1 expression. Furthermore, induction of p53 by p38 was dependent on c-Jun down-regulation. Accordingly, RNAi down-regulation of BHLHB3 or p53 interrupted tumor cell quiescence, while down-regulation of c-Jun or FoxM1 or overexpression of BHLHB3 in malignant cells mimicked the onset of quiescence. Our results identify components of the regulatory mechanisms driving p38-induced cancer cell quiescence. These may regulate dormancy of residual disease that usually precedes the onset of metastasis in many cancers.

Design of a single plasmid-based modified yeast one-hybrid system for investigation of in vivo protein-protein and protein-DNA interactions

We have developed a modified yeast one-hybrid system (MY1H) useful for in vivo investigation of protein-protein and protein-DNA interactions. Our single-plasmid expression system is capable of differential protein expression levels; in addition to a GAL4 activation domain (AD) fusion protein, a second protein can be coexpressed at either comparable or higher transcriptional levels from expression vectors pCETT or pCETF, respectively. This second protein can play a structural, modifying, or inhibitory role that restores or blocks reporter gene expression. Our MY1H was validated by use of the well-characterized DNA-binding protein p53 and its inhibitory partners, large T antigen (LTAg) and 53BP2. By coexpressing LTAg or 53BP2 at comparable or higher levels than the GAL4AD-p53 fusion in the MY1H, we show that DNA binding of p53 decreases by different, measurable extents dependent on the expression level of inhibitory partner. As with the traditional Y1H, our system could also be used to investigate proteins that provide coactivational or bridging functions and to identify novel protein- or DNA-binding partners through library screening. Our MY1H provides a system for investigation of simultaneous protein-protein and protein-DNA interactions, and thus is a useful addition to current methods for in vivo investigation of such interactions.

Crystal structure of human 53BP1 BRCT domains bound to p53 tumour suppressor

The BRCT (BRCA1 C-terminus) is an evolutionary conserved protein-protein interacting module found as single, tandem or multiple repeats in a diverse range of proteins known to play roles in the DNA-damage response. The BRCT domains of 53BP1 bind to the tumour suppressor p53. To investigate the nature of this interaction, we have determined the crystal structure of the 53BP1 BRCT tandem repeat in complex with the DNA-binding domain of p53. The structure of the 53BP1-p53 complex shows that the BRCT tandem repeats pack together through a conserved interface that also involves the inter-domain linker. A comparison of the structure of the BRCT region of 53BP1 with the BRCA1 BRCT tandem repeat reveals that the interdomain interface and linker regions are remarkably well conserved. 53BP1 binds to p53 through contacts with the N-terminal BRCT repeat and the inter-BRCT linker. The p53 residues involved in this binding are mutated in cancer and are also important for DNA binding. We propose that BRCT domains bind to cellular target proteins through a conserved structural element termed the 'BRCT recognition motif'.

Structure of the 53BP1 BRCT region bound to p53 and its comparison to the Brca1 BRCT structure

Brca1 C-terminal (BRCT) domains are a common protein-protein interaction motif in proteins involved in the DNA damage response and DNA repair. The DNA-damage response protein 53BP1 has two BRCT domains that bind to the DNA-binding domain of p53. The 53BP1 tandem-BRCT region is homologous to the tandem-BRCT region of Brca1, which is involved in double-strand break repair and homologous recombination and which binds BACH1, a member of the DEAH helicase family. Here we report the structures of a human 53BP1-p53 complex and of the rat Brca1 BRCT repeats. The 53BP1-p53 structure shows that the two BRCT repeats are arranged tandemly and pack extensively through an interface that also involves the inter-repeat linker. The first BRCT repeat and the linker together bind p53 on a region that overlaps with the DNA-binding surface of p53 and involves p53 residues that are mutated in cancer and are important for DNA binding. Comparison with the structure of the tandem-BRCT region of Brca1 shows a remarkable conservation of the repeat arrangement and of the inter-BRCT repeat interface. Analysis of human BRCA1 tumor-derived mutations and conservation identifies a potential protein-binding site that we show through mutagenesis is involved in BACH1 binding. The BACH1-binding region of Brca1 consists of a unique insertion in the first BRCT repeat and the inter-repeat linker and is analogous to the region of 53BP1 that binds p53.

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.

Strategies for manipulating the p53 pathway in the treatment of human cancer

Human cancer progression is driven in part by the mutation of oncogenes and tumour-suppressor genes which, under selective environmental pressures, give rise to evolving populations of biochemically altered cells with enhanced tumorigenic and metastatic potential. Given that human cancers are biologically and pathologically quite distinct, it has been quite surprising that a common event, perturbation of the p53 pathway, occurs in most if not all types of human cancers. The central role of p53 as a tumour-suppressor protein has fuelled interest in defining its mechanism of function and regulation, determining how its inactivation facilitates cancer progression, and exploring the possibility of restoring p53 function for therapeutic benefit. This review will highlight the key biochemical properties of p53 protein that affect its tumour-suppressor function and the experimental strategies that have been developed for the re-activation of the p53 pathway in cancers.

p53 binding protein 1 (53BP1) is an early participant in the cellular response to DNA double-strand breaks

p53 binding protein 1 (53BP1), a protein proposed to function as a transcriptional coactivator of the p53 tumor suppressor, has BRCT domains with high homology to the Saccharomyces cerevisiae Rad9p DNA damage checkpoint protein. To examine whether 53BP1 has a role in the cellular response to DNA damage, we probed its intracellular localization by immunofluorescence. In untreated primary cells and U2OS osteosarcoma cells, 53BP1 exhibited diffuse nuclear staining; whereas, within 5-15 min after exposure to ionizing radiation (IR), 53BP1 localized at discreet nuclear foci. We propose that these foci represent sites of processing of DNA double-strand breaks (DSBs), because they were induced by IR and chemicals that cause DSBs, but not by ultraviolet light; their peak number approximated the number of DSBs induced by IR and decreased over time with kinetics that parallel the rate of DNA repair; and they colocalized with IR-induced Mre11/NBS and gamma-H2AX foci, which have been previously shown to localize at sites of DSBs. Formation of 53BP1 foci after irradiation was not dependent on ataxia-telangiectasia mutated (ATM), Nijmegen breakage syndrome (NBS1), or wild-type p53. Thus, the fast kinetics of 53BP1 focus formation after irradiation and the lack of dependency on ATM and NBS1 suggest that 53BP1 functions early in the cellular response to DNA DSBs.

Proapoptotic p53-interacting protein 53BP2 is induced by UV irradiation but suppressed by p53

p53 is an important mediator of the cellular stress response with roles in cell cycle control, DNA repair, and apoptosis. 53BP2, a p53-interacting protein, enhances p53 transactivation, impedes cell cycle progression, and promotes apoptosis through unknown mechanisms. We now demonstrate that endogenous 53BP2 levels increase following UV irradiation induced DNA damage in a p53-independent manner. In contrast, we found that the presence of a wild-type (but not mutant) p53 gene suppressed 53BP2 steady-state levels in cell lines with defined p53 genotypes. Likewise, expression of a tetracycline-regulated wild-type p53 cDNA in p53-null fibroblasts caused a reduction in 53BP2 protein levels. However, 53BP2 levels were not reduced if the tetracycline-regulated p53 cDNA was expressed after UV damage in these cells. This suggests that UV damage activates cellular factors that can relieve the p53-mediated suppression of 53BP2 protein. To address the physiologic significance of 53BP2 induction, we utilized stable cell lines with a ponasterone A-regulated 53BP2 cDNA. Conditional expression of 53BP2 cDNA lowered the apoptotic threshold and decreased clonogenic survival following UV irradiation. Conversely, attenuation of endogenous 53BP2 induction with an antisense oligonucleotide resulted in enhanced clonogenic survival following UV irradiation. These results demonstrate that 53BP2 is a DNA damage-inducible protein that promotes DNA damage-induced apoptosis. Furthermore, 53BP2 expression is highly regulated and involves both p53-dependent and p53-independent mechanisms. Our data provide new insight into 53BP2 function and open new avenues for investigation into the cellular response to genotoxic stress.

Retinoblastoma susceptibility protein, Rb, possesses multiple BRCT-Ws, BRCA1 carboxyl-terminus-related W regions with DNA break-binding activity

The BRCT region, the carboxyl-terminus of BRCA1 (the breast cancer susceptibility gene 1 product), is ubiquitous in several proteins that participate in cell cycle checkpoints and DNA repair. We have previously shown that the BRCT regions of TopBP1 (DNA topoisomerase II binding protein 1) and BRCA1 bound DNA breaks. A BRCT-related region, BRCT-W1, in the retinoblastoma susceptibility gene product (Rb) also could bind DNA fragments, independently of DNA sequences. Five BRCT-W regions were found in the Rb family. All BRCT-Ws of Rb bound DNA fragments. Electron microscopy and treatment with an exonuclease showed that BRCT-Ws bound double-strand DNA breaks. Since some BRCTs are exceptional common relating elements in tumor suppression, our findings reveal novel aspects of the tumor suppression mechanism.

Quantitative analysis of residual folding and DNA binding in mutant p53 core domain: definition of mutant states for rescue in cancer therapy

The tumour suppressor p53 is mutated in half of all human cancers, most frequently with missense substitutions in its core domain. We present a new assessment of the mutation database based on quantitative folding and DNA-binding studies of the isolated core domain. Our data identify five distinct mutant classes that correlate with four well-defined regions of the core domain structure. On extrapolation to 37 degrees C the wild-type protein has a stability of 3.0 kcal/mol. This also emerges as an oncogenic threshold: all beta-sandwich mutants destabilized by this amount (50% denatured) are expected to promote cancer. Other weakly destabilizing mutations are restricted to loop 3 in the DNA-binding region. Drugs that stabilize mutant p53 folding have the potential to reactivate apoptotic signalling pathways in tumour cells either by transactivation-dependent or independent pathways. Using an affinity ligand as a proof of principle we have recovered the thermodynamic stability of the hotspot G245S. With reference states for the five mutant classes as a guide, future therapeutic strategies may similarly stabilize partially structured or binding states of mutant p53 that restore limited p53 pathways to tumour suppression.

Conserved BRCT regions of TopBP1 and of the tumor suppressor BRCA1 bind strand breaks and termini of DNA

The BRCT region, the carboxyl-terminus of BRCA1 (the breast cancer susceptibility gene 1 product), is a ubiquitous region homologous to regions in DNA repair enzymes and cell cycle regulators. We showed that the BRCT regions bound DNA fragments, using the TopBP1 protein (topoisomerase II binding protein 1), with eight BRCTs as a model protein. The bindings were independent of DNA sequences, forms of DNA termini and energy. The BRCT-DNA complex showed resistance to an exonuclease, indicating that BRCT bound DNA breaks. The BRCTs also bound DNA nicks, suggesting that BRCTs play an important role in detection of both single- and double-strand DNA breaks or ends. On the other hand, BRCTs did not bind circular intact DNA. BRCTs of BRCA1 also bound DNA termini. Since some BRCTs are unique general elements in some tumor suppressions, these findings will reveal novel aspects of the tumor suppression mechanism.

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

Nuclear factor kappaB (NF-kappaB) is a transcription factor that controls the expression of many cellular and viral genes. The p65 (RelA) subunit plays a critical role as a transcriptional activator and recent observations have highlighted its role in the control of apoptosis. Here we report that 53BP2, a protein previously identified by interaction with wild type p53 and Bcl-2, also binds to p65 in a yeast two-hybrid system. This specific interaction was confirmed by pull-down assay in vitro and by a mammalian two-hybrid assay in vivo. We observed that full-length 53BP2 fused to GFP had a punctate distribution in cytoplasm, predominantly in perinuclear region whereas the N-terminal 53BP2 localized in cytoplasm and C-terminal 53BP2 localized in the nucleus. Furthermore, we found that overexpression of GFP-53BP2 induced apoptosis in transiently transfected cells. Neither the N-terminal nor the C-terminal of 53BP2 fused to GFP induced cell death. Interestingly, co-transfection with a p65 expression plasmid significantly inhibited 53BP2-induced cell death. The previous findings that 53BP2 bound to p53 and Bcl-2 together with our present observations suggest that 53BP2 may play a central role in the regulation of apoptosis and cell growth.

Proteolytic cleavage of p53 mutants in response to mismatched DNA

Interaction of p53 with mismatched DNA induces proteolytic cleavage with release of a 35-kDa protein fragment from the p53-DNA complexes. The 35-kDa cleavage product is activated for specific biochemical function(s) and may play a role in the cellular response to DNA damage (Molinari et al (1996) Oncogene 13: 2077-2086; Okorokov et al (1997) EMBO J 16: 6008-6017). In the present study we have asked if mutants of p53 retain the ability to undergo similar proteolytic cleavage, and compared sequence-specific 'DNA contact' with 'structural' mutants commonly found in human cancer. In addition, a series of phosphorylation site mutants were generated to investigate the possible effects of phosphorylation/dephosphorylation on the proteolytic cleavage of p53. All mutants tested bound to a mismatched DNA target in vitro. Moreover, studies in vitro and in vivo indicate that p53 mutants with intact conformational structure (as determined by immunoreactivity with PAb246 and PAb1620) retain the ability to undergo proteolytic cleavage similar, if not identical, to the wild-type p53 protein. Our results suggest that the capacity for p53 to bind mismatched DNA is independent of structural conformation of the central core domain. Proteolytic cleavage, however, is crucially dependent upon a wild-type conformation of the protein.

Cyclin E2, a novel G1 cyclin that binds Cdk2 and is aberrantly expressed in human cancers

A novel cyclin gene was discovered by searching an expressed sequence tag database with a cyclin box profile. The human cyclin E2 gene encodes a 404-amino-acid protein that is most closely related to cyclin E. Cyclin E2 associates with Cdk2 in a functional kinase complex that is inhibited by both p27(Kip1) and p21(Cip1). The catalytic activity associated with cyclin E2 complexes is cell cycle regulated and peaks at the G1/S transition. Overexpression of cyclin E2 in mammalian cells accelerates G1, demonstrating that cyclin E2 may be rate limiting for G1 progression. Unlike cyclin E1, which is expressed in most proliferating normal and tumor cells, cyclin E2 levels were low to undetectable in nontransformed cells and increased significantly in tumor-derived cells. The discovery of a novel second cyclin E family member suggests that multiple unique cyclin E-CDK complexes regulate cell cycle progression.

Identification of positive and negative regulatory regions involved in regulating expression of the human cytomegalovirus UL94 late promoter: role of IE2-86 and cellular p53 in mediating negative regulatory function

The human cytomegalovirus (HCMV) UL94 gene product is a herpesvirus-common virion protein that is expressed with true late kinetics. To identify the important cis- and trans-acting factors which contribute to UL94 transcriptional regulation, we have cloned, sequenced, and analyzed UL94 promoter function by transient transfection analysis. Transfection of UL94 promoter-reporter gene constructs into permissive human fibroblasts or U373(MG) cells indicated that promoter activity was detected following infection with HCMV. Point mutations within a TATA-like element located upstream of the RNA start site significantly reduced UL94 promoter activity. Deletion mutagenesis of the promoter indicated that a positive regulatory element (PRE) was likely to exist downstream of the UL94 mRNA start site, while a negative regulatory element (NRE) was present upstream of the TATA box. At late times of infection, the PRE appeared to have a dominant effect over the NRE to stimulate maximum levels of UL94 promoter activity, while at earlier times of infection, no activity associated with the PRE could be detected. The NRE, however, appeared to cause constitutive down-regulation of UL94 promoter activity. Binding sites for the cellular p53 protein located within the NRE appeared to contribute to NRE function, and NRE function could be recapitulated in cotransfection assays by concomitant expression of p53 and HCMV IE2-86 protein. Our results suggest a novel mechanism by which the cellular protein p53, which is involved in both transcriptional regulation and progression of cellular DNA synthesis, plays a central role in the regulation of a viral promoter which is not activated prior the onset of viral DNA replication.

p53 mutation is a poor prognostic indicator for survival in patients with hepatocellular carcinoma undergoing surgical tumour ablation

Forty-two patients with hepatocellular carcinoma (HCC) were resected and their tumours were analysed for p53 mutations by GC-clamped denaturing gradient gel electrophoresis (DGGE), single-strand conformation polymorphism (SSCP) and gene sequencing. All the exons have been analysed in this study. Eight of 12 HCCs with cirrhosis due to viral hepatitis and the two patients with sarcomatoid changes displayed p53 mutations. In contrast, no mutation was observed in the fibrolamellar variant (n = 9), non-cirrhotics (n = 13) and alcoholic cirrhosis (n = 6). The mutations observed were in exons 5-8. Two mutations were observed in codons 136 and 213 as well as a T insertion between residues 156 and 157 (exon 5) and these are reported for the first time in HCC. Likewise, the silent mutation polymorphism in codon 213 was noticed in 3 of the 42 patients. Survival analysis of these patients after surgery showed the mean and median survival in patients with wild-type p53 to be 60 and 43 months respectively. In the group with p53 mutations, the mean and median survival was 15 and 12 months. The difference was statistically significant (P= 0.003).

A proline-rich motif in p53 is required for transactivation-independent growth arrest as induced by Gas1

The involvement of p53 in regulating diverse cellular processes dictates that it must respond to multiple signaling mechanisms, thus coordinating the response to various "stress conditions." Genotoxic stress has served as a paradigm to dissect the transactivation-dependent branch of the pathway by which p53 can induce growth arrest. Alternate mechanisms have been invoked to explain transactivation-independent effects of p53, especially in the context of apoptosis. We have identified a p53-dependent pathway initiated by the gas1 product, a plasma membrane protein highly expressed during G0, which activates a transactivation-independent p53 growth arrest function. Through a detailed deletional analysis and site-specific mutagenesis of p53 we show that the Gas1-dependent signal transduction relies on a proline-rich region (amino acids 63-85) of murine p53. In vivo competition experiments using combinations of such mutants implicate this functional domain of p53 as a docking site in the transmission of antiproliferative signals.

Use of genetic suppressor elements to dissect distinct biological effects of separate p53 domains

p53 is a multifunctional tumor suppressor protein involved in the negative control of cell growth. Mutations in p53 cause alterations in cellular phenotype, including immortalization, neoplastic transformation, and resistance to DNA-damaging drugs. To help dissect distinct functions of p53, a set of genetic suppressor elements (GSEs) capable of inducing different p53-related phenotypes in rodent embryo fibroblasts was isolated from a retroviral library of random rat p53 cDNA fragments. All the GSEs were 100-300 nucleotides long and were in the sense orientation. They fell into four classes, corresponding to the transactivator (class I), DNA-binding (class II), and C-terminal (class III) domains of the protein and the 3'-untranslated region of the mRNA (class IV). GSEs in all four classes promoted immortalization of primary cells, but only members of classes I and III cooperated with activated ras to transform cells, and only members of class III conferred resistance to etoposide and strongly inhibited transcriptional transactivation by p53. These observations suggest that processes related to control of senescence, response to DNA damage, and transformation involve different functions of the p53 protein and furthermore indicate a regulatory role for the 3'-untranslated region of p53 mRNA.

The p53-binding protein 53BP2 also interacts with Bc12 and impedes cell cycle progression at G2/M

Using the yeast two-hybrid system, we have isolated a cDNA (designated BBP, for Bcl2-binding protein) for a protein (Bbp) that interacts with Bcl2. Bbp is identical to 53BP2, a partial clone of which was previously isolated in a two-hybrid screen for proteins that interact with p53. In this study, we show that specific interactions of Bbp/53BP2 with either Bcl2 or p53 require its ankyrin repeats and SH3 domain. These interactions can be reproduced in vitro with bacterially expressed fusion proteins, and competition experiments indicate that Bcl2 prevents p53 from binding to Bbp/53BP2. BBP/53BP2 mRNA is abundant in most cell lines examined, but the protein cannot be stably expressed in a variety of cell types by transfection. In transiently transfected cells, Bbp partially colocalizes with Bcl2 in the cytoplasm and results in an increased number of cells at G2/M, possibly accounting for the inability to obtain stable transfectants expressing the protein. These results demonstrate that a single protein can interact with either Bcl2 or p53 both in yeast cells and in vitro. The in vivo significance of these interactions and their potential consequences for cell cycle progression and cell death remain to be determined.

Discrimination of DNA binding sites by mutant p53 proteins

Critical determinants of DNA recognition by p53 have been identified by a molecular genetic approach. The wild-type human p53 fragment containing amino acids 71 to 330 (p53(71-330)) was used for in vitro DNA binding assays, and full-length human p53 was used for transactivation assays with Saccharomyces cerevisiae. First, we defined the DNA binding specificity of the wild-type p53 fragment by using systematically altered forms of a known consensus DNA site. This refinement indicates that p53 binds with high affinity to two repeats of PuGPuCA.TGPyCPy, a further refinement of an earlier defined consensus half site PuPuPuC(A/T).(T/A) GPyPyPy. These results were further confirmed by transactivation assays of yeast by using full-length human p53 and systematically altered DNA sites. Dimers of the pentamer AGGCA oriented either head-to-head or tail-to-tail bound efficiently, but transactivation was facilitated only through head-to-head dimers. To determine the origins of specificity in DNA binding by p53, we identified mutations that lead to altered specificities of DNA binding. Single-amino-acid substitutions were made at several positions within the DNA binding domain of p53, and this set of p53 point mutants were tested with DNA site variants for DNA binding. DNA binding analyses showed that the mutants Lys-120 to Asn, Cys-277 to Gln or Arg, and Arg-283 to Gln bind to sites with noncanonical base pair changes at positions 2, 3, and 1 in the pentamer (PuGPuCA), respectively. Thus, we implicate these residues in amino acid-base pair contacts. Interestingly, mutant Cys-277 to Gln bound a consensus site as two and four monomers, as opposed to the wild-type p53 fragment, which invariably binds this site as four monomers.