Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P43146 (tumour suppressor)
 5,935 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The tumour suppressor p53 is a transcription factor with high affinity for specific DNA target sequences. Wild type p53 has a very short half life in normal cells but the protein shows transient accumulation in response to DNA damage, accompanied by up-regulation of target genes such as p21 and induction of growth arrest in G1 of the cell cycle. The rapid turnover of p53 may involve the ubiquitin-dependent proteolytic pathway. In order to investigate p53 turnover we have employed an in vitro system with rabbit reticulocyte lysate, in which ubiquitin-dependent degradation of p53 is mediated by the oncoprotein E6 of human papilloma virus type 16 (HPV-16). Using this system we have previously shown that E6-mediated degradation is preferential for p53 in the 1620+ conformation (reactive with the monoclonal antibody PAb1620). p53-1620+ is a pre-requisite for specific DNA binding and we have now asked if p53 in complex with DNA remains susceptible to ubiquitin-dependent proteolysis in the presence of E6. Our results indicate that p53-DNA complexes are resistant to degradation, whereas the 'free' protein is completely degraded within 20 min. Moreover, E6 did not complex with p53-DNA, possibly due to masking of sites recognised either by E6 or by the E6-associated protein (E6-AP) which facilitates E6-p53 interaction. Preincubation with E6 inhibited the DNA binding capacity of p53 and this effect could be explained, at least in part, by ubiquitination of the p53 protein.
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PMID:p53 in complex with DNA is resistant to ubiquitin-dependent proteolysis in the presence of HPV-16 E6. 775 60

The E6 proteins originating from the tumour-associated Human Papillomavirus (HPV) types 16 and 18 have been shown to bind to and target the tumour suppressor protein, p53, for ubiquitin-mediated degradation. However, in cell lines derived from cervical neoplasias, the predominant early region transcripts are spliced and encode truncated forms of E6, termed E6*. We report here that HPV-18 E6* protein will interact both with the full-length E6 proteins from HPV-16 and HPV-18 and also with E6-AP, and subsequently blocks the association of full length E6 protein with p53. We also show that, as a result of this block, E6* can inhibit E6-mediated degradation of p53 both in vitro and in vivo. The biological consequences of this are increased transcriptional activity on p53-responsive promoters and an inhibition of cell growth in cells transfected with E6*. This is the first report of a potential biological function for this polypeptide and may represent a means by which HPV is able to modulate the activity of the full-length E6 protein with respect to p53 during viral infection.
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PMID:Alternatively spliced HPV-18 E6* protein inhibits E6 mediated degradation of p53 and suppresses transformed cell growth. 923 60

Human papillomavirus (HPV) 16 E6 induces the degradation of the tumour suppressor protein p53 by the ubiquitin-dependent proteolysis pathway. In vitro, this process involves the formation of a trimolecular complex between E6, p53 and a cellular protein E6-associated protein (E6-AP). However, an analysis of their potential interactions in vivo has not been carried out. We have established a model for the expression and analysis of the interactions of these three proteins in insect cells, a eukaryotic system where potentially crucial modifications of the proteins will occur. In baculovirus-infected cells the degradation of p53 can occur. However, p53 is only degraded early in the infectious cycle due to a lack of ATP at later times. Consequently, substantial quantities of material can be produced in this system for further analysis. Evidence is also provided that, in vivo, E6 can interact with p53 in the absence of E6-AP and that E6-AP can interact with p53 in the absence of E6. Furthermore, analysis of the subcellular localization of the proteins using both biochemical fractionation and indirect immunofluorescence suggests that the degradation of p53 occurs in the perinuclear region of the cell.
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PMID:Characterization of the interactions of human papillomavirus type 16 E6 with p53 and E6-associated protein in insect and human cells. 951 27

An important characteristic of the E6 proteins derived from oncogenic associated human papillomaviruses (HPVs) is their ability to target the cellular tumour suppressor protein, p53, for ubiquitin mediated degradation. Several studies have attempted to address the important characteristics of both E6 and p53 for this activity in vitro, but the equivalent determinants have not been extensively assessed in vivo. Indeed, recent studies indicate differences between the in vitro and the in vivo degradation assays. We have performed an extensive analysis of the ability of a range of HPV-18 E6 mutants to direct p53 degradation in vivo. In addition, we have also compared the ability of HPV-18 E6 to direct the degradation of different oligomeric forms of p53 both in human and in murine cells. The results of these studies show that mutants of E6 exhibit very similar phenotypes both in vitro and in vivo. In contrast, mutants of p53 show markedly different susceptibilities in vitro and in vivo to E6-induced degradation, and this is further affected by the nature of the cell type in which the assays are performed. Finally, using a cell line temperature sensitive for the E1 ubiquitin-activating enzyme we have been able to show directly that this enzyme is involved in the process of E6-mediated degradation of p53 in vivo.
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PMID:Comparison of human papillomavirus type 18 (HPV-18) E6-mediated degradation of p53 in vitro and in vivo reveals significant differences based on p53 structure and cell type but little difference with respect to mutants of HPV-18 E6. 971 44

The E6 proteins derived from tumour associated papillomavirus types target the cellular tumour suppressor protein p53 for ubiquitin mediated degradation. In cell lines derived from cervical tumours the p53 protein is present in very low amounts, but it can be activated by appropriate DNA damaging agents, indicating that functional p53 is present within these lines. Recent studies have also shown that different polymorphic forms of the p53 protein are differentially susceptible to E6 mediated degradation. Therefore we have been interested in analysing the effects of different HPV E6 proteins upon p53 levels in a variety of cervical tumour derived cell lines. We show that inhibition of E6 mediated degradation of p53 frequently results in increased levels of p53 expression. However, there are notable exceptions to this where increased p53 levels are only obtained following DNA damage and proteasome inhibition. We also show in E6 expressing cells, that as well as p53 being targeted for degradation, the localization of p53 to the nucleus is also inhibited, consistent with previous observations which indicate that degradation of p53 is not essential for E6 mediated inhibition of p53 function. These results have important implications for any potential therapies which might aim to block E6 mediated degradation of p53.
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PMID:Inhibition of E6 induced degradation of p53 is not sufficient for stabilization of p53 protein in cervical tumour derived cell lines. 1036 51

Levels of the tumour suppressor protein p53 are increased in response to a variety of DNA damaging agents. DNA damage-induced phosphorylation of p53 occurs at serine-15 in vivo. Phosphorylation of p53 at serine-15 leads to a stabilization of the polypeptide by inhibiting its interaction with Mdm2, a protein that targets p53 for ubiquitin-dependent degradation. However, the mechanisms by which DNA damage is signalled to p53 remain unclear. Here, we report the identification of a novel DNA-activated protein kinase that phosphorylates p53 on serine-15. Fractionation of HeLa nuclear extracts and biochemical analyses indicate that this kinase is distinct from the DNA-dependent protein kinase (DNA-PK) and corresponds to the human cell cycle checkpoint protein ATR. Immunoprecipitation studies of recombinant ATR reveal that catalytic activity of this polypeptide is required for DNA-stimulated phosphorylation of p53 on serine-15. These data suggest that ATR may function upstream of p53 in a signal transduction cascade initiated upon DNA damage and provide a biochemical assay system for ATR activity.
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PMID:The ataxia-telangiectasia related protein ATR mediates DNA-dependent phosphorylation of p53. 1043 22

The p53 tumour suppressor protein is regulated by ubiquitin-mediated proteasomal degradation. In normal cells p53 is constitutively ubiquitylated by the Mdm2 ubiquitin ligase. When the p53 response is activated by stress signals p53 levels rise due to inhibition of this degradative pathway. Here we show that p53 is modified by the small ubiquitin-like protein SUMO-1 at a single site, K386, in the C-terminus of the protein. Modification in vitro requires only SUMO-1, the SUMO-1 activating enzyme and ubc9. SUMO-1 and ubiquitin modification do not compete for the same lysine acceptor sites in p53. Overexpression of SUMO-1 activates the transcriptional activity of wild-type p53, but not K386R p53 where the SUMO-1 acceptor site has been mutated. The SUMO-1 modification pathway therefore acts as a potential regulator of the p53 response and may represent a novel target for the development of therapeutically useful modulators of the p53 response.
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PMID:SUMO-1 modification activates the transcriptional response of p53. 1056 57

The human cullin protein CUL-2 functions in a ubiquitin-ligase complex with the von Hippel-Lindau (VHL) tumour suppressor protein. Here we show that, in Caenorhabditis elegans, cul-2 is expressed in proliferating cells and is required at two distinct points in the cell cycle, the G1-to-S-phase transition and mitosis. cul-2 mutant germ cells undergo a G1-phase arrest that correlates with accumulation of CKI-1, a member of the CIP/KIP family of cyclin-dependent-kinase inhibitors. In cul-2 mutant embryos, mitotic chromosomes are unable to condense, leading to unequal DNA segregation, chromosome bridging and the formation of multiple nuclei.
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PMID:CUL-2 is required for the G1-to-S-phase transition and mitotic chromosome condensation in Caenorhabditis elegans. 1058 44

The mdm2 protein interacts with a number of proteins involved in cell growth control. Such interactions favour cell proliferation and may explain the oncogenic potential of mdm2 when over-expressed in cells. Interaction with the tumour suppressor p53 involves the N-terminus of mdm2 and targets p53 for rapid degradation by the ubiquitin pathway. We now describe a novel, highly conserved exon of mdm2 (exon alpha) which includes an in-frame UGA stop codon. Expression of exon alpha disrupts in vitro translation of the p53 binding domain of mdm2. We propose that exon alpha induces translation re-initiation at an internal AUG codon within the mdm2 alpha mRNA isoform. The putative mdm2 alpha protein lacks the N-terminus of mdm2 and shows little, if any, binding capacity for p53. Mdm2 alpha mRNA is expressed in a tissue-specific manner and is observed predominantly in testis and peripheral blood lymphocytes. We propose that mdm2 alpha expression may provide a mechanism for uncoupling mdm2-p53 interaction in certain cell types and/or under specific conditions of cell growth.
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PMID:A novel exon within the mdm2 gene modulates translation initiation in vitro and disrupts the p53-binding domain of mdm2 protein. 1059 3

The Discs Large (DLG) tumour suppressor protein is targeted for ubiquitin mediated degradation by the high risk human papillomavirus (HPV) E6 proteins. In this study we have used a mutational analysis of E6 in order to investigate the mechanism by which this occurs. We first show that the differences in the affinities of HPV-16 and of HPV-18 E6 proteins for binding DLG is reflected in their respective abilities to target DLG for degradation. A mutational analysis of HPV-18 E6 has enabled us to define regions within the carboxy terminal half of the protein which are essential for the ability of E6 to direct the degradation of DLG. Mutants within the amino terminal portion of E6 which have lost the ability to bind the E6-AP ubiquitin ligase, as measured by their ability to degrade p53, nonetheless retain the ability to degrade DLG. Significant levels of DLG degradation are also obtained using wheat germ extracts which lack E6-AP. Finally, we show that the transfer of the DLG binding domain onto the low risk HPV-6 E6 confers DLG binding activity to that protein and, most significantly, allows HPV-6 E6 to target DLG for degradation. These results indicate that E6 mediated degradation of DLG does not involve the E6-AP ubiquitin ligase and, in addition, shows that the high and low risk HPV E6 proteins most likely share a common cellular intermediary in the ubiquitin pathway.
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PMID:HPV E6 targeted degradation of the discs large protein: evidence for the involvement of a novel ubiquitin ligase. 1069 89


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