Insights into newly discovered marks and readers of epigenetic information

The field of chromatin biology has been advancing at an accelerated pace. Recent discoveries of previously uncharacterized sites and types of post-translational modifications (PTMs) and the identification of new sets of proteins responsible for the deposition, removal, and reading of these marks continue raising the complexity of an already exceedingly complicated biological phenomenon. In this Perspective article we examine the biological importance of new types and sites of histone PTMs and summarize the molecular mechanisms of chromatin engagement by newly discovered epigenetic readers. We also highlight the imperative role of structural insights in understanding PTM–reader interactions and discuss future directions to enhance the knowledge of PTM readout

Regulation of Methyllysine Readers through Phosphorylation

Methyllysine post-translational modifications (PTMs) of histones create binding sites for evolutionarily conserved reader domains that link nuclear host proteins and chromatin-modifying complexes to specific genomic regions. In the context of these events, adjacent histone PTMs are capable of altering the binding activity of readers toward their target marks. This provides a mechanism of "combinatorial readout" of PTMs that can enhance, decrease, or eliminate the association of readers with chromatin. In this Perspective, we focus on recent studies describing the impact of dynamic phospho-serine/threonine/tyrosine marks on the interaction of methyllysine readers with histones, summarize mechanistic aspects of the phospho/methyl readout, and highlight the significance of crosstalk between these PTMs. We also demonstrate that in addition to inhibiting binding and serving as a true switch, promoting dissociation of the methyllysine readers from chromatin, the phospho/methyl combination can act together in a cooperative manner--thus adding a new layer of regulatory information that can be encoded in these dual histone PTMs

Molecular mechanism of MLL PHD3 and RNA recognition by the Cyp33 RRM domain

The nuclear protein cyclophilin 33 (Cyp33) is a peptidyl-prolyl cis-trans isomerase that catalyzes cis-trans isomerization of the peptide bond preceding a proline and promotes folding and conformational changes in folded and unfolded proteins. The N-terminal RNA-recognition motif (RRM) domain of Cyp33 has been found to associate with the third plant homeodomain (PHD3) finger of the mixed lineage leukemia (MLL) proto-oncoprotein and a poly(A) RNA sequence. Here, we report a 1.9 A resolution crystal structure of the RRM domain of Cyp33 and describe the molecular mechanism of PHD3 and RNA recognition. The Cyp33 RRM domain folds into a five-stranded antiparallel beta-sheet and two alpha-helices. The RRM domain, but not the catalytic module of Cyp33, binds strongly to PHD3, exhibiting a 2 muM affinity as measured by isothermal titration calorimetry. NMR chemical shift perturbation (CSP) analysis and dynamics data reveal that the beta strands and the beta2-beta3 loop of the RRM domain are involved in the interaction with PHD3. Mutations in the PHD3-binding site or deletions in the beta2-beta3 loop lead to a significantly reduced affinity or abrogation of the interaction. The RNA-binding pocket of the Cyp33 RRM domain, mapped on the basis of NMR CSP and mutagenesis, partially overlaps with the PHD3-binding site, and RNA association is abolished in the presence of MLL PHD3. Full-length Cyp33 acts as a negative regulator of MLL-induced transcription and reduces the expression levels of MLL target genes MEIS1 and HOXA9. Together, these in vitro and in vivo data provide insight into the multiple functions of Cyp33 RRM and suggest a Cyp33-dependent mechanism for regulating the transcriptional activity of MLL

The Taf14 YEATS domain is a reader of histone crotonylation

The discovery of new histone modifications is unfolding at startling rates, however, the identification of effectors capable of interpreting these modifications has lagged behind. Here we report the YEATS domain as an effective reader of histone lysine crotonylation – an epigenetic signature associated with active transcription. We show that the Taf14 YEATS domain engages crotonyllysine via a unique π-π-π-stacking mechanism and that other YEATS domains have crotonyllysine binding activity

A triple action CDK4/6-PI3K-BET inhibitor with augmented cancer cell cytotoxicity

National Institutes of Health GM125195, GM135671, CA192656, FD00511

Chromatin condensation and recruitment of PHD finger proteins to histone H3K4me3 are mutually exclusive

Histone post-translational modifications, and specific combinations they create, mediate a wide range of nuclear events. However, the mechanistic bases for recognition of these combinations have not been elucidated. Here, we characterize crosstalk between H3T3 and H3T6 phosphorylation, occurring in mitosis, and H3K4me3, a mark associated with active transcription. We detail the molecular mechanisms by which H3T3ph/K4me3/T6ph switches mediate activities of H3K4me3-binding proteins, including those containing plant homeodomain (PHD) and double Tudor reader domains. Our results derived from nuclear magnetic resonance chemical shift perturbation analysis, orthogonal binding assays and cell fluorescence microscopy studies reveal a strong anti-correlation between histone H3T3/T6 phosphorylation and retention of PHD finger proteins in chromatin during mitosis. Together, our findings uncover the mechanistic rules of chromatin engagement for H3K4me3-specific readers during cell division

Molecular Mechanism of Membrane Docking by the Vam7p PX Domain

The Vam7p t-SNARE is an essential component of the vacuole fusion machinery that mediates membrane trafficking and protein sorting in yeast. Vam7p is recruited to vacuoles by its N-terminal PX domain that specifically recognizes PtdIns(3)P in the bilayers, however the precise mechanism of membrane anchoring remains unclear. Here we describe a molecular basis for membrane targeting and penetration by the Vam7p PX domain based on structural and quantitative analysis of its interactions with lipids and micelles. Our results derived from in vitro binding measurements using NMR, monolayer surface tension experiments and mutagenesis reveal a multivalent membrane docking mechanism involving specific PtdIns(3)P recognition that is facilitated by electrostatic interactions and accompanying hydrophobic insertion. Both the hydrophobic and electrostatic components enhance the Vam7p PX domain association with PtdIns(3)P-containing membranes. The inserting Val70, Leu71, and Trp75 residues located next to the PtdIns(3)P binding pocket are surrounded by a basic patch, which is involved in nonspecific electrostatic contacts with acidic lipids, such as PtdSer. Substitution of the insertion residues significantly reduces the binding and penetrating power of the Vam7p PX domain and leads to cytoplasmic redistribution of the EGFP-tagged protein. The affinities of the PX domain for PtdIns(3)P and other lipids reveal a remarkable synergy within the multivalent complex that stably anchors Vam7p at the vacuolar membrane

An Acetyl-Methyl Switch Drives a Conformational Change in p53

Individual posttranslational modifications (PTMs) of p53 mediate diverse p53-dependent responses, however much less is known about the combinatorial action of adjacent modifications. Here, we describe crosstalk between the early DNA damage response mark p53K382me2 and the surrounding PTMs that modulate binding of p53 co-factors, including 53BP1 and p300. The 1.8 Å resolution crystal structure of the tandem Tudor domain (TTD) of 53BP1 in complex with p53 peptide acetylated at K381 and dimethylated at K382 (p53K381acK382me2) reveals that the dual PTM induces a conformational change in p53. The α-helical fold of p53K381acK382me2 positions the side chains of R379, K381ac, and K382me2 to interact with TTD concurrently, reinforcing a modular design of double PTM mimetics. Biochemical and NMR analyses show that other surrounding PTMs, including phosphorylation of serine/threonine residues of p53, affect association with TTD. Our findings suggest a novel PTM-driven conformation switch-like mechanism that may regulate p53 interactions with binding partners

Tandem PHD Fingers of MORF/MOZ Acetyltransferases Display Selectivity for Acetylated Histone H3 and Are Required for the Association with Chromatin

MORF (monocytic leukemia zinc-finger protein (MOZ)-related factor) and MOZ are catalytic subunits of histone acetyltransferase (HAT) complexes essential in hematopoiesis, neurogenesis, skeletogenesis and other developmental programs and implicated in human leukemias. The canonical HAT domain of MORF/MOZ is preceded by a tandem of plant homeodomain (PHD) fingers whose biological roles and requirements for MORF/MOZ activity are unknown. Here we demonstrate that the tandem PHD1/2 fingers of MORF recognize the N-terminal tail of histone H3. Acetylation of Lys9 (H3K9ac) or Lys14 (H3K14ac) enhances binding of MORF PHD1/2 to unmodified H3 peptides two to three fold. The selectivity for acetylated H3 tail is conserved in the double PHD1/2 fingers of MOZ. This interaction requires the intact N-terminus of histone H3 and is inhibited by trimethylation of Lys4. Biochemical analysis using NMR, fluorescence spectroscopy and mutagenesis identified key amino acids of MORF PHD1/2 necessary for the interaction with histones. Fluorescence microscopy and immunoprecipitation experiments reveal that both PHD fingers are required for binding to H3K14ac in vivo and localization to chromatin. The HAT assays indicate that the interaction with H3K14ac may promote enzymatic activity in trans. Together, our data suggest that the PHD1/2 fingers play a role in MOZ/MORF HATs association with the chromatic regions enriched in acetylated marks

KAP1 Is a Chromatin Reader that Couples Steps of RNA Polymerase II Transcription to Sustain Oncogenic Programs

Precise control of the RNA polymerase II (RNA Pol II) cycle, including pausing and pause release, maintains transcriptional homeostasis and organismal functions. Despite previous work to understand individual transcription steps, we reveal a mechanism that integrates RNA Pol II cycle transitions. Surprisingly, KAP1/TRIM28 uses a previously uncharacterized chromatin reader cassette to bind hypo-acetylated histone 4 tails at promoters, guaranteeing continuous progression of RNA Pol II entry to and exit from the pause state. Upon chromatin docking, KAP1 first associates with RNA Pol II and then recruits a pathway-specific transcription factor (SMAD2) in response to cognate ligands, enabling gene-selective CDK9-dependent pause release. This coupling mechanism is exploited by tumor cells to aberrantly sustain transcriptional programs commonly dysregulated in cancer patients. The discovery of a factor integrating transcription steps expands the functional repertoire by which chromatin readers operate and provides mechanistic understanding of transcription regulation, offering alternative therapeutic opportunities to target transcriptional dysregulation