The biochemical and genetic discovery of the SAGA complex.

One of the major advances in our understanding of gene regulation in eukaryotes was the discovery of factors that regulate transcription by controlling chromatin structure. Prominent among these discoveries was the demonstration that Gcn5 is a histone acetyltransferase, establishing a direct connection between transcriptional activation and histone acetylation. This breakthrough was soon followed by the purification of a protein complex that contains Gcn5, the SAGA complex. In this article, we review the early genetic and biochemical experiments that led to the discovery of SAGA and the elucidation of its multiple activities.

Analysis of DNA Processing Enzyme FEN1 and Its Regulation by Protein Lysine Acetylation.

Cellular proteins are modified by lysine acetylation wherein an acetyltransferase transfers an acetyl group from acetyl co enzyme A onto the e-amino group of lysine residues. This modification is extremely dynamic and can be reversed by a deacetylase that removes the acetyl group. Addition of acetyl group to the lysine residue neutralizes its positive charge, thereby functioning as a molecular switch in regulating the enzymatic functions of the protein, its stability, and it cellular localization. Since this modification is extremely dynamic within the cell, biochemical studies characterizing changes in protein function are imperative to understand how this modification alters protein function in a specific cellular pathway. This unit describes in detail expression and purification of a recombinant nuclease and acetyltransferase, in vitro acetylation of the recombinant protein and biochemical assays to study the changes in enzymatic activity of the in vitro acetylated nuclease.

Analysis of p300 occupancy at the early stage of stem cell differentiation by chromatin immunoprecipitation.

Chromatin immunoprecipitation (ChIP) is an invaluable method to study the specific interaction of regulatory proteins with genomic DNA. Since its first development, it has been modified extensively to make it applicable to many different cell types and experimental systems. The cross-linking of regulatory proteins to genomic DNA requires monolayer cells or single cell suspensions. Here, we describe a ChIP protocol using embryoid bodies formed at the early stage differentiation of pluripotent stem cells, which we have used to determine long-range p300-dependent regulatory elements of myogenic-specific genes.

Functional phosphosite screening for targeted protein-protein interactions by combining phosphoproteomics strategies and mammalian two-hybrid assays.

An effective new platform for phosphosite mapping and subsequent functional screening was developed to analyze the targeted protein-protein interactions of p300 and CBP with ß-catenin. Two novel functional phosphosites, Ser12 of p300 and Ser92 of CBP, were revealed to modulate p300/ß-catenin and CBP/ß-catenin interactions, respectively.

Autoacetylation induced specific structural changes in histone acetyltransferase domain of p300: probed by surface enhanced Raman spectroscopy.

Reversible acetylation of histone and non-histone proteins plays an important role in the regulation of gene expression and cellular homeostasis. A balance between acetylation and deacetylation of these proteins are maintained by histone acetyltransferases (HATs) and histone deacetylases (HDACs). Among different HATs, p300/CBP is the most widely studied chromatin modifying enzymes. p300 is involved in several physiological processes like cell growth, regulation of gene expression, development, and tumor suppressor, and therefore its dysfunction causes different diseases. The autoacetylation of p300 is one of the key regulators of its catalytic activity. Mechanistically, autoacetylation induced structural changes in the p300 HAT domain acts as a master switch. In this report, we have shown that the natural HAT inhibitor garcinol could potently inhibit the autoacetylation activity. Furthermore, for the first time, we demonstrate that indeed autoacetylation induces structural changes in p300 HAT domain, as probed by surface-enhanced Raman scattering. Presumably, SERS will be a very useful tool to find out the structural changes in the other self-modifying enzymes like kinases and methyltransferases.

Reconstitution of chromatin transcription with purified components reveals a chromatin-specific repressive activity of p300.

Here we describe an in vitro chromatin transcription system in which chromatin assembly and transcription are carried out with purified and defined factors. With basal (also known as general) transcription factors and sequence-specific DNA-binding activators, we observed chromatin-specific, activation domain-dependent transcription. We then examined the biochemical function of purified p300 in the absence of the endogenous factor and other related activities and found, unexpectedly, that p300 has a chromatin-specific, transcriptional repression activity that can be relieved by the addition of acetyl-CoA. This p300-mediated repression is reversible, requires the p300 bromodomain but not the acetyltransferase region, and does not involve the formation of a stable, nuclease-resistant nucleoprotein complex. Hence, the mechanism of transcriptional repression by p300 is distinct from that of histone H1, PARP-1 or Sir2. These findings reveal a novel chromatin-specific repressive function of p300.