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p300/CBP Transcriptional Coactivator

David Sowa '12 and Sonam Lhaki '12


I. Introduction

p300/CBP (CREB-binding protein) is a histone acetyltransferase that promotes transcription through acetylation of the histones, thereby undoing the nucleosome and exposing DNA regions for transcription. The p300/CBP protein consists of two subdomains known as the N and C subdomains, with a portion of the C domain containing a loop that caps both ends of the N domain. Another important feature of the structure is the presence of a tunnel and a pocket that interact with lysine side chains, which leads to acetylation of these lysines in the histone tail.

p300 activates transcription through direct interaction with RNA polymerase II and transcription factors. Since p300/CBP acetyltransferases play important roles in many biological processes, such as cell proliferation and differentiation, much research is being undertaken to see how they function in cancer. For example, research has shown that p300/CBP may contribute to p53 degradation and therefore help cells resist apoptosis, which would therefore lead to cancer (4).

Because of the association of mutant p300/CBP with cancer, as well as its function in other diseases including cardiac disease and diabetes, inhibition of these proteins may represent an avenue for therapeutic treatments.  In this tutorial, p300/CBP is shown complexed with Lys-CoA, a conjugate of lysine and coenzyme A which specifically targets p300 and inhibits its histone acetylase function.

II. General Structure

.p300 consists of two components known as the C and the N subdomains. The C subdomain runs along the entire structure and caps the ends of the larger N subdomain using a loop that spans the structure. 

The HAT domain of p300 consists of a central β-sheet composed of seven β-strands surrounded by nine α-helices. 3 of the α-helices and the last β-sheet extend off of the C subdomain.

Here, p300 is shown binding the Lys-CoA substrate. Lys-CoA consists of a lysine portion and a coenzyme A portion, both of which assist in association with p300. This substrate selectively and potently inhibits p300 acetylation activity (5).

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III. p300/Lys-CoA Interactions

Several interactions contribute to p300's effective binding to the substrate. In one case, the L1 loop of the N subdomain buries 266 Å2 of the lysine portion of Lys-CoA and contributes to Lys-CoA/p300  binding

Another important interaction is a hydrogen bond formation between the oxygen of p300's Trp1436 residue and a nitrogen on the substrate's lysine side chain. 

Furthermore, the aliphatic portion of the lysine interacts with a hydrophobic tunnel of p300. This tunnel is composed of residues Tyr1397, Trp1436, Tyr1446 and Cys1438

Additionally, the CoA portion of the inhibitor contributes to protein-inhibitor association. Key interactions include hydrogen bonds between CoA phosphates and the Arg1410 residue of p300 . One of these bonds is a water-mediated contact, but the water molecule is not shown here.  In a mutagenic study, it was found that an R1410C mutation was a factor in human lung cancer, probably due to the resulting reduction in p300 HAT activity. This study highlights the importance of the Arg residue. 

Finally, a shallow, electronegative groove connects this substrate-binding site to a nearby electronegative pocket that is also thought to effect efficient p300 HAT activity. This pocket is formed by side-chains of the main-chain residues Thr1357, Glu1505, Asp1625, and Asp1628. .Amino-acid mutagenesis on these residues diminishes HAT activity, which indicates that they must support the protein-substrate interaction...

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IV. Comparison of p300 with Other HATs

In some ways, p300/CBP is structurally similar to other HAT enzymes. There is structural conservation in the central core region associated with acetyl-CoA cofactor binding (as is the case with the p300, yeast Gcn5, and yeast Esa1 HAT domains). Yet, there's plenty that sets p300 apart.

One unique feature of the p300 HAT domain is that it has the unusually long L1 loop that interacts with the Lys-CoA inhibitor. This loop's position allows the protein to effectively bind the Lys-CoA substrate.

In addition, the substrate-binding pockets of the p300 HAT domain are more shallow and acidic. Gcn5 and Esa1, on the other hand, have deeper and more apolar pockets. Comparison of p300, Gcn5, and Esa1 electrostatic surface diagrams reveals the greater negative character of the p300 HAT domain. This leads to the selective binding of the basic (lysine) portion of Lys-CoA.

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V. Therapeutic Implications


Recent research has examined the potential role of histone acetyltransferases in various diseases, including cancer, asthma, COPD, and viral infections.
Because different types of histone acetyltransferases catalyze acetylation on certain types of lysine residues, many HATs are now being considered as targets for therapy (3). p300/CBP is being researched for these very reasons--it may be a contributing factor in human cancers.

Currently, a challenge facing researchers is developing  small-molecule inhibitors of HATs that can be used as potential drugs. One example in the case of p300 is C646 , which was found to competitively inhibit p300 moreso than it inhibited other acetyltransferases. C646 is an intriguing potential drug because it will normalize cell growth even better than other potent inhibitors (2).

Finally, other methods for inhibiting HATs that are being considered include alteration of substrate specificity, targeting of specific loci, restricting access of non-target proteins, and coordinating the multiple enzyme activites of the complex (1). Only further research will reveal if one of these is the key to treating cancer.

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VI. References

(1) Berndsen, Christopher E. and John M. Denu. 2008. Catalysis and substrate selection by histone/protein lysine acetyltransferases. Current Opinion in Structural Biology 18:682-689.

(2) Bowers, Erin M., Gai Yan, Chandrani Mukherjee, Andrew Orry, Ling Wang, Marc A. Holbert, Nicholas T. Crump, Catherine A. Hazzalin, Glen Liszczak, Hua Yan, Cecilia Larocca, S. Adrian Saldanha, Ruben Abagyan, Yan Sun, David J. Meyers, Ronen Marmorstein, Louis C. Mahadevan, Rhoda M. Alani, and Philip A. Cole. 2010. Virtual Ligand Screening of the p300/CBP Histone  acetyltransferase: Identification of a Selective Small Molecule Inhibitor. Chemistry & Biology 17(5): 471-482.

(3) Dekker, Frank J. and Hidde J. Haisma. 2009. Histone acetyltransferases as emerging drug targets. Drug Discovery Today 14(19-20):942-948.

(4) Goodman, Richard H. and Sarah Smolik. 2000.CBP/p300 in cell growth, transformation, and development. Genes & Dev 14:1553-1577.

(5) Lau, Ontario D., Tapas K. Kundu, Raymond E. Soccio, Slimane Ait-Si-Ali, Ehab M. Khalil, Alex Vassilev, Alan P. Wolffe, Toshihiro Nakatani, Robert G. Roeder, and Philip A. Cole. 2000. HATs off: selective synthetic inhibitors of the histone acetyltransferases p300 and PCAF. Molecular Cell 5:589-595.

(6) Liu, X., Ling Wang, Kehao Zhao, Paul R. Thompson, Yousang Hwang, Ronen Marmorstein, and Philip A. Cole. 2007. The structural basis of protein acetylation by the p300/CBP transcriptional coactivator. Nature 451:846-850.

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