Transcription coregulator

Histone acetyltransferases

Nuclear DNA is normally tightly wrapped around histones rendering the DNA inaccessible to the general transcription machinery and hence this tight association prevents transcription of DNA. At physiological pH, the phosphate component of the DNA backbone is deprotonated which gives DNA a net negative charge. Histones are rich in lysine residues which at physiological pH are protonated and therefore positively charged. The electrostatic attraction between these opposite charges is largely responsible for the tight binding of DNA to histones.

Many coactivator proteins have intrinsic histone acetyltransferase (HAT) catalytic activity or recruit other proteins with this activity to promoters. These HAT proteins are able to acetylate the amine group in the sidechain of histone lysine residues which makes lysine much less basic, not protonated at physiological pH, and therefore neutralizes the positive charges in the histone proteins. This charge neutralization weakens the binding of DNA to histones causing the DNA to unwind from the histone proteins and thereby significantly increases the rate of transcription of this DNA.

Many corepressors can recruit histone deacetylase (HDAC) enzymes to promoters. These enzymes catalyze the hydrolysis of acetylated lysine residues restoring the positive charge to histone proteins and hence the tie between histone and DNA. PELP-1 can act as a transcriptional corepressor for transcription factors in the nuclear receptor family such as glucocorticoid receptors.

Nuclear receptor coactivators

Nuclear receptors bind to coactivators in a ligand-dependent manner. A common feature of nuclear receptor coactivators is that they contain one or more LXXLL binding motifs (a contiguous sequence of 5 amino acids where L = leucine and X = any amino acid) referred to as NR (nuclear receptor) boxes. The LXXLL binding motifs have been shown by X-ray crystallography to bind to a groove on the surface of ligand binding domain of nuclear receptors. Examples include:

• ARA (androgen receptor associated protein)
  • ARA54 ()
  • ARA55 ()
  • ARA70 ()
• AIRE
• BCAS3 (breast carcinoma amplified sequence 3)
• CREB-binding protein
• CRTC (CREB regulated transcription coactivator)
  • CRTC1 ()
  • CRTC2 ()
  • CRTC3 ()
• CARM1 (coactivator-associated arginine methyltransferase 1)
• Nuclear receptor coactivator (NCOA)
  • NCOA1/SRC-1 (steroid receptor coactivator-1)/
  • NCOA2/GRIP1 (glucocorticoid receptor interacting protein 1)/ TIF2 (transcriptional intermediary factor 2)
  • NCOA3/AIB1 (amplified in breast)
  • NCOA4/ARA70 (androgen receptor associated protein 70)
  • NCOA5 ()
  • NCOA6 ()
  • NCOA7 ()
• p300
• PCAF (p300/CBP associating factor)
• PGC1 (proliferator activated receptor gamma coactivator 1)
  • PPARGC1A ()
  • PPARGC1B ()
• PNRC (proline-rich nuclear receptor coactivator 1)
  • PNRC1 ()
  • PNRC2 ()

Nuclear receptor corepressors

Corepressor proteins also bind to the surface of the ligand binding domain of nuclear receptors, but through a LXXXIXXX(I/L) motif of amino acids (where L = leucine, I = isoleucine and X = any amino acid). In addition, compressors bind preferentially to the apo (ligand free) form of the nuclear receptor (or possibly antagonist bound receptor).

• CtBP 602618 (associates with class II histone deacetylases)
• LCoR (ligand-dependent corepressor)
• Nuclear receptor CO-Repressor (NCOR)
  • NCOR1 ()
  • NCOR2 ()/SMRT (Silencing Mediator (co-repressor) for Retinoid and Thyroid-hormone receptors) (associates with histone deacetylase-3)
• Rb (retinoblastoma protein) (associates with histone deacetylase-1 and -2)
• RCOR (REST corepressor)
  • RCOR1 ()
  • RCOR2 ()
  • RCOR3 ()
• Sin3
  • SIN3A ()
  • SIN3B ()
• TIF1 (transcriptional intermediary factor 1)
  • TRIM24 Tripartite motif-containing 24 ()
  • TRIM28 Tripartite motif-containing 28 ()
  • TRIM33 Tripartite motif-containing 33 ()

Dual function activator/repressors

• NSD1 ()
• PELP-1 (proline, glutamic acid and leucine rich protein 1)
• RIP140 (receptor-interacting protein 140)
• YAP
• WWTR1 (TAZ)

ATP-dependent remodeling factors

• SWI/SNF family
• chromatin structure remodeling complex
• ISWI protein ,

References

References

1. (2000). "The coregulator exchange in transcriptional functions of nuclear receptors". Genes Dev.
2. (Jan 2011). "TcoF-DB: dragon database for human transcription co-factors and transcription factor interacting proteins". Nucleic Acids Res..
3. (1999). "ATP-dependent remodeling and acetylation as regulators of chromatin fluidity". Genes Dev.
4. (2004). "The transcriptional corepressor, PELP1, recruits HDAC2 and masks histones using two separate domains". J Biol Chem.
5. (1998). "The structural basis of estrogen receptor/coactivator recognition and the antagonism of this interaction by tamoxifen". Cell.
6. (2001). "Molecular cloning and characterization of PELP1, a novel human coregulator of estrogen receptor alpha". J Biol Chem.
7. (2002). "Structural basis for antagonist-mediated recruitment of nuclear co-repressors by PPARalpha". Nature.