Chemistry of acetyl transfer by histone modifying enzymes: structure, mechanism and implications for effector design.

The post-translational modification of histones plays an important role in chromatin regulation, a process that insures the fidelity of gene expression and other DNA transactions. Of the enzymes that mediate post-translation modification, the histone acetyltransferase (HAT) and histone deacetylase (HDAC) proteins that add and remove acetyl groups to and from target lysine residues within histones, respectively, have been the most extensively studied at both the functional and structural levels. Not surprisingly, the aberrant activity of several of these enzymes have been implicated in human diseases such as cancer and metabolic disorders, thus making them important drug targets. Significant mechanistic insights into the function of HATs and HDACs have come from the X-ray crystal structures of these enzymes both alone and in liganded complexes, along with associated enzymatic and biochemical studies. In this review, we will discuss what we have learned from the structures and related biochemistry of HATs and HDACs and the implications of these findings for the design of protein effectors to regulate gene expression and treat disease.

Histone acetyltransferases: function, structure, and catalysis.

Histone acetyltransferases (HATs) directly link chromatin modification to gene activation. Recent structure/function studies provide insights into HAT catalysis and histone binding, and genetic studies suggest cross-talk between acetylation and other histone modifications. Developmental aberrations in mice and certain human cancers are associated with HAT mutations, further highlighting the importance of these enzymes to normal cell growth and differentiation.

DNA sequence preferences of GAL4 and PPR1: how a subset of Zn2 Cys6 binuclear cluster proteins recognizes DNA.

Biophysical and genetic experiments have defined how the Saccharomyces cerevisiae protein GAL4 and a subset of related proteins recognize specific DNA sequences. We assessed DNA sequence preferences of GAL4 and a related protein, PPR1, in an in vitro DNA binding assay. For GAL4, the palindromic CGG triplets at the ends of the 17-bp recognition site are essential for tight binding, whereas the identities of the internal 11 bp are much less important, results consistent with the GAL4-DNA crystal structure. Small reductions in affinity due to mutations at the center-most 5 bp are consistent with the idea that an observed constriction in the minor groove in the crystalline GAL4-DNA complex is sequence dependent. The crystal structure suggests that this sequence dependence is due to phosphate contacts mediated by arginine 51, as part of a network of hydrogen bonds. Here we show that the mutant protein GAL4(1-100)R51A fails to discriminate sites with alterations in the center of the site from the wild-type site. PPR1, a relative of GAL4, also recognizes palindromic CGG triplets at the ends of its 12-bp recognition sequence. The identities of the internal 6 bp do not influence the binding of PPR1. We also show that the PPR1 site consists of a 12-bp duplex rather than 16 bp as reported previously: the two T residues immediately 5' to the CGG sequence in each half site, although highly conserved, are not important for binding by PPR1. Thus, GAL4 and PPR1 share common CGG half sites, but they prefer DNA sequences with the palindromic CGG separated by the appropriate number of base pairs, 11 for GAL4 and 6 for PPR1.

p53 sites acetylated in vitro by PCAF and p300 are acetylated in vivo in response to DNA damage.

The p53 tumor suppressor protein is a sequence-specific transcription factor that modulates the response of cells to DNA damage. Recent studies suggest that full transcriptional activity of p53 requires the coactivators CREB binding protein (CBP)/p300 and PCAF. These coactivators interact with each other, and both possess intrinsic histone acetyltransferase activity. Furthermore, p300 acetylates p53 to activate its sequence-specific DNA binding activity in vitro. In this study, we demonstrate that PCAF also acetylates p53 in vitro at a lysine residue distinct from that acetylated by p300 and thereby increases p53's ability to bind to its cognate DNA site. We have generated antibodies to acetylated p53 peptides at either of the two lysine residues that are targeted by PCAF or p300 and have demonstrated that these antibodies are highly specific for both acetylation and the particular site. Using these antibodies, we detect acetylation of these sites in vivo, and interestingly, acetylation at both sites increases in response to DNA-damaging agents. These data indicate that site-specific acetylation of p53 increases under physiological conditions that activate p53 and identify CBP/p300 and PCAF as the probable enzymes that modify p53 in vivo.

Structure of HAP1-PC7 bound to DNA: implications for DNA recognition and allosteric effects of DNA-binding on transcriptional activation.

HAP1 is a transcription factor in yeast whose DNA-binding domain has been implicated in directly affecting transcriptional activation. Two separate mutations in the DNA-binding domain, S63G (HAP1-PC7) and S63R (HAP1-18), retain wild-type binding affinity. However, HAP1-PC7 is transcriptionally silent while HAP1-18 shows highly elevated levels of transcription. We have determined the X-ray crystal structure of the DNA-binding domain of HAP1-PC7 bound to its DNA target, UAS(CYC7), and compared it to the previously solved HAP1-wt and HAP1-18 complexes to UAS(CYC7). Additionally, we have quantitatively compared the DNA-binding affinity and specificity of the HAP1-PC7, HAP1-18 and HAP1-wt DNA-binding domains. We show that, although the DNA-binding domains of these three proteins bind UAS(CYC7) with comparable affinity and specificity, the protein-DNA interactions are dramatically different between the three complexes. Conserved protein-DNA interactions are largely restricted to an internal DNA sequence that excludes one of the two conserved DNA half-sites of UAS(CYC7) suggesting a mode of recognition distinct from other HAP1 family members. Alternative protein-DNA interactions result in divergent DNA configurations between the three complexes. These results suggest that the differential transcriptional activities of the HAP1, HAP1-18 and HAP1-PC7 proteins are due, at least in part, to alternative protein-DNA contacts, and implies that HAP1-DNA interactions have direct allosteric effects on transcriptional activation.

Structure of histone deacetylases: insights into substrate recognition and catalysis.

Histone deacetylases catalyze the removal of the acetyl moiety from acetyl-lysine within histones to promote gene repression and silencing. These enzymes fall into distinct families based on primary sequence homology and functional properties in vivo. Recent structural studies of histone deacetylases and their homologs from bacteria have provided important insights into the mode of substrate recognition and catalysis by these enzymes.

Structure-function studies of the BTB/POZ transcriptional repression domain from the promyelocytic leukemia zinc finger oncoprotein.

The evolutionarily conserved BTB/POZ domain from the promyelocytic leukemia zinc finger (PLZF) oncoprotein mediates transcriptional repression through the recruitment of corepressor proteins containing histone deacetylases in acute promyelocytic leukemia. We have determined the 2.0 A crystal structure of the BTB/POZ domain from PLZF (PLZF-BTB/POZ), and have carried out biochemical analysis of PLZF-BTB/POZ harboring site-directed mutations to probe structure-function relationships. The structure reveals a novel alpha/beta homodimeric fold in which dimer interactions occur along two surfaces of the protein subunits. The conservation of BTB/POZ domain residues at the core of the protomers and at the dimer interface implies an analogous fold and dimerization mode for BTB/POZ domains from otherwise functionally unrelated proteins. Unexpectedly, the BTB/POZ domain forms dimer-dimer interactions in the crystals, suggesting a mode for higher-order protein oligomerization for BTB/POZ-mediated transcriptional repression. Biochemical characterization of PLZF-BTB/POZ harboring mutations in conserved residues involved in protein dimerization reveals that the integrity of the dimer interface is exquisitely sensitive to mutation and that dimer formation is required for wild-type levels of transcriptional repression. Interestingly, similar mutational analysis of residues within a pronounced protein cleft along the dimer interface, which had been implicated previously for interaction with corepressors, has negligible effects on dimerization or transcriptional repression. Together, these studies form a structure-function framework for understanding BTB/POZ-mediated oligomerization and transcriptional repression properties.

In Vitro Activity Assays for MYST Histone Acetyltransferases and Adaptation for High-Throughput Inhibitor Screening.

Lysine acetylation is a posttranslational modification that is carried out by acetyltransferases. The MYST proteins form the largest and most diverse family of acetyltransferases, which regulate gene expression, DNA repair, and cell cycle homeostasis, among other activities, by acetylating both histone and nonhistone proteins. This chapter will describe methods for the preparation and biochemical characterization of MYST family acetyltransferases, including protocols for the preparation of recombinant protein, enzyme assays for measuring steady-state parameters, and binding assays to measure cofactor and inhibitor binding. We also provide details on adapting these assays for high-throughput screening for small molecule MYST inhibitors. This chapter seeks to prepare researchers for some hurdles that they may encounter when studying the MYST proteins so that there may be better opportunity to plan appropriate controls and obtain high-quality data.

Structure of histone acetyltransferases.

Histone acetyltranferase (HAT) enzymes are the catalytic subunits of multisubunit protein complexes that acetylate specific lysine residues on the N-terminal regions of the histone components of chromatin to promote gene activation. These enzymes, which now include more than 20 members, fall into distinct families that generally have high sequence similarity and related substrate specificity within families, but have divergent sequence and substrate specificity between families. Significant insights into the mode of catalysis and histone substrate binding have been provided by the structure determination of the divergent HAT enzymes Hat1, Gcn5/PCAF and Esa1. A comparison of these structures reveals a structurally conserved central core domain that mediates extensive interactions with the acetyl-coenzyme A cofactor, and structurally divergent N and C-terminal domains. A correlation of these structures with other studies reveals that the core domain plays a particularly important role in histone substrate catalysis and that the N and C-terminal domains play important roles in histone substrate binding. These correlations imply a related mode of catalysis and histone substrate binding by a diverse group of HAT enzymes.

Crystal structure of a ternary SAP-1/SRF/c-fos SRE DNA complex.

Combinatorial DNA binding by proteins for promoter-specific gene activation is a common mode of DNA regulation in eukaryotic organisms, and occurs at the promoter of the c-fos proto-oncogene. The c-fos promoter contains a serum response element (SRE) that mediates ternary complex formation with the Ets proteins SAP-1 or Elk-1 and the MADS-box protein, serum response factor (SRF). Here, we report the crystal structure of a ternary SAP-1/SRF/c-fos SRE DNA complex containing the minimal DNA-binding domains of each protein. The structure of the complex reveals that the SAP-1 monomer and SRF dimer are bound on opposite faces of the DNA, and that the DNA recognition helix of SAP-1 makes direct contact with the DNA recognition helix of one of the two SRF subunits. These interactions facilitate an 82 degrees DNA bend around SRF and a modulation of protein-DNA contacts by each protein when compared to each of the binary DNA complexes. A comparison with a recently determined complex containing SRF, an idealized DNA site, and a SAP-1 fragment containing a SRF-interacting B-box region, shows a similar overall architecture but also shows important differences. Specifically, the comparison suggests that the B-box region of the Ets protein does not significantly influence DNA recognition by either of the proteins, and that the sequence of the DNA target effects the way in which the two proteins cooperate for DNA recognition. These studies have implications for how DNA-bound SRF may modulate the DNA-binding properties of other Ets proteins such as Elk-1, and for how other Ets proteins may modulate the DNA-binding properties of other DNA-bound accessory factors to facilitate promoter-specific transcriptional responses.

Structure and function of bromodomains in chromatin-regulating complexes.

Specific changes in chromatin structure are associated with transcriptional regulation. These chromatin alterations include both covalent modifications of the amino termini of histones as well as ATP-dependent non-covalent remodeling of nucleosomes. Certain protein domains, such as the bromodomains, are commonly associated with both of these classes of enzymes that alter chromatin. This review discusses recent advances in understanding the structure and function of bromodomains. Most significantly, a role of bromodomains has been revealed in binding to acetylated lysine residues in histone tails. Interactions between bromodomains and modified histones may be an important mechanism underlying chromatin structural changes and gene regulation.

Generation and characterization of monoclonal antibodies against the E6 and E7 oncoproteins of HPV.

Generation of three monoclonal antibodies (MAbs) to the major oncoproteins of human papillomavirus (HPV) was accomplished by an intense prime/boost regimen. Mice were primed with expression vectors expressing either the E6 or E7 oncoproteins of HPV-16 followed by boosting with a vaccinia virus construct and a replication-defective E1-deleted adenoviral recombinant of the human strain 5, and last, with baculovirus-derived HPV-16 E6 and E7 proteins in incomplete Freunds' adjuvant. Splenocytes were then fused with a myeloma cell line. The vaccination protocol generated one anti-E7 MAb of the IgM isotype and two anti-E6 MAbs of the IgG1 subisotype. The MAbs were tested for functionality in standard laboratory assays and found to detect the E6 and E7 proteins, respectively. The E7 MAb cross-reacted with the HPV-1a E7 oncoprotein. The binding sites of the MAbs were mapped to defined regions of each viral protein.

Crystal structure of a p53 core tetramer bound to DNA.

The tumor suppressor p53 regulates downstream genes in response to many cellular stresses and is frequently mutated in human cancers. Here, we report the use of a crosslinking strategy to trap a tetrameric p53 DNA-binding domain (p53DBD) bound to DNA and the X-ray crystal structure of the protein/DNA complex. The structure reveals that two p53DBD dimers bind to B form DNA with no relative twist and that a p53 tetramer can bind to DNA without introducing significant DNA bending. The numerous dimer-dimer interactions involve several strictly conserved residues, thus suggesting a molecular basis for p53DBD-DNA binding cooperativity. Surface residue conservation of the p53DBD tetramer bound to DNA highlights possible regions of other p53 domain or p53 cofactor interactions.

Structure and chemistry of the Sir2 family of NAD+-dependent histone/protein deactylases.

The yeast Sir2 (silent information regulator-2) protein functions as an NAD(+)-dependent histone deacetylase to silence gene expression from the mating-type locus, tolomeres and rDNA and also promotes longevity and genome stability in response to calorie restriction. Homologues of yeast Sir2 have been identified in the three domains of bacteria, archaea and eukaryotes; in mammalian cells, Sir2 proteins also deacetylate non-histone proteins such as the p53 tumour suppressor protein, alpha-tubulin and forkhead transcription factors to mediate diverse biological processes including metabolism, cell motility and cancer. We have determined the X-ray crystal structure of a Sir2 homologue from yeast Hst2 (yHst2), in various liganded forms, including the yHst2/acetyl-Lys-16 histone H4/NAD(+) ternary complex; we have also performed related biochemical studies to address the conserved mode of catalysis by these enzymes as well as the distinguishing features that allow different members of the family to target their respective cognate substrates. These studies have implications for the structure-based design of Sir2-specific small molecule compounds, which might modulate Sir2 function for therapeutic application.

Protein modules that manipulate histone tails for chromatin regulation.

Histones are the predominant protein components of chromatin and are subject to specific post-translational modifications that are correlated with transcriptional competence. Among these histone modifications are acetylation, phosphorylation and methylation, and recent studies reveal that conserved protein modules mediate the attachment, removal or recognition of these modifications. It is becoming clear that appropriate coordination of histone modifications and their manipulations by conserved protein modules are integral to gene-specific transcriptional regulation within chromatin.

Structure and function of histone acetyltransferases.

Histone acetyltranferase (HAT) enzymes are the catalytic subunit of large multisubunit HAT complexes that acetylate the epsilon-amino group of specific lysine residues on histone tails to promote transcriptional activation. Recent structural and functional studies on the divergent HAT enzymes Gcn5/PCAF, Esa1 and Hat1 have provided new insights into the underlying mechanism of histone binding and acetylation by HAT proteins. The three HAT enzymes contain a structurally conserved core domain that plays a functionally conserved role in binding the coenzyme A cofactor and in harboring the putative general base for catalysis. Structurally variable N- and C-terminal domains appear to contain a related scaffold that mediates histone substrate binding. These data provide a framework for understanding the structure and function of other more divergent HAT proteins such as TAF(II)250 and CBP/p300, and provides a starting point for understanding how HAT proteins may cooperate with other factors within in vivo HAT complexes to promote transcriptional activation.

6-Nitro-L-tryptophan: a novel spectroscopic probe of trp aporepressor and human serum albumin.

The binding of 6-nitro-L-tryptophan to trp aporepressor and human serum albumin has been examined by visible difference spectroscopy and circular dichroism. 6-Nitro-L-tryptophan, prepared by nitration of L-tryptophan with nitric acid in glacial acetic acid, exhibits a visible and near-uv absorption spectrum with lambda max at about 330 nm (epsilon = 7 X 10(3) M-1 cm-1) and a shoulder near 380 nm in H2O. In the presence of trp aporepressor, the visible absorption intensity is sharply diminished. Visible difference spectral titration data give KD = 1.27 X 10(-4) M and n = 0.95 per subunit at 25 degrees C. While 6-nitro-L-tryptophan exhibits no significant circular dichroism between 300 and 500 nm, the complex with trp aporepressor exhibits strong circular dichroism signals, with a negative maximum at 386 nm (delta epsilon = -7.5 M-1 cm-1) and a positive maximum at 310 nm (delta epsilon = +6 M-1 cm-1). Circular dichroism titration data give KD = 1.69 X 10(-4) M and n = 0.90 per subunit at 25 degrees C. The KD values determined spectroscopically are in excellent agreement with that determined by equilibrium dialysis, KD = 1.5 X 10(-4) M at 25 degrees C. In the presence of human serum albumin, the spectrum of 6-nitro-L-tryptophan exhibits a blue shift and an increase in absorption intensity; similar changes are observed in solvents of low dielectric contrast such as 80% aqueous dioxane. Visible difference spectral titration data give KD = 8.0 X 10(-5) M and n = 0.95 for human serum albumin. The complex of 6-nitro-L-tryptophan with human serum albumin exhibits a strong positive circular dichroism maximum at 380 nm (delta epsilon = +9.8 M-1 cm-1) with a shoulder at 310-320 nm. Circular dichroism titration data give KD = 6.4 X 10(-5) M and n = 0.83, in good agreement with the visible difference spectral results. Taken together, our results demonstrate the utility of 6-nitro-L-tryptophan as a spectroscopic probe for tryptophan-binding proteins.

Crystal structure of the CDK4/6 inhibitory protein p18INK4c provides insights into ankyrin-like repeat structure/function and tumor-derived p16INK4 mutations.

p18INK4c is a member of a family of INK4 proteins that function to arrest the G1 to S cell cycle transition by inhibiting the activity of the cyclin-dependent kinases 4 and 6. The X-ray crystal structure of the human p18INK4c protein to a resolution of 1.95 A reveals an elongated molecule comprised of five contiguous 32- or 33-residue ankyrin-like repeat units. Each ankyrin-like repeat contains a beta-strand helix-turn-helix extended strand beta-strand motif that associates with neighboring motifs through beta-sheet, and helical bundle interactions. Conserved ankyrin-like repeat residues function to facilitate the ankyrin repeat fold and the tertiary interactions between neighboring repeat units. A large percentage of residues that are conserved among INK4 proteins and that map to positions of tumor-derived p16INK4 mutations play important roles in protein stability. A subset of these residues suggest an INK4 binding surface for the cyclin-dependent kinases 4 and 6. This surface is centered around a region that shows structural features uncharacteristic of ankyrin-like repeat units.

Stereochemical effects of L-tryptophan and its analogues on trp repressor's affinity for operator-DNA.

We have employed a filter binding assay to help study the mechanism by which bound L-tryptophan enables the Escherichia coli trp repressor to bind its operators. We have prepared variants of the trp repressor using structural analogues of the natural corepressor, L-tryptophan, and measured the affinity of these variants for a 20-base pair oligonucleotide duplex containing a symmetrical idealization of the trp operator from the E. coli trpEDCBA operon. By normalizing for each analogue's previously determined affinity for the trp aporepressor, we have estimated the extent to which each of the functional groups of bound L-tryptophan contributes to operator affinity. We discuss the likely role of these functional groups in the context of the crystal structures of the inactive, unliganded trp aporepressor, the liganded, active repressor, an inactive pseudorepressor (Pseudorepressors are formed by analogues of L-tryptophan that bind at the tryptophan-binding site but form near isomorphs of the repressor that have poor affinity for operator-DNA.) and the trp repressor/operator complex. We find that the alpha-amino group and an unsubstituted amino (-NH-) nitrogen of L-tryptophan's indole ring are essential for operator affinity. The former properly orients the corepressor and the latter interacts directly with the DNA. The alpha-carboxyl group, on the other hand, greatly enhances but is not essential for operator binding. The alpha-carboxylate's role, which is dependent on the corepressor's orientation in the binding pocket, is apparently to position the guanidinium group of Arg-84 for favorable contacts with the operator's sugar-phosphate backbone.

Crystal structure of a PPR1-DNA complex: DNA recognition by proteins containing a Zn2Cys6 binuclear cluster.

PPR1 is a yeast transcription factor that contains a six-cysteine, two-zinc (Zn) domain, homologous to a similar structure in GAL4. Like GAL4, it binds to DNA sites with conserved CGG triplets symmetrically placed near each end. Whereas the GAL4 site has 11 intervening base pairs, the PPR1 site has 6. The crystal structure of a 95-residue fragment of PPR1 in specific complex with DNA shows that the protein binds to a symmetrical 14-bp recognition site as a nonsymmetrical homodimer. An amino-terminal Zn domain interacts with a conserved CGG triplet near each end of the site through major groove contacts, and the carboxy-terminal residues mediate dimerization through a coiled-coil element and an extended strand. A linker region, connecting the Zn domain and the coiled-coil, folds into a beta-hairpin. This hairpin packs differently on the two subunits and leads to a striking asymmetry, which is largely restricted to the dimerization and linker regions of the protein. Comparison with the GAL4-DNA structure shows that their specificities for sites of different length are determined by the preferred folds of their respective linker segments and by residues at the amino-terminal ends of their coiled-coils. None of these residues contact DNA in PPR1, and they contact only the sugar phosphate backbone in GAL4. We propose that this novel mode of DNA site selection is employed by other proteins that contain a Zn2Cys6 binuclear cluster.