Nucleosome Competition Reveals Processive Acetylation by the SAGA
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Nucleosome competition reveals processive PNAS PLUS acetylation by the SAGA HAT module Alison E. Ringela, Anne M. Cieniewiczb,c, Sean D. Tavernab,c, and Cynthia Wolbergera,c,1 aDepartment of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD 21205; bDepartment of Pharmacology and Molecular Sciences, Johns Hopkins University School of Medicine, Baltimore, MD 21205; and cCenter for Epigenetics, Johns Hopkins University School of Medicine, Baltimore, MD 21205 Edited by Carl Wu, Howard Hughes Medical Institute, Ashburn, VA, and approved September 1, 2015 (received for review April 30, 2015) The Spt-Ada-Gcn5 acetyltransferase (SAGA) coactivator complex histone acetyltransferase (HAT) complexes also contain reader hyperacetylates histone tails in vivo in a manner that depends modules that recognize the H3K4me3 mark (16, 17). upon histone 3 lysine 4 trimethylation (H3K4me3), a histone mark The SAGA (Spt-Ada-Gcn5 acetyltransferase) complex is a enriched at promoters of actively transcribed genes. SAGA contains highly conserved transcriptional coactivator (18) that is involved in a separable subcomplex known as the histone acetyltransferase the transcription of nearly all yeast genes (19, 20) and mediates (HAT) module that contains the HAT, Gcn5, bound to Sgf29, Ada2, crosstalk between H3K4me3 and histone hyperacetylation. The and Ada3. Sgf29 contains a tandem Tudor domain that recognizes HAT activity of SAGA resides in a four-protein subcomplex H3K4me3-containing peptides and is required for histone hyper- known as the “HAT module” (21, 22), which contains the catalytic acetylation in vivo. However, the mechanism by which H3K4me3 subunit Gcn5 (23–25), Ada2, Ada3 (26, 27), and Sgf29 (21, 28). recognition leads to lysine hyperacetylation is unknown, as in The association of Gcn5 with the other HAT module subunits vitro studies show no effect of the H3K4me3 modification on modulates Gcn5 activity and specificity. Gcn5 on its own acetylates histone peptide acetylation by Gcn5. To determine how H3K4me3 free histones but not nucleosomes (22, 23, 25) and preferentially binding by Sgf29 leads to histone hyperacetylation by Gcn5, we modifies histone H3 at K14 (25, 29). A complex containing Gcn5 used differential fluorescent labeling of histones to monitor acety- bound to Ada2 and Ada3 is more active overall and modifies lation of individual subpopulations of methylated and unmodified nucleosomal substrates (22, 25). Binding to Ada2/Ada3 also nucleosomes in a mixture. We find that the SAGA HAT module broadens the lysine specificity of Gcn5 to acetylate multiple lysine BIOCHEMISTRY preferentially acetylates H3K4me3 nucleosomes in a mixture con- residues on the histone H3 tail (22, 25). Sgf29 contains a tandem taining excess unmodified nucleosomes and that this effect requires Tudor domain that binds to H3K4me3 (17, 30) and is required for the Tudor domain of Sgf29. The H3K4me3 mark promotes processive, maintaining wild-type levels of histone H3 acetylation in vivo multisite acetylation of histone H3 by Gcn5 that can account for the (30–32). However, in vitro studies comparing acetylation kinetics different acetylation patterns established by SAGA at promoters by the SAGA complex on peptide substrates in the presence or versus coding regions. Our results establish a model for Sgf29 absence of Sgf29 do not show a rate enhancement when the function at gene promoters and define a mechanism governing H3K4me3 modification is present (30). Because these experi- crosstalk between histone modifications. ments were limited in scope to peptide substrates and measured acetylation using relatively insensitive end-point assays (30), acetyltransferase | histone crosstalk | histone modifications | the full impact of H3K4me3 on HAT module activity may transcription | coactivator Significance he different patterns of histone posttranslational modifications Tdistributed across the genome are proposed to constitute a Crosstalk between histone modifications regulates transcription “histone code” that orchestrates distinct transcriptional programs by establishing spatial and temporal relationships between his- by recruiting specific effector proteins (1–4). The phenome- tone marks. Despite discoveries of reader domains that physically non of histone code “crosstalk,” whereby one type of histone associate with chromatin-modifying enzymes, the mechanisms by modification directs the establishment of another, or through which recognition of one modification triggers other kinds of which several histone modifications are recognized in tandem, has modifications have remained elusive. Gcn5 is the catalytic subunit emerged as an important and widespread mechanism regulating of the Spt-Ada-Gcn5 acetyltransferase (SAGA) histone acetyl- chromatin-templated processes (5, 6). The multifunctional com- transferase (HAT) module, which also recognizes histone 3 lysine 4 plexes that activate transcription are thought to mediate crosstalk trimethylation (H3K4me3) through the tandem Tudor domain- through “reader” domains that recognize particular chromatin containing protein Sgf29. Although previous studies could not marks as well as through catalytic subunits that deposit or remove connect H3K4me3 recognition to differences in acetylation by histone modifications (5, 7, 8). As a result, distinct combinations of Gcn5, we report enhanced processivity by the HAT module on histone posttranslational modifications that correlate with tran- methylated substrates using a previously unpublished histone scriptional output cluster across the genome (9–11). High levels of color-coding assay. Our work defines a mechanism for histone histone 3 lysine 4 trimethylation (H3K4me3) and histone H3 crosstalk that may account for genome-wide patterns of Gcn5- hyperacetylation are present at the promoters of actively tran- mediated acetylation. scribed genes (9, 11, 12), and multiple lines of evidence support the Author contributions: A.E.R., S.D.T., and C.W. designed research; A.E.R. and A.M.C. per- existence of regulatory mechanisms coupling the two modifica- formed research; A.E.R., A.M.C., S.D.T., and C.W. analyzed data; and A.E.R. and C.W. tions. Studies using tandem mass spectrometry to sequence whole wrote the paper. histone tails have shown that H3K4me3 is highly correlated with The authors declare no conflict of interest. hyperacetylation of the same H3 tail (13, 14). Deleting the yeast This article is a PNAS Direct Submission. Set1 methyltransferase, which trimethylates H3K4, leads to dra- Freely available online through the PNAS open access option. matically lower levels of histone H3 tail acetylation overall (13, 15). 1To whom correspondence should be addressed. Email: [email protected]. These results are consistent with a role for H3K4 methylation in This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. triggering hyperacetylation of histone H3; indeed, a number of 1073/pnas.1508449112/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1508449112 PNAS Early Edition | 1of10 Downloaded by guest on September 26, 2021 HAT module and variants lacking either the Sgf29 tandem Tudor domain or the Gcn5 bromodomain were expressed as complexes in Escherichia coli and purified to homogeneity (Fig. 1). We first confirmed that S. pombe Sgf29 interacts specifically with H3K4me3 peptides in a pull-down assay. As shown in Fig. 2A, streptavidin beads coated with H3K4me3 peptides efficiently re- tain Sgf29, whereas unmodified H3 peptides do not. To determine the effect of H3K4me3 on the kinetics of pep- tide acetylation by Gcn5, we measured initial acetylation rates by the HAT module at increasing concentrations of either un- modified or H3K4me3 peptides in the presence of saturating amounts of acetyl-CoA. Compared with unmodified peptides, Fig. 1. Domain architecture of HAT module complexes. (Left) SDS/PAGE gel the presence of the H3K4me3 modification increases kcat/Km by of purified S. pombe HAT module complexes containing different domain 3.1-fold (Fig. 2B and Table 1). This change in kcat/Km is caused truncations in which all four subunits are present in roughly equimolar solely by a decrease in Km from 250 ± 40 μMto77± 10 μM, with quantities. (Right) Location of the catalytic domain, tandem Tudor domain, no corresponding change in kcat (Fig. 2B and Table 1). Deletion and bromodomain within the HAT module. of the Sgf29 tandem Tudor domain (ΔTudor) eliminates the difference in acetylation kinetics on H3K4me3 versus un- be much greater than previously determined. Thus, although modified peptides (Fig. 2C and Table 1), as is consistent with a k K multiple lines of evidence suggest that H3K4me3 regulates role for H3K4me3 binding in the observed difference in cat/ m. histone acetylation by SAGA in yeast and mammalian cells (17, We also ruled out the possibility that the H3K4me3 modification k K D 30, 31, 33), in vitro studies have failed to explain how H3K4me3 affects cat or m for acetyl-CoA (Fig. 2 and Table 2); our recognition by Sgf29 gives rise to different patterns of histone acet- finding is consistent with the proposed Gcn5 reaction mecha- ylation. As a result, the underlying mechanism governing crosstalk nism, in which the cofactor binds before the peptide (34, 35). between H3K4 trimethylation and acetylation by SAGA remains To confirm that acetylation by Gcn5 is not directly affected by poorly understood. truncating the Sgf29 tandem Tudor domain, we compared kcat K Δ To elucidate the mechanism underlying crosstalk between