|March 26, 2016|
p53 (also known as protein 53 or tumor protein 53 ), is a tumor suppressor protein that in humans is encoded by the TP53 gene.
The name p53 is in reference to its apparent molecular mass: It runs as a 53- kilodalton (kDa) protein on SDS-PAGE. But, based on calculations from its amino acid residues, p53's mass is actually only 43.7 kDa. This difference is due to the high number of proline residues in the protein, which slow its migration on SDS-PAGE, thus making it appear heavier than it actually is. This effect is observed with p53 from a variety of species, including humans, rodents, frogs, and fish.
P53 is also known as:
In humans, p53 is encoded by the TP53 gene located on the short arm of chromosome 17 (17p13.1). have been identified in most mammals for which complete genome data are available.
In humans, the two most common polymorphisms to occur involve the substitution of an Arginine base for a Proline base. This polymorphism arises out of a SNP mutation on the 72 codon, where a guanine base is replaced by a cytosine.
For these mammals, the gene is located on different chromosomes:
( Italics are used to denote the TP53 gene name and distinguish it from the protein it encodes.)
Human p53 is 393 amino acids long and has seven domains :
A tandem of nine-amino-acid transactivation domains (9aaTAD) was identified in the AD1 and AD2 regions of transcription factor p53.
9aaTADs mediate p53 interaction with general coactivators - TAF9, CBP/p300 (all four domains KIX, TAZ1, TAZ2 and IBiD), GCN5 and PC4, regulatory protein MDM2 and replication protein A (RPA).
Mutations that deactivate p53 in cancer usually occur in the DBD. Most of these mutations destroy the ability of the protein to bind to its target DNA sequences, and thus prevents transcriptional activation of these genes. As such, mutations in the DBD are recessive loss-of-function mutations. Molecules of p53 with mutations in the OD dimerise with wild-type p53, and prevent them from activating transcription. Therefore OD mutations have a dominant negative effect on the function of p53.
Wild-type p53 is a labile protein, comprising folded and unstructured regions that function in a synergistic manner.
P53 has many anticancer mechanisms, and plays a role in apoptosis, genetic stability, and inhibition of angiogenesis. In its anti-cancer role, p53 works through several mechanisms:
Activated p53 binds DNA and activates expression of several genes including WAF1/CIP1 encoding for p21. p21 (WAF1) binds to the G1 - S / CDK ( CDK2) and S/CDK complexes (molecules important for the G1/S transition in the cell cycle) inhibiting their activity.
When p21(WAF1) is complexed with CDK2 the cell cannot continue to the next stage of cell division. A mutant p53 will no longer bind DNA in an effective way, and, as a consequence, the p21 protein will not be available to act as the "stop signal" for cell division. Thus, cells will divide uncontrollably, and form tumors.
Recent research has also linked the p53 and RB1 pathways, via p14ARF, raising the possibility that the pathways may regulate each other.
P53 by regulating LIF has been shown to facilitate implantation in the mouse model and possibly in humans.
P53 expression can be stimulated by UV light, which also causes DNA damage. In this case, p53 can initiate events leading to tanning .
P53 becomes activated in response to a myriad of stress types, which include but are not limited to DNA damage (induced by either UV, IR , or chemical agents such as hydrogen peroxide), oxidative stress, osmotic shock, ribonucleotide depletion, and deregulated oncogene expression. This activation is marked by two major events. First, the half-life of the p53 protein is increased drastically, leading to a quick accumulation of p53 in stressed cells. Second, a conformational change forces p53 to be activated as a transcription regulator in these cells. The critical event leading to the activation of p53 is the phosphorylation of its N-terminal domain. The N-terminal transcriptional activation domain contains a large number of phosphorylation sites and can be considered as the primary target for protein kinases transducing stress signals.
The protein kinases that are known to target this transcriptional activation domain of p53 can be roughly divided into two groups. A first group of protein kinases belongs to the MAPK family (JNK1-3, ERK1-2, p38 MAPK), which is known to respond to several types of stress, such as membrane damage, oxidative stress, osmotic shock, heat shock, etc. A second group of protein kinases (ATR, ATM , CHK1 and CHK2 , DNA-PK, CAK) is implicated in the genome integrity checkpoint, a molecular cascade that detects and responds to several forms of DNA damage caused by genotoxic stress. Oncogenes also stimulate p53 activation, mediated by the protein p14ARF.
In unstressed cells, p53 levels are kept low through a continuous degradation of p53. A protein called Mdm2 (also called HDM2 in humans), which is itself a product of p53, binds to p53, preventing its action and transports it from the nucleus to the cytosol. Also Mdm2 acts as ubiquitin ligase and covalently attaches ubiquitin to p53 and thus marks p53 for degradation by the proteasome. However, ubiquitylation of p53 is reversible. A ubiquitin specific protease, USP7 (or HAUSP ), can cleave ubiquitin off p53, thereby protecting it from proteasome-dependent degradation. This is one means by which p53 is stabilized in response to oncogenic insults.
Phosphorylation of the N-terminal end of p53 by the above-mentioned protein kinases disrupts Mdm2-binding. Other proteins, such as Pin1, are then recruited to p53 and induce a conformational change in p53, which prevents Mdm2-binding even more. Phosphorylation also allows for binding of trancriptional coactivators, like p300 or PCAF, which then acetylate the carboxy-terminal end of p53, exposing the DNA binding domain of p53, allowing it to activate or repress specific genes. Deacetylase enzymes, such as Sirt1 and Sirt7, can deacetylate p53, leading to an inhibition of apoptosis. Some oncogenes can also stimulate the transcription of proteins which bind to MDM2 and inhibit its activity.
If the TP53 gene is damaged, tumor suppression is severely reduced. People who inherit only one functional copy of the TP53 gene will most likely develop tumors in early adulthood, a disease known as Li-Fraumeni syndrome. The TP53 gene can also be damaged in cells by mutagens ( chemicals , radiation, or viruses), increasing the likelihood that the cell will begin decontrolled division. More than 50 percent of human tumors contain a mutation or deletion of the TP53 gene. Loss of p53 creates genomic instability that most often results in the aneuploidy phenotype.
Certain pathogens can also affect the p53 protein that the TP53 gene expresses. One such example, human papillomavirus (HPV), encodes a protein, E6, which binds the p53 protein and inactivates it. This, in synergy with the inactivation of another cell cycle regulator, pRb, by the HPV protein E7, allows for repeated cell division manifested in the clinical disease of warts. Certain HPV types, in particular types 16 and 18, can also lead to progression from a benign wart to low or high-grade cervical dysplasia, which are reversible forms of precancerous lesions. Persistent infection of the cervix over the years can cause irreversible changes leading to carcinoma in situ and eventually invasive cervical cancer. This results from the effects of HPV genes, particularly those encoding E6 and E7, which are the two viral oncoproteins that are preferentially retained and expressed in cervical cancers by integration of the viral DNA into the host genome.
In healthy humans, the p53 protein is continually produced and degraded in the cell. The degradation of the p53 protein is, as mentioned, associated with MDM2 binding. In a negative feedback loop, MDM2 is itself induced by the p53 protein. However, mutant p53 proteins often do not induce MDM2, and are thus able to accumulate at very high concentrations. Worse, mutant p53 protein itself can inhibit normal p53 protein levels.
P53 was identified in 1979 by Lionel Crawford, David P. Lane, Arnold Levine, and Lloyd Old, working at Imperial Cancer Research Fund (UK) Princeton University/UMDNJ (Cancer Institute of New Jersey), and Sloan-Kettering Memorial Hospital, respectively. It had been hypothesized to exist before as the target of the SV40 virus, a strain that induced development of tumors. The TP53 gene from the mouse was first cloned by Peter Chumakov of the Russian Academy of Sciences in 1982,
It was initially presumed to be an oncogene due to the use of mutated cDNA following purification of tumour cell mRNA. Its character as a tumor suppressor gene was finally revealed in 1989 by Bert Vogelstein working at Johns Hopkins School of Medicine.
Warren Maltzman, of the Waksman Institute of Rutgers University first demonstrated that TP53 was responsive to DNA damage in the form of ultraviolet radiation.
In 1992, Wafik El-Deiry when he was working with Bert Vogelstein at Johns Hopkins University identified the consensus sequence, to which human p53 could bind, by immunoprecipitating human genomic DNA that could be bound by baculovirus-produced human p53 protein. This sequence was published in the first issue of the journal Nature Genetics in 1992 in work that is highly cited. The consensus sequence is 5'-RRRCWWGYYY-N(0-13)-RRRCWWGYYY-3' and is located in the regulatory regions of genes that are activated by the p53 transcription factor. The presence of p53 response elements in or around genes (promoters, upstream sequences, introns) is a powerful predictor of regulation and activation of a particular gene by p53.
In 1993, p53 was voted molecule of the year by Science magazine.
That same year, 1993, Wafik El-Deiry when he was working with Bert Vogelstein at Johns Hopkins University discovered p21(WAF1) as a gene regulated directly by p53. This work was reported in the most highly cited paper ever published in the journal Cell, and provided a molecular mechanism by which mammalian cells undergo growth arrest when damaged. The p21(WAF1) protein binds directly to cyclin-CDK complexes that drive forward the cell cycle and inhibits their kinase activity thereby causing cell cycle arrest to allow repair to take place. p21 can also mediate growth arrest associated with differentiation and a more permanent growth arrest associated with cellular senescence. The p21 gene contains several p53 response elements that mediate direct binding of the p53 protein, resulting in transcriptional activation of the gene encoding the p21(WAF1) protein.
P53 has been shown to interact with
Image:DAXX Pathway.svg|The DAXX Pathway
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