Mdm2 is an important negative regulator of the
p53p53 , is a tumor suppressor protein that in humans is encoded by the TP53 gene. p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer...
tumor suppressor. It is the name of a
geneA gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
as well as the
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
encoded by that gene. Mdm2 protein functions both as an
E3 ubiquitin ligaseA ubiquitin ligase is a protein that in combination with an E2 ubiquitin-conjugating enzyme causes the attachment of ubiquitin to a lysine on a target protein via an isopeptide bond; the E3 ubiquitin ligase targets specific protein substrates for degradation by the proteasome...
that recognizes the N-terminal trans-activation domain (TAD) of the
p53p53 , is a tumor suppressor protein that in humans is encoded by the TP53 gene. p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer...
tumor suppressor and an inhibitor of
p53p53 , is a tumor suppressor protein that in humans is encoded by the TP53 gene. p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer...
transcriptional activation.
Discovery and expression in tumor cells
The murine
double minuteDouble minutes are small fragments of extrachromosomal DNA, which have been observed in a large number of human tumors including breast, lung, ovary, colon, and most notably, neuroblastoma. They are a manifestation of gene amplification during the development of tumors, which give the cells...
(
mdm2)
oncogeneAn oncogene is a gene that has the potential to cause cancer. In tumor cells, they are often mutated or expressed at high levels.An oncogene is a gene found in the chromosomes of tumor cells whose activation is associated with the initial and continuing conversion of normal cells into cancer...
, which codes for the Mdm2 protein, was originally cloned, along with two other genes (mdm1 and mdm3) from the transformed mouse cell line 3T3-DM. Mdm2 overexpression, in cooperation with oncogenic Ras, promotes transformation of primary rodent fibroblasts, and
mdm2 expression led to tumor formation in nude mice. The human homologue of this protein was later identified and is sometimes called Hdm2. Further supporting the role of mdm2 as an
oncogeneAn oncogene is a gene that has the potential to cause cancer. In tumor cells, they are often mutated or expressed at high levels.An oncogene is a gene found in the chromosomes of tumor cells whose activation is associated with the initial and continuing conversion of normal cells into cancer...
, several human
tumorA tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...
types have been shown to have increased levels of Mdm2, including soft tissue sarcomas and osteosarcomas as well as breast tumors. An additional Mdm2 family member, Mdm4 (also called MdmX), has been discovered and is also an important negative regulator of
p53p53 , is a tumor suppressor protein that in humans is encoded by the TP53 gene. p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer...
.
Ubiquitination target: p53
The key target of Mdm2 is the
p53p53 , is a tumor suppressor protein that in humans is encoded by the TP53 gene. p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer...
tumor suppressor. Mdm2 has been identified as a p53 interacting protein that represses p53 transcriptional activity. Mdm2 achieves this repression by binding to and blocking the N-terminal trans-activation domain of p53. Mdm2 is a p53 responsive gene—that is, its transcription can be activated by p53. Thus when p53 is stabilized, the transcription of Mdm2 is also induced, resulting in higher Mdm2 protein levels.
E3 ligase activity
Mdm2 also acts as an
E3 ubiquitin ligaseA ubiquitin ligase is a protein that in combination with an E2 ubiquitin-conjugating enzyme causes the attachment of ubiquitin to a lysine on a target protein via an isopeptide bond; the E3 ubiquitin ligase targets specific protein substrates for degradation by the proteasome...
, targeting both itself and p53 for degradation by the
proteasomeProteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
(see also
UbiquitinUbiquitin is a small regulatory protein that has been found in almost all tissues of eukaryotic organisms. Among other functions, it directs protein recycling.Ubiquitin can be attached to proteins and label them for destruction...
). Several
lysineLysine is an α-amino acid with the chemical formula HO2CCH4NH2. It is an essential amino acid, which means that the human body cannot synthesize it. Its codons are AAA and AAG....
residues in p53 C-terminus have been identified as the sites of ubiquitination, and it has been shown that p53 protein levels are downregulated by Mdm2 in a proteasome-dependent manner. Mdm2 is capable of auto-polyubiquitination, and in complex with p300, a cooperating
E3 ubiquitin ligaseA ubiquitin ligase is a protein that in combination with an E2 ubiquitin-conjugating enzyme causes the attachment of ubiquitin to a lysine on a target protein via an isopeptide bond; the E3 ubiquitin ligase targets specific protein substrates for degradation by the proteasome...
, is capable of polyubiquitinating p53. In this manner, Mdm2 and p53 are the members of a negative feedback control loop that keeps the level of p53 low in the absence of p53-stabilizing signals. This loop can be interfered with by kinases and genes like
p14arfp14ARF is an alternate reading frame product of the CDKN2A locus. Both p16INK4a and p14ARF are involved in cell cycle regulation. p14ARF inhibits mdm2, thus promoting p53, which promotes p21 activation, which then binds and inactivates certain cyclin-CDK complexes, which would otherwise promote...
when p53 activation signals, including
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
damage, are high.
Structure and function
The full-length transcript of the mdm2 gene encodes a protein of 491 amino acids with a predicted molecular weight of 56kDa. This protein contains several conserved structural domains including an N-terminal p53 interaction domain, the structure of which has been solved using
x-ray crystallographyX-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...
. The Mdm2 protein also contains a central acidic domain (residues 230-300). The
phosphorylationPhosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
of residues within this domain appears to be important for regulation of Mdm2 function. In addition, this region contains nuclear export and import signals that are essential for proper nuclear-cytoplasmic trafficking of Mdm2. Another conserved domain within the Mdm2 protein is a
Zinc fingerZinc fingers are small protein structural motifs that can coordinate one or more zinc ions to help stabilize their folds. They can be classified into several different structural families and typically function as interaction modules that bind DNA, RNA, proteins, or small molecules...
domain, the function of which is poorly understood.
Mdm2 also contains a C-terminal RING domain (amino acid resdiues 430-480), which contains a Cis3-His2-Cis3 consensus that coordinates two molecules of
zincZinc , or spelter , is a metallic chemical element; it has the symbol Zn and atomic number 30. It is the first element in group 12 of the periodic table. Zinc is, in some respects, chemically similar to magnesium, because its ion is of similar size and its only common oxidation state is +2...
. These residues are required for zinc binding, which is essential for proper folding of the RING domain. The RING domain of Mdm2 confers
E3 ubiquitin ligaseA ubiquitin ligase is a protein that in combination with an E2 ubiquitin-conjugating enzyme causes the attachment of ubiquitin to a lysine on a target protein via an isopeptide bond; the E3 ubiquitin ligase targets specific protein substrates for degradation by the proteasome...
activity and is sufficient for E3 ligase activity in Mdm2 RING autoubiquitination. The RING domain of Mdm2 is unique in that it incorporates a conserved
Walker A or P-loopThe Walker A and Walker B motifs are protein sequence motifs. These were first reported in ATP-binding proteins by Walker and co-workers in 1982.-Walker A motif:...
motif characteristic of
nucleotideNucleotides are molecules that, when joined together, make up the structural units of RNA and DNA. In addition, nucleotides participate in cellular signaling , and are incorporated into important cofactors of enzymatic reactions...
binding proteins, as well as a nucleolar localization sequence. The RING domain also binds specifically to
RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
, although the function of this is poorly understood.
Regulation
There are several known mechanisms for regulation of Mdm2. One of these mechanisms is
phosphorylationPhosphorylation is the addition of a phosphate group to a protein or other organic molecule. Phosphorylation activates or deactivates many protein enzymes....
of the Mdm2 protein. Mdm2 is phosphorylated at multiple sites in cells. Following
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
damage, phosphorylation of Mdm2 leads to changes in protein function and stabilization of
p53p53 , is a tumor suppressor protein that in humans is encoded by the TP53 gene. p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer...
. Additionally, phosphorylation at certain residues within the central acidic domain of Mdm2 may stimulate its ability to target p53 for degradation. The induction of the
p14arfp14ARF is an alternate reading frame product of the CDKN2A locus. Both p16INK4a and p14ARF are involved in cell cycle regulation. p14ARF inhibits mdm2, thus promoting p53, which promotes p21 activation, which then binds and inactivates certain cyclin-CDK complexes, which would otherwise promote...
protein, the alternate reading frame product of the p16INK4a locus, is also a mechanism of negatively regulating the p53-Mdm2 interaction.
p14arfp14ARF is an alternate reading frame product of the CDKN2A locus. Both p16INK4a and p14ARF are involved in cell cycle regulation. p14ARF inhibits mdm2, thus promoting p53, which promotes p21 activation, which then binds and inactivates certain cyclin-CDK complexes, which would otherwise promote...
directly interacts with Mdm2 and leads to up-regulation of p53 transcriptional response. ARF sequesters Mdm2 in the
nucleolusThe nucleolus is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA is transcribed and assembled within the nucleolus...
, resulting in inhibition of nuclear export and activation of p53, since nuclear export is essential for proper p53 degradation.
Inhibitors of the MDM2-p53 interaction include the cis-imidazoline analog
nutlinNutlins are cis-imidazoline analogs which inhibit the interaction between mdm2 and tumour suppressor p53, and were discovered by screening a chemical library by Vassilev et al. Nutlin-1, Nutlin-2 and Nutlin-3 were all identified in the same screen, however Nutlin-3 is the compound most commonly...
.
Levels and stability of Mdm2 are also modulated by ubiquitylation. Mdm2 auto ubiquitylates itself, which allows for its degradation by the
proteasomeProteasomes are very large protein complexes inside all eukaryotes and archaea, and in some bacteria. In eukaryotes, they are located in the nucleus and the cytoplasm. The main function of the proteasome is to degrade unneeded or damaged proteins by proteolysis, a chemical reaction that breaks...
. Mdm2 also interacts with a ubiquitin specific protease,
USP7Ubiquitin-specific-processing protease 7 also known as ubiquitin carboxyl-terminal hydrolase 7 or herpesvirus-associated ubiquitin-specific protease is an enzyme that in humans is encoded by the USP7 gene....
, which can reverse Mdm2-ubiquitylation and prevent it from being degraded by the proteasome. It is interesting to note that
USP7Ubiquitin-specific-processing protease 7 also known as ubiquitin carboxyl-terminal hydrolase 7 or herpesvirus-associated ubiquitin-specific protease is an enzyme that in humans is encoded by the USP7 gene....
also protects from degradation the p53 protein, which is a major target of Mdm2. Thus Mdm2 and USP7 form an intricate circuit to finely regulate the stability and activity of p53, whose levels are critical for its function.
Interactions
Mdm2 has been shown to
interactProtein–protein interactions occur when two or more proteins bind together, often to carry out their biological function. Many of the most important molecular processes in the cell such as DNA replication are carried out by large molecular machines that are built from a large number of protein...
with
HDAC1Histone deacetylase 1 is an enzyme that in humans is encoded by the HDAC1 gene.-Interactions:HDAC1 has been shown to interact with RAD9A, DNMT3L, MTA1, FKBP3, CDC20, CDH1, Host cell factor C1, BUB3, Nuclear receptor co-repressor 2, IKZF1, Prohibitin, EVI1, DDX5, DNMT3A, Mothers against...
,
RPL2660S ribosomal protein L26 is a protein that in humans is encoded by the RPL26 gene.-Further reading:...
,
FKBP3FK506-binding protein 3 is a protein that in humans is encoded by the FKBP3 gene.-Interactions:FKBP3 has been shown to interact with YY1, HDAC1, Histone deacetylase 2 and Mdm2.-Further reading:...
,
CCNG1Cyclin-G1 is a protein that in humans is encoded by the CCNG1 gene.-Interactions:CCNG1 has been shown to interact with P16, Mdm2, PPP2R4 and P53.-Further reading:...
,
HTATIPHistone acetyltransferase KAT5 is an enzyme that in humans is encoded by the KAT5 gene.- Function :The protein encoded by this gene belongs to the MYST family of histone acetyl transferases and was originally isolated as an HIV-1 TAT-interactive protein...
,
GNL3Guanine nucleotide-binding protein-like 3 also known as nucleostemin is a protein that in humans is encoded by the GNL3 gene. It is found within the nucleolus that binds p53. Nucleostemin regulates the cell cycle and affects cell differentiation, decreasing in amount as this differentiation...
,
Death associated protein 6Death-associated protein 6 also known as DAXX is a protein that in humans is encoded by the DAXX gene.- Function :DAXX, a death-domain-associated protein, was first discovered through its cytoplasmic interaction with the classical death receptor Fas...
,
PSME3Proteasome activator complex subunit 3 is a protein that in humans is encoded by the PSME3 gene.-Interactions:PSME3 has been shown to interact with P53 and Mdm2.-Further reading:...
, Insulin-like growth factor 1 receptor,
RRM2BRibonucleoside-diphosphate reductase subunit M2 B is an enzyme that in humans is encoded by the RRM2B gene.-Interactions:RRM2B has been shown to interact with Mdm2 and Ataxia telangiectasia mutated.-Further reading:...
,
FOXO4Forkhead box protein O4 is a protein that in humans is encoded by the FOXO4 gene.-Interactions:FOXO4 has been shown to interact with PIN1 and Mdm2.-Further reading:...
, Ribosomal protein L5,
Abl geneV-abl Abelson murine leukemia viral oncogene homolog 1 also known as ABL1 is a protein that, in humans, is encoded by the ABL1 gene located on chromosome 9.- Function :...
,
RYBPRING1 and YY1-binding protein is a protein that in humans is encoded by the RYBP gene.-Interactions:RYBP has been shown to interact with RING1, YY1, Caspase 10, E2F3, E2F2, Mdm2, Abl gene and CBX2.-Further reading:...
,
HIF1AHypoxia-inducible factor 1, alpha subunit , also known as HIF1A, is a protein that in humans is encoded by the HIF1A gene...
,
PCAFP300/CBP-associated factor , also known as K acetyltransferase 2B , is a human gene and trancriptional coactivator associated with p53.-Structure:...
,
TATA binding proteinThe TATA-binding protein is a general transcription factor that binds specifically to a DNA sequence called the TATA box. This DNA sequence is found about 35 base pairs upstream of the transcription start site in some eukaryotic gene promoters...
,
P73p73 is a protein related to the p53 tumor protein. Because of its structural resemblance to p53, it has also been considered a tumor suppressor. It is involved in cell cycle regulation, and induction of apoptosis. Like p53, p73 is characterized by the presence of different isoforms of the protein...
,
CTBP2C-terminal-binding protein 2 also known as CtBP2 is a protein that in humans is encoded by the CTBP2 gene.- Function :This gene produces alternative transcripts encoding two distinct proteins. One protein is a transcriptional repressor , while the other isoform is a major component of specialized...
,
NUMBProtein numb homolog is a protein that in humans is encoded by the NUMB gene.-Interactions:NUMB has been shown to interact with Adaptor-related protein complex 2, alpha 1, Mdm2, L1, DPYSL2, SIAH1, P53 and LNX1.-Further reading:...
,
P53p53 , is a tumor suppressor protein that in humans is encoded by the TP53 gene. p53 is crucial in multicellular organisms, where it regulates the cell cycle and, thus, functions as a tumor suppressor that is involved in preventing cancer...
,
P16Cyclin-dependent kinase inhibitor 2A, also known as multiple tumor suppressor 1 , is a tumor suppressor protein, that in humans is encoded by the CDKN2A gene...
,
PSMD1026S proteasome non-ATPase regulatory subunit 10 is an enzyme that in humans is encoded by the PSMD10 gene.-Interactions:PSMD10 has been shown to interact with PAAF1, Mdm2 and PSMC4.-Further reading:...
,
EP300E1A binding protein p300 also known as EP300 or p300 is a protein that, in humans, is encoded by the EP300 gene. This protein regulates the activity of many genes in tissues throughout the body...
,
CTBP1C-terminal-binding protein 1 also known as CtBP1 is a protein that in humans is encoded by the CTBP1 gene.- Function :The CtBP1 protein binds to the C-terminus of adenovirus E1A proteins. This phosphoprotein is a transcriptional repressor and may play a role during cellular proliferation...
,
MDM4Protein Mdm4 is a protein that in humans is encoded by the MDM4 gene.-Interactions:MDM4 has been shown to interact with E2F1, Mdm2 and P53.-Further reading:...
,
RPL1160S ribosomal protein L11 is a protein that in humans is encoded by the RPL11 gene.-Interactions:RPL11 has been shown to interact with P16, BLMH, Mdm2, Promyelocytic leukemia protein and P53.-External links:*...
,
Promyelocytic leukemia proteinProbable transcription factor PML is a tumor suppressor protein that in humans is encoded by the PML gene.-Interactions:Promyelocytic leukemia protein has been shown to interact with Retinoic acid receptor alpha, HDAC1, Nerve Growth factor IB, SKI protein, Zinc finger and BTB domain-containing...
,
Dihydrofolate reductase- Function :Dihydrofolate reductase converts dihydrofolate into tetrahydrofolate, a methyl group shuttle required for the de novo synthesis of purines, thymidylic acid, and certain amino acids...
,
Arrestin beta 2Beta-arrestin-2, also known as arrestin beta-2, is an intracellular protein that in humans is encoded by the ARRB2 gene.Members of arrestin/beta-arrestin protein family are thought to participate in agonist-mediated desensitization of G protein-coupled receptors and cause specific dampening of...
,
Arrestin beta 1Arrestin, beta 1, also known as ARRB1, is a protein which in humans is encoded by the ARRB1 gene.- Function :Members of arrestin/beta-arrestin protein family are thought to participate in agonist-mediated desensitization of G protein-coupled receptors and cause specific dampening of cellular...
and
Ubiquitin CUbiquitin is a protein that in humans is encoded by the UBC gene.-Interactions:Ubiquitin C has been shown to interact with SCNN1A, SCNN1G, Parkin , P70-S6 Kinase 1, TRAF6, HDAC3, SFPQ, S100A10, Mothers against decapentaplegic homolog 3, NOTCH1, HIF1A, Epidermal growth factor receptor, E2F1,...
.
External links