![Mediator4TC](https://upload.wikimedia.org/wikipedia/commons/5/54/Mediator4TC.jpg)
Mediator is a
multiprotein complex that functions as a
transcriptional coactivator
A coactivator is a type of transcriptional coregulator that binds to an activator (a transcription factor) to increase the rate of transcription of a gene or set of genes. The activator contains a DNA binding domain that binds either to a DNA ...
in all
eukaryotes
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
. It was discovered in 1990 in the lab of
Roger D. Kornberg
Roger David Kornberg (born April 24, 1947) is an American biochemist and professor of structural biology at Stanford University School of Medicine. Kornberg was awarded the Nobel Prize in Chemistry in 2006 for his studies of the process by which ...
, recipient of the 2006
Nobel Prize in Chemistry
)
, image = Nobel Prize.png
, alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
. Mediator complexes interact with
transcription factors
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The func ...
and
RNA polymerase II
RNA polymerase II (RNAP II and Pol II) is a multiprotein complex that transcribes DNA into precursors of messenger RNA (mRNA) and most small nuclear RNA (snRNA) and microRNA. It is one of the three RNAP enzymes found in the nucleus of eukaryo ...
. The main function of mediator complexes is to transmit signals from the transcription factors to the polymerase.
Mediator complexes are variable at the evolutionary, compositional and conformational levels.
The first image shows only one "snapshot" of what a particular mediator complex might be composed of, but it certainly does not accurately depict the conformation of the complex ''in vivo''. During evolution, mediator has become more complex. The yeast ''
Saccharomyces cerevisiae
''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have been o ...
'' (a simple
eukaryote
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
) is thought to have up to 21 subunits in the core mediator (exclusive of the CDK module), while mammals have up to 26.
Individual subunits can be absent or replaced by other subunits under different conditions. Also, there are many
intrinsically disordered regions in mediator proteins, which may contribute to the conformational flexibility seen both with and without other bound proteins or protein complexes. A more realistic model of a mediator complex without the CDK module is shown in the second figure.
The mediator complex is required for the successful
transcription
Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including:
Genetics
* Transcription (biology), the copying of DNA into RNA, the fir ...
by RNA polymerase II. Mediator has been shown to make contacts with the polymerase in the
transcription preinitiation complex
The preinitiation complex (abbreviated PIC) is a complex of approximately 100 proteins that is necessary for the transcription of protein-coding genes in eukaryotes and archaea. The preinitiation complex positions RNA polymerase II at gene transc ...
.
A recent model showing the association of the polymerase with mediator in the absence of DNA is shown in the figure to the left.
[ In addition to RNA polymerase II, mediator must also associate with transcription factors and DNA. A model of such interactions is shown in the figure to the right. Note that the different morphologies of mediator do not necessarily mean that one of the models is correct; rather those differences may reflect the flexibility of mediator as it interacts with other molecules. For example, after binding the enhancer and core promoter, the mediator complex undergoes a compositional change in which the ]kinase
In biochemistry, a kinase () is an enzyme that catalyzes the transfer of phosphate groups from high-energy, phosphate-donating molecules to specific substrates. This process is known as phosphorylation, where the high-energy ATP molecule don ...
module dissociates from the complex to allow association with RNA polymerase II
RNA polymerase II (RNAP II and Pol II) is a multiprotein complex that transcribes DNA into precursors of messenger RNA (mRNA) and most small nuclear RNA (snRNA) and microRNA. It is one of the three RNAP enzymes found in the nucleus of eukaryo ...
and transcriptional activation.
The Mediator complex is located within the cell nucleus
The cell nucleus (pl. nuclei; from Latin or , meaning ''kernel'' or ''seed'') is a membrane-bound organelle found in eukaryotic cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, h ...
. It is required for the successful transcription
Transcription refers to the process of converting sounds (voice, music etc.) into letters or musical notes, or producing a copy of something in another medium, including:
Genetics
* Transcription (biology), the copying of DNA into RNA, the fir ...
of nearly all class II gene promoters in yeast. It works in the same manner in mammals. The mediator functions as a coactivator and binds to the C-terminal domain
The C-terminus (also known as the carboxyl-terminus, carboxy-terminus, C-terminal tail, C-terminal end, or COOH-terminus) is the end of an amino acid chain (protein or polypeptide), terminated by a free carboxyl group (-COOH). When the protein is ...
of RNA polymerase II
RNA polymerase II (RNAP II and Pol II) is a multiprotein complex that transcribes DNA into precursors of messenger RNA (mRNA) and most small nuclear RNA (snRNA) and microRNA. It is one of the three RNAP enzymes found in the nucleus of eukaryo ...
holoenzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. ...
, acting as a bridge between this enzyme and transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...
s.
Structure
The yeast mediator complex is approximately as massive as a small subunit of a eukaryotic ribosome. The yeast mediator is composed of 25 subunits, while the mammalian mediator complexes are slightly larger.[ Mediator can be divided into 4 main parts: The head, middle, tail, and the transiently associated CDK8 kinase module.]
Mediator subunits have many intrinsically disordered regions called "splines", which may be important to allow the structural changes of the mediator that change the function of the complex.[ The figure shows how the splines of the Med 14 subunit connect a large portion of the complex together while still allowing flexibility.][
Mediator complexes that lack a subunit have been found or produced. These smaller mediators can still function normally in some activity, but lack other capabilities.][ This indicates a somewhat independent function of some of the subunits while being part of the larger complex.
Another example of structural variability is seen in vertebrates, in which 3 ]paralogues
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a s ...
of subunits of the cyclin-dependent kinase module have evolved by 3 independent gene duplication
Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene. ...
events followed by sequence divergence.[ ]
There is a report that mediator forms stable associations with a particular type of non-coding RNA
A non-coding RNA (ncRNA) is a functional RNA molecule that is not Translation (genetics), translated into a protein. The DNA sequence from which a functional non-coding RNA is transcribed is often called an RNA gene. Abundant and functionally im ...
, ncRNA-a. These stable associations have also been shown to regulate gene expression ''in vivo'', and are prevented by mutations in MED12 that produce the human disease FG syndrome
FG syndrome (FGS) is a rare genetic syndrome caused by one or more recessive genes located on the X chromosome and causing physical anomalies and developmental delays. FG syndrome was named after the first letters of the surnames of the first pat ...
. Thus, the structure of a mediator complex can be augmented by RNA as well as proteinaceous transcription factors.[
]
Function
Mediator was originally discovered because it was important for RNA polymerase II function, but it has many more functions than just interactions at the transcription start site.[
]
RNA polymerase II-Mediator core initiation complex
Mediator is a crucial component for transcription initiation. Mediator interacts with the pre-initiation complex, composed of RNA Polymerase II and general transcription factors TFIIB, TFIID, TFIIE, TFIIF, and TFIIH to stabilize and initiate transcription. Studies of Mediator-RNA Pol II contacts in budding yeast have emphasized the importance of TFIIB-Mediator contacts in the formation of the complex. Interactions of Mediator with TFIID in the initiation complex has been shown.
The Structure of a core Mediator (cMed) that's associated with a core pre-initiation complex was elucidated.
RNA synthesis
The preinitiation complex, which contains mediator, transcription factors, a nucleosome and RNA polymerase II, is important to position the polymerase for the start of transcription. Before RNA synthesis can occur, the polymerase must dissociate from mediator. This appears to be accomplished by phosphorylation of part of the polymerase by a kinase. Importantly, mediator and transcription factors do not dissociate from the DNA at the time polymerase begins transcription. Rather, the complex remains at the promoter to recruit another RNA polymerase to begin another round of transcription.
There is some evidence to suggest that mediator in a yeast is involved in regulating RNA polymerase III
In eukaryote cells, RNA polymerase III (also called Pol III) is a protein that transcribes DNA to synthesize ribosomal 5S rRNA, tRNA and other small RNAs.
The genes transcribed by RNA Pol III fall in the category of "housekeeping" genes whose e ...
(Pol III) transcripts of tRNA
Transfer RNA (abbreviated tRNA and formerly referred to as sRNA, for soluble RNA) is an adaptor molecule composed of RNA, typically 76 to 90 nucleotides in length (in eukaryotes), that serves as the physical link between the mRNA and the amino ac ...
s In support of that evidence, an independent report showed specific association of mediator with Pol III in ''Saccharomyces cerevisiae''. Those authors also reported specific associations with RNA polymerase I
RNA polymerase 1 (also known as Pol I) is, in higher eukaryotes, the polymerase that only transcribes ribosomal RNA (but not 5S rRNA, which is synthesized by RNA polymerase III), a type of RNA that accounts for over 50% of the total RNA synthesize ...
and proteins involved in transcription elongation and RNA processing, supporting other evidence of mediator's involvement in elongation and processing.[
]
Chromatin organization
Mediator is involved in "looping" of chromatin
Chromatin is a complex of DNA and protein found in eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser structures. This prevents the strands from becoming tangled and also plays important roles in r ...
, which brings distant regions of a chromosome into closer physical proximity. The ncRNA-a mentioned above is involved in such looping. Enhancer RNAs
Enhancer RNAs (eRNAs) represent a class of relatively long non-coding RNA molecules (50-2000 nucleotides) transcribed from the DNA sequence of enhancer regions. They were first detected in 2010 through the use of genome-wide techniques such as RN ...
(eRNAs) can function similarly.
In addition to the looping of euchromatin
Euchromatin (also called "open chromatin") is a lightly packed form of chromatin ( DNA, RNA, and protein) that is enriched in genes, and is often (but not always) under active transcription. Euchromatin stands in contrast to heterochromatin, whic ...
, mediator appears to be involved in formation or maintenance of heterochromatin
Heterochromatin is a tightly packed form of DNA or '' condensed DNA'', which comes in multiple varieties. These varieties lie on a continue between the two extremes of constitutive heterochromatin and facultative heterochromatin. Both play a role ...
at centromere
The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
s and telomere
A telomere (; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Although there are different architectures, telomeres, in a broad sense, are a widespread genetic feature mos ...
s.
Signal transduction
TGFβ signaling at the cell membrane results in 2 different intracellular pathways. One of them depends on MED15, while the other is independent of MED15. In both human cells and ''Caenorhabditis elegans
''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (ro ...
'' MED15 is involved in lipid homeostasis through the pathway involving SREBPs In the model plant ''Arabidopsis thaliana
''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land.
A winter a ...
'' the ortholog
Sequence homology is the biological homology between DNA, RNA, or protein sequences, defined in terms of shared ancestry in the evolutionary history of life. Two segments of DNA can have shared ancestry because of three phenomena: either a spec ...
of MED15 is required for signaling by the plant hormone
Plant hormone (or phytohormones) are signal molecules, produced within plants, that occur in extremely low concentrations. Plant hormones control all aspects of plant growth and development, from embryogenesis, the regulation of organ size, pa ...
Salicylic acid, while MED25 is required for the transcriptional activation of jasmonate and shade signalling responses. Two components of the CDK module (MED12 and MED13) are involved in the Wnt signaling pathway
The Wnt signaling pathways are a group of signal transduction pathways which begin with proteins that pass signals into a cell through cell surface receptors. The name Wnt is a portmanteau created from the names Wingless and Int-1. Wnt signaling p ...
[ MED23 is involved in ]RAS
Ras or RAS may refer to:
Arts and media
* RAS Records Real Authentic Sound, a reggae record label
* Rundfunk Anstalt Südtirol, a south Tyrolese public broadcasting service
* Rás 1, an Icelandic radio station
* Rás 2, an Icelandic radio stati ...
/MAPK/ERK pathway
The MAPK/ERK pathway (also known as the Ras-Raf-MEK-ERK pathway) is a chain of proteins in the cell that communicates a signal from a receptor on the surface of the cell to the DNA in the nucleus of the cell.
The signal starts when a signaling ...
[ This abbreviated review shows the versatility of individual mediator subunits, and leads to the idea that mediator is an end-point of signaling pathways.][
]
Human disease
Involvement of mediator in various human diseases has been reviewed. Since inhibiting one interaction of a disease-causing signaling pathway with a subunit of mediator may not inhibit general transcription needed for normal function, mediator subunits are attractive candidates for therapeutic drugs.[
]
Interactions
A method employing very gentle cell lysis in yeast followed by co- immunoprecipitation with an antibody to a mediator subunit (Med 17) has confirmed almost all previously reported or predicted interactions and revealed many previously unsuspected specific interactions of various proteins with mediator.[
]
MED 1
A discussion of all mediator subunits is beyond the scope of this article, but details of one of the subunits is illustrative of the types of information that may be gathered for other subunits.
regulation by Micro RNAs
Micro RNAs are involved in regulating the expression of many proteins. Med1 is targeted by miR-1, which is important in gene regulation in cancers. The tumor suppressor
A tumor suppressor gene (TSG), or anti-oncogene, is a gene that regulates a cell during cell division and replication. If the cell grows uncontrollably, it will result in cancer. When a tumor suppressor gene is mutated, it results in a loss or red ...
miR-137 also regulates MED1.
Mouse embryonic development
Null mutants die at an early gestational age (embryonic day 11.5). By investigating hypomorphic mutants (which can survive 2 days longer), it was found that placental defects were primarily lethal and that there were also defects in cardiac and hepatic development, but many other organs were normal[
]
Mouse cells and tissues
Conditional mutations can be produced in mice which affect only specific cells or tissues at specific times, so that the mouse can develop to adulthood and the adult phenotype
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...
can be studied. In one case, MED1 was found to participate in controlling the timing of events of meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...
in male mice. Conditional mutants in keratinocyte
Keratinocytes are the primary type of Cell (biology), cell found in the epidermis (skin), epidermis, the outermost layer of the skin. In humans, they constitute 90% of epidermal skin cells.
Basal cells in the stratum basale, basal layer (''str ...
s show differences in skin wound healing. A conditional mutant in mice was found to change dental epithelium
Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
into epidermal
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...
epithelium, which caused hair to grow associated with the incisors.
Subunit composition
The Mediator complex is composed at least 31 subunits in all eukaryotes studied: MED1
Mediator of RNA polymerase II transcription subunit 1 also known as DRIP205 or Trap220 is a subunit of the Mediator complex and is a protein that in humans is encoded by the ''MED1'' gene. MED1 functions as a nuclear receptor coactivator.
Func ...
, MED4, MED6
Mediator of RNA polymerase II transcription subunit 6 is one of the subunits of the Mediator complex. It is an enzyme that in humans is encoded by the ''MED6'' gene.
Protein family
This family of proteins represent the transcriptional mediat ...
, MED7
Mediator of RNA polymerase II transcription subunit 7 is an enzyme that in humans is encoded by the ''MED7'' gene.
The activation of gene transcription is a multistep process that is triggered by factors that recognize transcriptional enhancer s ...
, MED8
Mediator of RNA polymerase II transcription subunit 8 is an enzyme that in humans is encoded by the ''MED8'' gene.
Function
This gene encodes a protein that is one of more than 20 subunits of the mediator complex, first identified in S. cerevi ...
, MED9
Mediator complex subunit 9 (Med9) is a protein that in humans is encoded by the MED9 gene.
Function
The multiprotein Mediator complex is a coactivator required for activation of RNA polymerase II transcription by DNA bound transcription fact ...
, MED10
Mediator complex subunit 10 (Med10) is a protein that in humans is encoded by the MED10 gene.
Function
Med10 is a component of the Mediator complex, which is a coactivator for DNA-binding factors that activate transcription via RNA polymerase ...
, MED11
Mediator complex subunit 11 (Med11) is a protein that in humans is encoded by the MED11 gene.
Function
Med11 is a component of the Mediator (coactivator), Mediator complex, which is a coactivator for DNA-binding factors that activate transcrip ...
, MED12
Mediator of RNA polymerase II transcription, subunit 12 homolog (S. cerevisiae), also known as MED12, is a human gene found on the X chromosome.
Clinical significance
Mutations in ''MED12'' are responsible for at least two different forms of X- ...
, MED13, MED13L, MED14
Mediator of RNA polymerase II transcription subunit 14 is an enzyme that in humans is encoded by the ''MED14'' gene.
The activation of gene transcription is a multistep process that is triggered by factors that recognize transcriptional enhancer ...
, MED15
Mediator of RNA polymerase II transcription subunit 15, also known as Gal11, Spt13 in yeast and PCQAP, ARC105, or TIG-1 in humans is a protein encoded by the ''MED15'' gene.
Function
MED15 is a general transcriptional cofactor of the mediator ...
, MED16
Mediator of RNA polymerase II transcription subunit 16 is an enzyme that in humans is encoded by the ''MED16'' gene.
Interactions
MED16 has been shown to interact with Thyroid hormone receptor alpha, Estrogen receptor alpha
Estrogen receptor ...
, MED17, MED18, MED19, MED20, MED21
Mediator of RNA polymerase II transcription subunit 21 is an enzyme that in humans is encoded by the ''MED21'' gene.
Interactions
MED21 has been shown to interact with:
* BRCA1,
* CDK8,
* GTF2F1,
* GTF2H4,
* MED6, and
* POLR2A
DNA-directed ...
, MED22
Mediator of RNA polymerase II transcription subunit 22 is an enzyme that in humans is encoded by the ''MED22'' gene.
Function
This gene is located in the surfeit gene cluster, a group of very tightly linked housekeeping genes that do not share ...
, MED23, MED24
Mediator of RNA polymerase II transcription subunit 24 is an enzyme that in humans is encoded by the ''MED24'' gene.
Function
This gene encodes a component of the mediator complex (also known as TRAP, SMCC, DRIP, or ARC), a transcriptional coa ...
, MED25
Mediator of RNA polymerase II transcription subunit 25 is an enzyme that in humans is encoded by the ''MED25'' gene.
Interactions
MED25 has been shown to Protein-protein interaction, interact with MED4.
References
Further reading
*
* ...
, MED26
Mediator of RNA polymerase II transcription subunit 26 is an enzyme that in humans is encoded by the ''MED26'' gene.
It forms part of the Mediator complex.
The activation of gene transcription is a multistep process that is triggered by factor ...
, MED27
Mediator of RNA polymerase II transcription subunit 27 is an enzyme that in humans is encoded by the ''MED27'' gene. It forms part of the Mediator complex.
The ubiquitous expression of Med27 mRNA suggests a universal requirement for Med27 in tra ...
, MED28
Mediator of RNA polymerase II transcription subunit 28 is an enzyme that in humans is encoded by the ''MED28'' gene. It forms part of the Mediator complex.
Function
Subunit Med28 of the Mediator may function as a scaffolding protein within ...
, MED29, MED30
Mediator of RNA polymerase II transcription subunit 30 is an enzyme that in humans is encoded by the ''MED30'' gene. It represents subunit Med30 of the Mediator_(coactivator), Mediator complex and is metazoan-specific, having no Homology (biology ...
, MED31, CCNC, and CDK8
Cell division protein kinase 8 is an enzyme that in humans is encoded by the ''CDK8'' gene.
Function
The protein encoded by this gene is a member of the cyclin-dependent protein kinase (CDK) family. CDK8 and cyclin C associate with the mediato ...
. There are three fungal-specific components, referred to as Med2, Med3 and Med5.
The subunits form at least three structurally
A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...
distinct submodules. The head and the middle modules interact
Advocates for Informed Choice, dba interACT or interACT Advocates for Intersex Youth, is a 501(c)(3) nonprofit organization using innovative strategies to advocate for the legal and human rights of children with intersex traits. The organizati ...
directly with RNA polymerase II, whereas the elongated tail module interact
Advocates for Informed Choice, dba interACT or interACT Advocates for Intersex Youth, is a 501(c)(3) nonprofit organization using innovative strategies to advocate for the legal and human rights of children with intersex traits. The organizati ...
s with gene-specific regulatory proteins
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
. Mediator containing the CDK8 module is less active than Mediator lacking this module in supporting transcriptional activation.
*The head module contains: MED6, MED8, MED11, SRB4/MED17, SRB5/MED18, ROX3/MED19, SRB2/MED20 and SRB6/MED22.
*The middle module contains: MED1, MED4, NUT1/MED5, MED7, CSE2/MED9, NUT2/MED10, SRB7/MED21 and SOH1/MED31. CSE2/MED9 interacts directly with MED4.
*The tail module contains: MED2, PGD1/MED3, RGR1/MED14, GAL11/MED15 and SIN4/MED16.
*The CDK8 module contains: MED12, MED13, CCNC and CDK8. Individual preparations of the Mediator complex lacking one or more distinct subunits have been variously termed ARC, CRSP, DRIP, PC2, SMCC and TRAP.
In other species
Below is a cross-species comparison of mediator complex subunits.[Gene names derived from ]
Notes
References
{{Transcription coregulators
Protein complexes
Gene expression