p300/CBP-associated factor histone acetyltransferase processing of a peptide substrate. Kinetic analysis of the catalytic mechanism

p300/CBP-associated factor (PCAF) is a histone acetyltransferase that plays an important role in the remodeling of chromatin and the regulation of gene expression. It has been shown to catalyze preferentially acetylation of the epsilon-amino group of lysine 14 in histone H3. In this study, the kinetic mechanism of PCAF was evaluated with a 20-amino acid peptide substrate derived from the amino terminus of histone H3 (H3-20) and recombinant bacterially expressed PCAF catalytic domain (PCAF(cat)). The enzymologic behavior of full-length PCAF and PCAF(cat) were shown to be similar. PCAF-catalyzed acetylation of the substrate H3-20 was shown to be specific for Lys-14, analogous to its behavior with the full-length histone H3 protein. Two-substrate kinetic analysis displayed an intersecting line pattern, consistent with a ternary complex mechanism for PCAF. The dead-end inhibitor analog desulfo-CoA was competitive versus acetyl-CoA and noncompetitive versus H3-20. The dead-end analog inhibitor H3-20 K14A was competitive versus H3-20 and uncompetitive versus acetyl-CoA. The potent bisubstrate analog inhibitor H3-CoA-20 was competitive versus acetyl-CoA and noncompetitive versus H3-20. Taken together, these inhibition patterns support an ordered BiBi kinetic mechanism for PCAF in which acetyl-CoA binding precedes H3-20 binding. Viscosity experiments suggest that diffusional release of product is not rate-determining for PCAF catalysis. These results provide a mechanistic framework for understanding the detailed catalytic behavior of an important subset of the histone acetyltransferases and have significant implications for molecular regulation of and inhibitor design for these enzymes.

Tyrosine analogues as alternative substrates for protein tyrosine kinase Csk: insights into substrate selectivity and catalytic mechanism

Protein tyrosine kinases are critical enzymes in cell signal transduction but relatively little is known about the molecular recognition of the tyrosine substrate by these enzymes. Details of tyrosine substrate specificity within the context of a short peptide were investigated for protein tyrosine kinase Csk. It was found that aryl ring functional group substitutions the size of methyl group or smaller were generally well tolerated by the protein tyrosine kinase Csk whereas larger groups caused a decline in substrate efficiency. Extension of the phenol from the peptide backbone by a single methylene was acceptable for phosphorylation whereas removal of a methylene nearly abolished reactivity. Only the L-tyrosine derivative was processed. A negative charge ortho to the phenol hydroxyl was incompatible with substrate reactivity, consistent with previous pH rate profiles which indicated the importance of the neutral phenol. Overall, these studies confirmed the interpretation of a previous linear free energy relationship analysis which suggested that the enzyme followed a dissociative transition state mechanism.

HATs off: selective synthetic inhibitors of the histone acetyltransferases p300 and PCAF

Histone acetyltransferases (HATs) play important roles in the regulation of gene expression. In this report, we describe the design, synthesis, and application of peptide CoA conjugates as selective HAT inhibitors for the transcriptional coactivators p300 and PCAF. Two inhibitors (Lys-CoA for p300 and H3-CoA-20 for PCAF) were found to be potent (IC(50) approximately = 0.5 microM) and selective (approximately 200-fold) in blocking p300 and PCAF HAT activities. These inhibitors were used to probe enzymatic and transcriptional features of HAT function in several assay systems. These compounds should be broadly useful as biological tools for evaluating the roles of HATs in transcriptional studies and may serve as lead agents for the development of novel antineoplastic therapeutics.

Two antiatherogenic effects of progesterone on human macrophages; inhibition of cholesteryl ester synthesis and block of its enhancement by glucocorticoids

The effects of progesterone and estradiol on cholesteryl ester (CE) formation by monocyte-derived human macrophages were examined. Formation was assessed from incorporation of 14C-cholesterol during a 20-h incubation with hormone and from that of 3H-oleate (3 h) after hormone removal. Progesterone inhibited cholesterol into CE and decreased CE cellular levels. Inhibition: 1) was reversed by progesterone removal; 2) was independent of the progesterone receptor (not blocked by the receptor antagonist RU40555); and 3) exhibited specific structural requirements; 11alpha-OH-progesterone was inhibitory, whereas its stereoisomer 11beta-OH-progesterone was not. In contrast to progesterone, estradiol was ineffective. We had reported that dexamethasone enhanced CE accumulation by human macrophages (1). In this study, we describe similar effects of the endogenous steroid, cortisol, and of the most widely prescribed glucocorticoid, prednisolone. Both steroids increased CE formation from two folds, in the presence of cholesterol-liposomes, to five folds, in the presence of modified low-density lipoprotein. Progesterone (0.1-1 micromol/L), added during glucocorticoid treatment, blocked this increase. The progesterone block: 1) was duplicated by the steroid receptor inhibitor RU40555; 2) was not reversed by hormone removal; and 3) reflected inhibition of glucocorticoid-induced increases in messenger RNA for acyl-CoA-cholesterol:acyl transferase. Thus, progesterone exerted two effects on macrophages: it acutely inhibited CE formation, and it prevented glucocorticoid-induced increases in acyl-CoA-cholesterol-acyl transferase gene expression and CE synthesis.