Mechanism for autoinhibition and activation of the MORC3 ATPase.

Microrchidia 3 (MORC3) is a human protein linked to autoimmune disorders, Down syndrome, and cancer. It is a member of a newly identified family of human ATPases with an uncharacterized mechanism of action. Here, we elucidate the molecular basis for inhibition and activation of MORC3. The crystal structure of the MORC3 region encompassing the ATPase and CW domains in complex with a nonhydrolyzable ATP analog demonstrates that the two domains are directly coupled. The extensive ATPase:CW interface stabilizes the protein fold but inhibits the catalytic activity of MORC3. Enzymatic, NMR, mutational, and biochemical analyses show that in the autoinhibited, off state, the CW domain sterically impedes binding of the ATPase domain to DNA, which in turn is required for the catalytic activity. MORC3 autoinhibition is released by disrupting the intramolecular ATPase:CW coupling through the competitive interaction of CW with histone H3 tail or by mutating the interfacial residues. Binding of CW to H3 leads to a marked rearrangement in the ATPase-CW cassette, which frees the DNA-binding site in active MORC3 (on state). We show that ATP-induced dimerization of the ATPase domain is strictly required for the catalytic activity and that the dimeric form of ATPase-CW might cooperatively bind to dsDNA. Together, our findings uncovered a mechanism underlying the fine-tuned regulation of the catalytic domain of MORC3 by the epigenetic reader, CW.

Guiding the HBO1 complex function through the JADE subunit.

JADE is a core subunit of the HBO1 acetyltransferase complex that regulates developmental and epigenetic programs and promotes gene transcription. Here we describe the mechanism by which JADE facilitates recruitment of the HBO1 complex to chromatin and mediates its enzymatic activity. Structural, genomic and complex assembly in vivo studies show that the PZP (PHD1-zinc-knuckle-PHD2) domain of JADE engages the nucleosome through binding to histone H3 and DNA and is necessary for the association with chromatin targets. Recognition of unmethylated H3K4 by PZP directs enzymatic activity of the complex toward histone H4 acetylation, whereas H3K4 hypermethylation alters histone substrate selectivity. We demonstrate that PZP contributes to leukemogenesis, augmenting transforming activity of the NUP98-JADE2 fusion. Our findings highlight biological consequences and the impact of the intact JADE subunit on genomic recruitment, enzymatic function and pathological activity of the HBO1 complex.

The winged helix domain of MORF binds CpG islands and the TAZ2 domain of p300.

Acetylation of histones by lysine acetyltransferases (KATs) provides a fundamental mechanism by which chromatin structure and transcriptional programs are regulated. Here, we describe a dual binding activity of the first winged helix domain of human MORF KAT (MORFWH1) that recognizes the TAZ2 domain of p300 KAT (p300TAZ2) and CpG rich DNA sequences. Structural and biochemical studies identified distinct DNA and p300TAZ2 binding sites, allowing MORFWH1 to independently engage either ligand. Genomic data show that MORF/MOZWH1 colocalizes with H3K18ac, a product of enzymatic activity of p300, on CpG rich promoters of target genes. Our findings suggest a functional cooperation of MORF and p300 KATs in transcriptional regulation.

MORF and MOZ acetyltransferases target unmethylated CpG islands through the winged helix domain.

Human acetyltransferases MOZ and MORF are implicated in chromosomal translocations associated with aggressive leukemias. Oncogenic translocations involve the far amino terminus of MOZ/MORF, the function of which remains unclear. Here, we identified and characterized two structured winged helix (WH) domains, WH1 and WH2, in MORF and MOZ. WHs bind DNA in a cooperative manner, with WH1 specifically recognizing unmethylated CpG sequences. Structural and genomic analyses show that the DNA binding function of WHs targets MORF/MOZ to gene promoters, stimulating transcription and H3K23 acetylation, and WH1 recruits oncogenic fusions to HOXA genes that trigger leukemogenesis. Cryo-EM, NMR, mass spectrometry and mutagenesis studies provide mechanistic insight into the DNA-binding mechanism, which includes the association of WH1 with the CpG-containing linker DNA and binding of WH2 to the dyad of the nucleosome. The discovery of WHs in MORF and MOZ and their DNA binding functions could open an avenue in developing therapeutics to treat diseases associated with aberrant MOZ/MORF acetyltransferase activities.

Structural and biophysical characterization of the nucleosome-binding PZP domain.

The core subunit of the MORF acetyltransferase complex BRPF1 contains a unique combination of zinc fingers, including a plant homeodomain (PHD) finger followed by a zinc knuckle and another PHD finger, which together form a PZP domain (BRPF1PZP). BRPF1PZP has been shown to bind to the nucleosome and make contacts with both histone H3 tail and DNA. Here, we describe biophysical and structural methods for characterization of the interactions between BRPF1PZP, H3 tail, DNA, and the intact nucleosome. For complete details on the use and execution of this protocol, please refer to Klein et al. (2020).

The role of the PZP domain of AF10 in acute leukemia driven by AF10 translocations.

Chromosomal translocations of the AF10 (or MLLT10) gene are frequently found in acute leukemias. Here, we show that the PZP domain of AF10 (AF10PZP), which is consistently impaired or deleted in leukemogenic AF10 translocations, plays a critical role in blocking malignant transformation. Incorporation of functional AF10PZP into the leukemogenic CALM-AF10 fusion prevents the transforming activity of the fusion in bone marrow-derived hematopoietic stem and progenitor cells in vitro and in vivo and abrogates CALM-AF10-mediated leukemogenesis in vivo. Crystallographic, biochemical and mutagenesis studies reveal that AF10PZP binds to the nucleosome core particle through multivalent contacts with the histone H3 tail and DNA and associates with chromatin in cells, colocalizing with active methylation marks and discriminating against the repressive H3K27me3 mark. AF10PZP promotes nuclear localization of CALM-AF10 and is required for association with chromatin. Our data indicate that the disruption of AF10PZP function in the CALM-AF10 fusion directly leads to transformation, whereas the inclusion of AF10PZP downregulates Hoxa genes and reverses cellular transformation. Our findings highlight the molecular mechanism by which AF10 targets chromatin and suggest a model for the AF10PZP-dependent CALM-AF10-mediated leukemogenesis.

Molecular Basis for the PZP Domain of BRPF1 Association with Chromatin.

The assembly of human histone acetyltransferase MOZ/MORF complexes relies on the scaffolding bromodomain plant homeodomain (PHD) finger 1 (BRPF1) subunit. The PHD-zinc-knuckle-PHD module of BRPF1 (BRPF1PZP) has been shown to associate with the histone H3 tail and DNA; however, the molecular mechanism underlying recognition of H3 and the relationship between the histone and DNA-binding activities remain unclear. In this study, we report the crystal structure of BRPF1PZP bound to the H3 tail and characterize the role of the bipartite interaction in the engagement of BRPF1PZP with the nucleosome core particle (NCP). We find that although both interactions of BRPF1PZP with the H3 tail and DNA are required for tight binding to NCP and for acetyltransferase function of the BRPF1-MORF-ING5-MEAF6 complex, binding to extranucleosomal DNA dominates. Our findings suggest that functionally active BRPF1PZP might be important in stabilization of the MOZ/MORF complexes at chromatin with accessible DNA.

Molecular mechanism of the MORC4 ATPase activation.

Human Microrchidia 4 (MORC4) is associated with acute and chronic pancreatitis, inflammatory disorders and cancer but it remains largely uncharacterized. Here, we describe the structure-function relationship of MORC4 and define the molecular mechanism for MORC4 activation. Enzymatic and binding assays reveal that MORC4 has ATPase activity, which is dependent on DNA-binding functions of both the ATPase domain and CW domain of MORC4. The crystal structure of the ATPaseCW cassette of MORC4 and mutagenesis studies show that the DNA-binding site and the histone/ATPase binding site of CW are located on the opposite sides of the domain. The ATPase and CW domains cooperate in binding of MORC4 to the nucleosome core particle (NCP), enhancing the DNA wrapping around the histone core and impeding binding of DNA-associated proteins, such as transcription factors, to the NCP. In cells, MORC4 mediates formation of nuclear bodies in the nucleus and has a role in the progression of S-phase of the cell cycle, and both these functions require CW and catalytic activity of MORC4. Our findings highlight the mechanism for MORC4 activation, which is distinctly different from the mechanisms of action observed in other MORC family members.

Accessibility of the histone H3 tail in the nucleosome for binding of paired readers.

Combinatorial polyvalent contacts of histone-binding domains or readers commonly mediate localization and activities of chromatin-associated proteins. A pair of readers, the PHD fingers of the protein CHD4, has been shown to bivalently recognize histone H3 tails. Here we describe a mechanism by which these linked but independent readers bind to the intact nucleosome core particle (NCP). Comprehensive NMR, chemical reactivity, molecular dynamics, and fluorescence analyses point to the critical roles of intra-nucleosomal histone-DNA interactions that reduce the accessibility of H3 tails in NCP, the nucleosomal DNA, and the linker between readers in modulating nucleosome- and/or histone-binding activities of the readers. We show that the second PHD finger of CHD4 initiates recruitment to the nucleosome, however both PHDs are required to alter the NCP dynamics. Our findings reveal a distinctive regulatory mechanism for the association of paired readers with the nucleosome that provides an intricate balance between cooperative and individual activities of the readers.

Covalent Modifications of Histone H3K9 Promote Binding of CHD3.

Chromatin remodeling is required for genome function and is facilitated by ATP-dependent complexes, such as nucleosome remodeling and deacetylase (NuRD). Among its core components is the chromodomain helicase DNA binding protein 3 (CHD3) whose functional significance is not well established. Here, we show that CHD3 co-localizes with the other NuRD subunits, including HDAC1, near the H3K9ac-enriched promoters of the NuRD target genes. The tandem PHD fingers of CHD3 bind histone H3 tails and posttranslational modifications that increase hydrophobicity of H3K9-methylation or acetylation (H3K9me3 or H3K9ac)-enhance this interaction. Binding of CHD3 PHDs promotes H3K9Cme3-nucleosome unwrapping in vitro and perturbs the pericentric heterochromatin structure in vivo. Methylation or acetylation of H3K9 uniquely alleviates the intra-nucleosomal interaction of histone H3 tails, increasing H3K9 accessibility. Collectively, our data suggest that the targeting of covalently modified H3K9 by CHD3 might be essential in diverse functions of NuRD.