Complex coactivator/mediator

Another important coactivator consists of 20 or more polypeptides in a protein complex called mediator (Fig. 28-27) the 20 core polypeptides are highly conserved from fungi to humans. Mediator binds tightly to the carboxyl-terminal domain (CTD) of the largest subunit of Pol II. The mediator complex is required for both basal and regulated transcription at promoters used by Pol II, and it also stimulates the phosphorylation of the CTD by TFIIH. Both mediator and TFIID are required at some promoters. As with TFIID, some DNA-binding transactivators interact with one or more components of the mediator complex. Coactivator complexes function at or near the promoter s TATA box. 

RNA polymerase II can be isolated from the cell in various forms. From yeast and from metazooans, holoenzyme forms of RNA polymerase have been purified that are composed of the core of RNA polymerase II and other accessory transcription factors, suggesting that a preassembled transcription apparatus can be assembled at the promotor in one step. These holoenzymes are formed from the core of RNA polymerase associated to a variable degree with the general transcription factors and with one or more other subunit complexes called mediators or coactivators. 

The pi60 coactivators themselves interact with other proteins or protein complexes with mediator function and histone acetylase activity. Particularly, an interaction with the CBP/p300 protein and the PCAF histone acetylase (see Section 1.4.6.) has been shown. It is thought that the primary function of the pl60 class of coactivators is... 

Muliprotein complexes that do not directly bind DNA, but are recruited by sequence-specific transcription factors and mediate their capacity to activate genes (coactivators) and to repress genes (corepressors). 

The transcriptional activity of NRs is also modulated by various posttranslational modifications of the receptors themselves or of their coregulatory proteins. Phosphorylation, as well as several other types of modification, such as acetylation, SUMOylation, ubiquitinylation, and methylation, has been reported to modulate the functions of NRs, potentially constituting an important cellular integration mechanism. In addition to the modifications of the receptors themselves, such modifications have been reported for their coactivators and corepressors. Therefore, these different modes of regulation reveal an unexpected complexity of the dynamics of NR-mediated transcription. 

Beside coactivators so-called corepressors exist that are bound to transcription factors such as nuclear receptors and inhibit the initiation of transcription. These factors include the nuclear receptor corepressor (NCoR) and the silencing mediator of retinoic acid and thyroid hormone receptor (SMRT), which interact with nuclear receptors and serve as platforms for complexes containing histone deacetylases (HDACs). These enzymes cause the reversal of histone acetylation of histones leading to a tightening of chromatin and enhancing its inaccessibility for RNA polymerase containing complexes. 

Figure 43-11. The hormone response transcription unit. The hormone response transcription unit is an assembly of DNA elements and bound proteins that interact, through protein-protein interactions, with a number of coactivator or corepressor molecules. An essential component is the hormone response element which binds the ligand (A)-bound receptor (R). Also Important are the accessory factor elements (AFEs) with bound transcription factors. More than two dozen of these accessory factors (AFs), which are often members of the nuclear receptor superfamily, have been linked to hormone effects on transcription. The AFs can interact with each other, with the liganded nuclear receptors, or with coregulators. These components communicate with the basal transcription complex through a coregulator complex that can consist of one or more members of the pi 60, corepressor, mediator-related, or CBP/p300 families (see Table 43-6).

A small number of proteins, including NCoR and SMRT, comprise the corepressor family. They function, at least in part, as described in Figure 43-2. Another family includes the TRAPs, DRIPs, and ARC (Table 43-6). These so-called mediator-related proteins range in size from 80 kDa to 240 kDa and are thought to be involved in linking the nuclear receptor-coactivator complex to RNA polymerase II and the other components of the basal transcription apparatus. 

In order for estrogen-mediated genomic activation to occur, the ligand-ER complex must bind to other nuclear proteins (coregulator proteins) that can either act as coactivators (stimulators of gene transcription) or corepressors (inhibitors of gene transcription) for access of the complex to the EREs (for... 

Some of the coactivators are ascribed a mediator function between the DNA-boimd transcriptional activators and the pre-initiation complex, while others are attributed an active role in the restructuring of the chromatin. 

Fig. 1.34 Activators and coactivators of transcription intiation. The figure shows the function of three groups of proteins that function as coactivators. The general cofactors mediate the interactions between the specific transcription activators and the TFIID complex as well as with various forms of the RNA polymerase II holoenzyme. The TAFs are components of the TFIID complex and serve as contact points for specific transcription activators. The mediators are components of various forms of holoenzymes of RNA polymerase II. SRB proteins belong to the class of mediators, which, among other things, interacts with the CTD of RNA polymerase. The simphfied diagram does not show the interactions with chromatin.

Common mediator Smads The mode of action of Smad 4 clearly differs from that of the other members of the Smad family. Smad 4 binds to phosphorylated pathway-restricted Smads and forms hetero-ohgomeric complexes. These translocate to the nucleus where they bind to related DNA elements and activate the transcription of target genes, e.g., the genes for the CDK inhibitors pl5 and p21 (see Chapter 13) (Fig. new). Activation of transcription often occurs in cooperation with other coactivators (see 1.4.3.2). 

The effects of DNA-binding transactivators on Pol II are mediated by coactivator protein complexes such as TFIID or mediator. The modular structures of the transactivators have distinct activation and DNA-binding domains. Other protein complexes, including histone acetyltransferases such as GCN5-ADA2-ADA3 and ATP-dependent complexes such as SWI/SNF and NURF, reversibly remodel chromatin structure. 

Mediators and coactivators. Transcriptional activators that act in a crude cell-free system often do not function with purified DNA, RNA polymerase, and the basal transcription factors as indicated in Eq. 28-5. Studies with yeast, Drosophila, and human cells revealed that additional large multisubunit complexes known as mediators are needed 272/346-348 A yeast mediator complex consists of 20 subunits.349-350b Many activator proteins bind to the DNA sequences known as enchancers, discussed in the next section. Mediator complexes may also interact with enhancer-bound activators. Individual proteins, such as the TAF subunits, that bind to and cooperate with activator proteins are often called coactivators.351... 

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