CREB-binding protein/p300 activates MyoD by acetylation

The myogenic protein MyoD requires two nuclear histone acetyltransferases, CREB-binding protein (CBP)/p300 and PCAF, to transactivate muscle promoters. MyoD is acetylated by PCAF in vitro, which seems to increase its affinity for DNA. We here show that MyoD is constitutively acetylated in muscle cells. In vitro, MyoD is acetylated both by CBP/p300 and by PCAF on two lysines located at the boundary of the DNA binding domain. MyoD acetylation by CBP/p300 (as well as by PCAF) increases its activity on a muscle-specific promoter, as assessed by microinjection experiments. MyoD mutants that cannot be acetylated in vitro are not activated in the functional assay. Our results provide direct evidence that MyoD acetylation functionally activates the protein and show that both PCAF and CBP/p300 are candidate enzymes for MyoD acetylation in vivo.     

Acetylation of estrogen receptor alpha by p300 at lysines 266 and 268 enhances the deoxyribonucleic acid binding and transactivation activities of the receptor

Using a variety of biochemical and cell-based approaches, we show that estrogen receptor alpha (ERalpha) is acetylated by the p300 acetylase in a ligand- and steroid receptor coactivator-dependent manner. Using mutagenesis and mass spectrometry, we identified two conserved lysine residues in ERalpha (Lys266 and Lys268) that are the primary targets of p300-mediated acetylation. These residues are acetylated in cells, as determined by immunoprecipitation-Western blotting experiments using an antibody that specifically recognizes ERalpha acetylated at Lys266 and Lys268. The acetylation of ERalpha by p300 is reversed by native cellular deacetylases, including trichostatin A-sensitive enzymes (i.e. class I and II deacetylases) and nicotinamide adenine dinucleotide-dependent/nicotinamide-sensitive enzymes (i.e. class III deacetylases, such as sirtuin 1). Acetylation at Lys266 and Lys268, or substitution of the same residues with glutamine (i.e. K266/268Q), a residue that mimics acetylated lysine, enhances the DNA binding activity of ERalpha in EMSAs. Likewise, substitution of Lys266 and Lys268 with glutamine enhances the ligand-dependent activity of ERalpha in a cell-based reporter gene assay. Collectively, our results implicate acetylation as a modulator of the ligand-dependent gene regulatory activity of ERalpha. Such regulation is likely to play a role in estrogen-dependent signaling outcomes in a variety of estrogen target tissues in both normal and pathological states.     

p300/CBP acetyl transferases interact with and acetylate the nucleotide excision repair factor XPG

Nucleotide excision repair (NER) is an important DNA repair mechanism through which cells remove bulky DNA lesions. Following DNA damage, the histone acetyltransferase (HAT) p300 (also referred to as lysine acetyltransferase or KAT) is known to associate with proliferating cell nuclear antigen (PCNA), a master regulator of DNA replication and repair processes. This interaction, which results in HAT inhibition, may be dissociated by the cell cycle inhibitor p21^CDKN1A, thereby restoring p300 activity; however, the role of this protein interplay is still unclear. Here, we report that silencing p300 or its homolog CREB-binding protein (CBP) by RNA interference (RNAi) significantly reduces DNA repair synthesis in human fibroblasts. In addition, we determined whether p300 and CBP may associate with and acetylate specific NER factors such as XPG, the 3′-endonuclease that is involved in the incision/excision step and is known to interact with PCNA. Our results show that p300 and CBP interact with XPG, which has been found to be acetylated in vivo. XPG is acetylated by p300 in vitro, and this reaction is inhibited by PCNA. Knocking down both p300/CBP by RNAi or by chemical inhibition with curcumin greatly reduced XPG acetylation, and a concomitant accumulation of the protein at DNA damage sites was observed. The ability of p21 to bind PCNA was found to regulate the interaction between p300 and XPG, and an abnormal accumulation of XPG at DNA damage sites was also found in p21^−/− fibroblasts. These results indicate an additional function of p300/CBP in NER through the acetylation of XPG protein in a PCNA–p21 dependent manner.     

The two DNA clamps Rad9/Rad1/Hus1 complex and proliferating cell nuclear antigen differentially regulate flap endonuclease 1 activity

DNA damage leads to activation of several mechanisms such as DNA repair and cell-cycle checkpoints. It is evident that these different cellular mechanisms have to be finely co-ordinated. Growing evidence suggests that the Rad9/Rad1/Hus1 cell-cycle checkpoint complex (9-1-1 complex), which is recruited to DNA lesion upon DNA damage, plays a major role in DNA repair. This complex has been shown to interact with and stimulate several proteins involved in long-patch base excision repair. On the other hand, the well-characterised DNA clamp-proliferating cell nuclear antigen (PCNA) also interacts with and stimulates several of these factors. In this work, we compared the effects of the 9-1-1 complex and PCNA on flap endonuclease 1 (Fen1). Our data suggest that PCNA and the 9-1-1 complex can independently bind to and activate Fen1. Finally, acetylation of Fen1 by p300-HAT abolished the stimulatory effect of the 9-1-1 complex but not that of PCNA, suggesting a possible mechanism of regulation of this important repair pathway.