Interpreting the molecular mechanisms of RBBP4/7 and their roles in human diseases (Review).

Histone chaperones serve a pivotal role in maintaining human physiological processes. They interact with histones in a stable manner, ensuring the accurate and efficient execution of DNA replication, repair and transcription. Retinoblastoma binding protein (RBBP)4 and RBBP7 represent a crucial pair of histone chaperones, which not only govern the molecular behavior of histones H3 and H4, but also participate in the functions of several protein complexes, such as polycomb repressive complex 2 and nucleosome remodeling and deacetylase, thereby regulating the cell cycle, histone modifications, DNA damage and cell fate. A strong association has been indicated between RBBP4/7 and some major human diseases, such as cancer, age­related memory loss and infectious diseases. The present review assesses the molecular mechanisms of RBBP4/7 in regulating cellular biological processes, and focuses on the variations in RBBP4/7 expression and their potential mechanisms in various human diseases, thus providing new insights for their diagnosis and treatment.

Overlapping functions of RBBP4 and RBBP7 in regulating cell proliferation and histone H3.3 deposition during mouse preimplantation development.

Preimplantation development is critical for reproductive successes in mammals. Thus, it is important to understand how preimplantation embryogenesis is regulated. As a key event of preimplantation development, epigenetic reprogramming has been widely studied, yet how epigenetic complexes regulate preimplantation development remains largely unknown. Retinoblastoma binding protein 4 (RBBP4) and 7 (RBBP7) are integral components of epigenetic complexes including SIN3A, NuRD, and CoREST. Here, we demonstrate that double knockdown of Rbbp4 and 7, but not individually, causes embryonic lethality during the morula-to-blastocyst transition. Mechanistically, depletion of RBBP4 and 7 results in dysregulation of genes related to cell cycle, lineage development, and regulation of transcription, which is accompanied by cell cycle block, disrupted lineage specification and chromatin structure. Interestingly, RBBP4/7 depletion leads to a dramatic increase in H3.3 and H3K27ac abundance during morula-to-blastocyst transition. ChIP-seq analysis in early embryos and embryonic stem cells reveals enrichment of H3.3 at the promoter regions of RBBP4/7 target genes. In summary, our studies demonstrate the compensatory role of RBBP4/7 and reveal its potential mechanisms in preimplantation development.Summary sentence:RBBP4 and RBBP7 play a compensatory role in regulating cell proliferation, apoptosis, and histone H3.3 deposition during preimplantation development.

Direct interaction between the PRDM3 and PRDM16 tumor suppressors and the NuRD chromatin remodeling complex.

Aberrant isoform expression of chromatin-associated proteins can induce epigenetic programs related to disease. The MDS1 and EVI1 complex locus (MECOM) encodes PRDM3, a protein with an N-terminal PR-SET domain, as well as a shorter isoform, EVI1, lacking the N-terminus containing the PR-SET domain (ΔPR). Imbalanced expression of MECOM isoforms is observed in multiple malignancies, implicating EVI1 as an oncogene, while PRDM3 has been suggested to function as a tumor suppressor through an unknown mechanism. To elucidate functional characteristics of these N-terminal residues, we compared the protein interactomes of the full-length and ΔPR isoforms of PRDM3 and its closely related paralog, PRDM16. Unlike the ΔPR isoforms, both full-length isoforms exhibited a significantly enriched association with components of the NuRD chromatin remodeling complex, especially RBBP4. Typically, RBBP4 facilitates chromatin association of the NuRD complex by binding to histone H3 tails. We show that RBBP4 binds to the N-terminal amino acid residues of PRDM3 and PRDM16, with a dissociation constant of 3.0 µM, as measured by isothermal titration calorimetry. Furthermore, high-resolution X-ray crystal structures of PRDM3 and PRDM16 N-terminal peptides in complex with RBBP4 revealed binding to RBBP4 within the conserved histone H3-binding groove. These data support a mechanism of isoform-specific interaction of PRDM3 and PRDM16 with the NuRD chromatin remodeling complex.

Targeting cancer addiction for SALL4 by shifting its transcriptome with a pharmacologic peptide.

Sal-like 4 (SALL4) is a nuclear factor central to the maintenance of stem cell pluripotency and is a key component in hepatocellular carcinoma, a malignancy with no effective treatment. In cancer cells, SALL4 associates with nucleosome remodeling deacetylase (NuRD) to silence tumor-suppressor genes, such as PTEN. Here, we determined the crystal structure of an amino-terminal peptide of SALL4(1-12) complexed to RBBp4, the chaperone subunit of NuRD, at 2.7 Å, and subsequent design of a potent therapeutic SALL4 peptide (FFW) capable of antagonizing the SALL4-NURD interaction using systematic truncation and amino acid substitution studies. FFW peptide disruption of the SALL4-NuRD complex resulted in unidirectional up-regulation of transcripts, turning SALL4 from a dual transcription repressor-activator mode to singular transcription activator mode. We demonstrate that FFW has a target affinity of 23 nM, and displays significant antitumor effects, inhibiting tumor growth by 85% in xenograft mouse models. Using transcriptome and survival analysis, we discovered that the peptide inhibits the transcription-repressor function of SALL4 and causes massive up-regulation of transcripts that are beneficial to patient survival. This study supports the SALL4-NuRD complex as a drug target and FFW as a viable drug candidate, showcasing an effective strategy to accurately target oncogenes previously considered undruggable.

Structural and functional characterization of the RBBP4-ZNF827 interaction and its role in NuRD recruitment to telomeres.

The nucleosome remodeling and histone deacetylase (NuRD) complex is an essential multi-subunit protein complex that regulates higher-order chromatin structure. Cancers that use the alternative lengthening of telomere (ALT) pathway of telomere maintenance recruit NuRD to their telomeres. This interaction is mediated by the N-terminal domain of the zinc-finger protein ZNF827. NuRD-ZNF827 plays a vital role in the ALT pathway by creating a molecular platform for recombination-mediated repair. Disruption of NuRD binding results in loss of ALT cell viability. Here, we present the crystal structure of the NuRD subunit RBBP4 bound to the N-terminal 14 amino acids of ZNF827. RBBP4 forms a negatively charged channel that binds to ZNF827 through a network of electrostatic interactions. We identify the precise amino acids in RBBP4 required for this interaction and demonstrate that disruption of these residues prevents RBBP4 binding to both ZNF827 and telomeres, but is insufficient to decrease ALT activity. These data provide insights into the structural and functional determinants of NuRD activity at ALT telomeres.

Unique Structural Platforms of Suz12 Dictate Distinct Classes of PRC2 for Chromatin Binding.

Developmentally regulated accessory subunits dictate PRC2 function. Here, we report the crystal structures of a 120 kDa heterotetrameric complex consisting of Suz12, Rbbp4, Jarid2, and Aebp2 fragments that is minimally active in nucleosome binding and of an inactive binary complex of Suz12 and Rbbp4. Suz12 contains two unique structural platforms that define distinct classes of PRC2 holo complexes for chromatin binding. Aebp2 and Phf19 compete for binding of a non-canonical C2 domain of Suz12; Jarid2 and EPOP occupy an overlapped Suz12 surface required for chromatin association of PRC2. Suz12 and Aebp2 progressively block histone H3K4 binding to Rbbp4, suggesting that Rbbp4 may not be directly involved in PRC2 inhibition by the active H3K4me3 histone mark. Nucleosome binding enabled by Jarid2 and Aebp2 is in part accounted for by the structures, which also reveal that disruption of the Jarid2-Suz12 interaction may underlie the disease mechanism of an oncogenic chromosomal translocation of Suz12.

Cryo-EM structures of PRC2 simultaneously engaged with two functionally distinct nucleosomes.

Epigenetic regulation is mediated by protein complexes that couple recognition of chromatin marks to activity or recruitment of chromatin-modifying enzymes. Polycomb repressive complex 2 (PRC2), a gene silencer that methylates lysine 27 of histone H3, is stimulated upon recognition of its own catalytic product and has been shown to be more active on dinucleosomes than H3 tails or single nucleosomes. These properties probably facilitate local H3K27me2/3 spreading, causing heterochromatin formation and gene repression. Here, cryo-EM reconstructions of human PRC2 bound to bifunctional dinucleosomes show how a single PRC2, via interactions with nucleosomal DNA, positions the H3 tails of the activating and substrate nucleosome to interact with the EED subunit and the SET domain of EZH2, respectively. We show how the geometry of the PRC2-DNA interactions allows PRC2 to accommodate varying lengths of the linker DNA between nucleosomes. Our structures illustrate how an epigenetic regulator engages with a complex chromatin substrate.

Probing the interaction between the histone methyltransferase/deacetylase subunit RBBP4/7 and the transcription factor BCL11A in epigenetic complexes.

The transcription factor BCL11A has recently been reported to be a driving force in triple-negative breast cancer (TNBC), contributing to the maintenance of a chemoresistant breast cancer stem cell (BCSC) population. Although BCL11A was shown to suppress γ-globin and p21 and to induce MDM2 expression in the hematopoietic system, its downstream targets in TNBC are still unclear. For its role in transcriptional repression, BCL11A was found to interact with several corepressor complexes; however, the mechanisms underlying these interactions remain unknown. Here, we reveal that BCL11A interacts with histone methyltransferase (PRC2) and histone deacetylase (NuRD and SIN3A) complexes through their common subunit, RBBP4/7. In fluorescence polarization assays, we show that BCL11A competes with histone H3 for binding to the negatively charged top face of RBBP4. To define that interaction, we solved the crystal structure of RBBP4 in complex with an N-terminal peptide of BCL11A (residues 2-16, BCL11A(2-16)). The crystal structure identifies novel interactions between BCL11A and the side of the ß-propeller of RBBP4 that are not seen with histone H3. We next show that BCL11A(2-16) pulls down RBBP4, RBBP7, and other components of PRC2, NuRD, and SIN3A from the cell lysate of the TNBC cell line SUM149. Furthermore, we demonstrate the therapeutic potential of targeting the RBBP4-BCL11A binding by showing that a BCL11A peptide can decrease aldehyde dehydrogenase-positive BCSCs and mammosphere formation capacity in SUM149. Together, our findings have uncovered a previously unidentified mechanism that BCL11A may use to recruit epigenetic complexes to regulate transcription and promote tumorigenesis.

The MTA1 subunit of the nucleosome remodeling and deacetylase complex can recruit two copies of RBBP4/7.

The nucleosome remodeling and deacetylase (NuRD) complex remodels the genome in the context of both gene transcription and DNA damage repair. It is essential for normal development and is distributed across multiple tissues in organisms ranging from mammals to nematode worms. In common with other chromatin-remodeling complexes, however, its molecular mechanism of action is not well understood and only limited structural information is available to show how the complex is assembled. As a step towards understanding the structure of the NuRD complex, we have characterized the interaction between two subunits: the metastasis associated protein MTA1 and the histone-binding protein RBBP4. We show that MTA1 can bind to two molecules of RBBP4 and present negative stain electron microscopy and chemical crosslinking data that allow us to build a low-resolution model of an MTA1-(RBBP4)2 subcomplex. These data build on our understanding of NuRD complex structure and move us closer towards an understanding of the biochemical basis for the activity of this complex.

Structural basis of plant homeodomain finger 6 (PHF6) recognition by the retinoblastoma binding protein 4 (RBBP4) component of the nucleosome remodeling and deacetylase (NuRD) complex.

The NuRD complex is a conserved transcriptional coregulator that contains both chromatin-remodeling and histone deacetylase activities. Mutations of PHF6 are found in patients with Börjeson-Forssman-Lehmann syndrome, T-cell acute lymphoblastic leukemia, or acute myeloid leukemia. Recently, PHF6 was identified to interact with the NuRD complex, and this interaction is mediated by the RBBP4 component. However, little is known about the molecular basis for the interaction. Here, we present the crystal structure of the complex of the NuRD subunit RBBP4 bound to the PHF6 peptide (residues 162-170). The PHF6 peptide binds to the top surface of the RBBP4 ß-propeller. A pair of positively charged residues of the PHF6 peptide insert into the negatively charged pocket of RBBP4, which is critical for the interaction between PHF6 and RBBP4. Corresponding PHF6 mutants impair this interaction in vitro and in vivo. Structural comparison shows that the PHF6-binding pocket overlaps with FOG1 and histone H3 on RBBP4/Nurf55, but it is distinct from the pocket recognizing histone H4, Su(z)12, and MTA1. We further show that the middle disordered region (residues 145-207, containing the RBBP4-binding motif) is sufficient for the transcriptional repression mediated by PHF6 on the GAL4 reporter, and knockdown of RBBP4 diminished the PHF6-mediated repression. Our RBBP4-PHF6 complex structure provides insights into the molecular basis of PHF6-NuRD complex interaction and implicates a role for PHF6 in chromatin structure modulation and gene regulation.

Lead discovery for Alzheimer's disease related target protein RbAp48 from traditional Chinese medicine.

Deficiency or loss of function of Retinoblastoma-associated proteins (RbAp48) is related with Alzheimer's disease (AD), and AD disease is associated with age-related memory loss. During normal function, RbAp48 forms a complex with the peptide FOG-1 (friend of GATA-1) and has a role in gene transcription, but an unstable complex may affect the function of RbAp48. This study utilizes the world's largest traditional Chinese medicine (TCM) database and virtual screening to provide potential compounds for RbAp48 binding. A molecular dynamics (MD) simulation was employed to understand the variations after protein-ligand interaction. FOG1 was found to exhibit low stability after RbAp48 binding; the peptide displayed significant movement from the initial docking position, a phenomenon which matched the docking results. The protein structure of the other TCM candidates was not variable during MD simulation and had a greater stable affinity for RbAp48 binding than FOG1. Our results reveal that the protein structure does not affect ligand binding, and the top three TCM candidates Bittersweet alkaloid II, Eicosandioic acid, and Perivine might resolve the instability of the RbAp48-FOG1 complex and thus be used in AD therapy.

Insight into the architecture of the NuRD complex: structure of the RbAp48-MTA1 subcomplex.

The nucleosome remodeling and deacetylase (NuRD) complex is a widely conserved transcriptional co-regulator that harbors both nucleosome remodeling and histone deacetylase activities. It plays a critical role in the early stages of ES cell differentiation and the reprogramming of somatic to induced pluripotent stem cells. Abnormalities in several NuRD proteins are associated with cancer and aging. We have investigated the architecture of NuRD by determining the structure of a subcomplex comprising RbAp48 and MTA1. Surprisingly, RbAp48 recognizes MTA1 using the same site that it uses to bind histone H4, showing that assembly into NuRD modulates RbAp46/48 interactions with histones. Taken together with other results, our data show that the MTA proteins act as scaffolds for NuRD complex assembly. We further show that the RbAp48-MTA1 interaction is essential for the in vivo integration of RbAp46/48 into the NuRD complex.

Structural plasticity of histones H3-H4 facilitates their allosteric exchange between RbAp48 and ASF1.

The mechanisms by which histones are disassembled and reassembled into nucleosomes and chromatin structure during DNA replication, repair and transcription are poorly understood. A better understanding of the processes involved is, however, crucial if we are to understand whether and how histone variants and post-translationally modified histones are inherited in an epigenetic manner. To this end we have studied the interaction of the histone H3-H4 complex with the human retinoblastoma-associated protein RbAp48 and their exchange with a second histone chaperone, anti-silencing function protein 1 (ASF1). Exchange of histones H3-H4 between these two histone chaperones has a central role in the assembly of new nucleosomes, and we show here that the H3-H4 complex has an unexpected structural plasticity, which is important for this exchange.

Histone methylation by PRC2 is inhibited by active chromatin marks.

The Polycomb repressive complex 2 (PRC2) confers transcriptional repression through histone H3 lysine 27 trimethylation (H3K27me3). Here, we examined how PRC2 is modulated by histone modifications associated with transcriptionally active chromatin. We provide the molecular basis of histone H3 N terminus recognition by the PRC2 Nurf55-Su(z)12 submodule. Binding of H3 is lost if lysine 4 in H3 is trimethylated. We find that H3K4me3 inhibits PRC2 activity in an allosteric fashion assisted by the Su(z)12 C terminus. In addition to H3K4me3, PRC2 is inhibited by H3K36me2/3 (i.e., both H3K36me2 and H3K36me3). Direct PRC2 inhibition by H3K4me3 and H3K36me2/3 active marks is conserved in humans, mouse, and fly, rendering transcriptionally active chromatin refractory to PRC2 H3K27 trimethylation. While inhibition is present in plant PRC2, it can be modulated through exchange of the Su(z)12 subunit. Inhibition by active chromatin marks, coupled to stimulation by transcriptionally repressive H3K27me3, enables PRC2 to autonomously template repressive H3K27me3 without overwriting active chromatin domains.

Chromatin-modifying complex component Nurf55/p55 associates with histones H3 and H4 and polycomb repressive complex 2 subunit Su(z)12 through partially overlapping binding sites.

Drosophila Nurf55 is a component of different chromatin-modifying complexes, including the PRC2 (Polycomb repressive complex 2). Based on the 1.75-Å crystal structure of Nurf55 bound to histone H4 helix 1, we analyzed interactions of Nurf55 (Nurf55 or p55 in fly and RbAp48/46 in human) with the N-terminal tail of histone H3, the first helix of histone H4, and an N-terminal fragment of the PRC2 subunit Su(z)12 using isothermal calorimetry and pulldown experiments. Site-directed mutagenesis identified the binding site of histone H3 at the top of the Nurf55 WD40 propeller. Unmodified or K9me3- or K27me3-containing H3 peptides were bound with similar affinities, whereas the affinity for K4me3-containing H3 peptides was reduced. Helix 1 of histone H4 and Su(z)12 bound to the edge of the ß-propeller using overlapping binding sites. Our results show similarities in the recognition of histone H4 and Su(z)12 and identify Nurf55 as a versatile interactor that simultaneously contacts multiple partners.

Insights into association of the NuRD complex with FOG-1 from the crystal structure of an RbAp48·FOG-1 complex.

Chromatin-modifying complexes such as the NuRD complex are recruited to particular genomic sites by gene-specific nuclear factors. Overall, however, little is known about the molecular basis for these interactions. Here, we present the 1.9 Å resolution crystal structure of the NuRD subunit RbAp48 bound to the 15 N-terminal amino acids of the GATA-1 cofactor FOG-1. The FOG-1 peptide contacts a negatively charged binding pocket on top of the RbAp48 ß-propeller that is distinct from the binding surface used by RpAp48 to contact histone H4. We further show that RbAp48 interacts with the NuRD subunit MTA-1 via a surface that is distinct from its FOG-binding pocket, providing a first glimpse into the way in which NuRD assembly facilitates interactions with cofactors. Our RbAp48·FOG-1 structure provides insight into the molecular determinants of FOG-1-dependent association with the NuRD complex and into the links between transcription regulation and nucleosome remodeling.

Histone H1 subtypes differentially modulate chromatin condensation without preventing ATP-dependent remodeling by SWI/SNF or NURF.

Although ubiquitously present in chromatin, the function of the linker histone subtypes is partly unknown and contradictory studies on their properties have been published. To explore whether the various H1 subtypes have a differential role in the organization and dynamics of chromatin we have incorporated all of the somatic human H1 subtypes into minichromosomes and compared their influence on nucleosome spacing, chromatin compaction and ATP-dependent remodeling. H1 subtypes exhibit different affinities for chromatin and different abilities to promote chromatin condensation, as studied with the Atomic Force Microscope. According to this criterion, H1 subtypes can be classified as weak condensers (H1.1 and H1.2), intermediate condensers (H1.3) and strong condensers (H1.0, H1.4, H1.5 and H1x). The variable C-terminal domain is required for nucleosome spacing by H1.4 and is likely responsible for the chromatin condensation properties of the various subtypes, as shown using chimeras between H1.4 and H1.2. In contrast to previous reports with isolated nucleosomes or linear nucleosomal arrays, linker histones at a ratio of one per nucleosome do not preclude remodeling of minichromosomes by yeast SWI/SNF or Drosophila NURF. We hypothesize that the linker histone subtypes are differential organizers of chromatin, rather than general repressors.