Activity
MED26 is a transcription elongation factor that increases the overall transcription rate of RNA polymerase II by reactivating transcription elongation complexes that have arrested transcription. It does this through recruiting ELL/EAF- and P-TEFb- containing complexes to promoters via a direct interaction with the N-terminal domain (NTD). The MED26 NTD also binds TFIID, and TFIID and elongation complexes interact with MED26 through overlapping binding sites. [7] MED26 NTD may function as a molecular switch contributing to the transition of Pol II into productive elongation.
The three structural domains of TFIIS are conserved from yeast to human. The 80 or so N-terminal residues form a protein interaction domain containing a conserved motif, which has been called the LW motif because of the invariant leucine and tryptophan residues it contains. Although the N-terminal domain is not needed for transcriptional activity, a similar sequence has been identified in other transcription factors and proteins that are predominantly nuclear localized. [8] [9] [10] Specific examples are listed below:
- MED26 (also known as CRSP70 and ARC70), a subunit of the Mediator complex, which is required for the activity of the enhancer-binding protein Sp1.
- Elongin A, a subunit of a transcription elongation factor previously known as SIII. It increases the rate of transcription by suppressing transient pausing of the elongation complex.
- PPP1R10, a nuclear regulatory subunit of protein phosphatase 1 that was previously known as p99, FB19 or PNUTS.
- PIBP, a small hypothetical protein that could be a phosphoinositide binding protein.
- IWS1, which is thought to function in both transcription initiation and elongation. [11]
- TFIIS, which rescues RNA polymerase II from backtracked pause states.
The N-terminal domain of MED26 is a protein fold known as a TFIIS N-terminal domain (or TND). [8] It is a compact five-helix bundle. The hydrophobic core residues of helices 2, 3, and 4 are well conserved among TFIIS domains, although helix 1 is less conserved. [10]
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