dRYBP counteracts chromatin-dependent activation and repression of transcription

Unveiling the Binding between the Armadillo-Repeat Domain of Plakophilin 1 and the Intrinsically Disordered Transcriptional Repressor RYBP

Plakophilin 1 (PKP1), a member of the p120ctn subfamily of the armadillo (ARM)-repeat-containing proteins, is an important structural component of cell-cell adhesion scaffolds although it can also be ubiquitously found in the cytoplasm and the nucleus. RYBP (RING 1A and YY1 binding protein) is a multifunctional intrinsically disordered protein (IDP) best described as a transcriptional regulator. Both proteins are involved in the development and metastasis of several types of tumors. We studied the binding of the armadillo domain of PKP1 (ARM-PKP1) with RYBP by using in cellulo methods, namely immunofluorescence (IF) and proximity ligation assay (PLA), and in vitro biophysical techniques, namely fluorescence, far-ultraviolet (far-UV) circular dichroism (CD), and isothermal titration calorimetry (ITC). We also characterized the binding of the two proteins by using in silico experiments. Our results showed that there was binding in tumor and non-tumoral cell lines. Binding in vitro between the two proteins was also monitored and found to occur with a dissociation constant in the low micromolar range (~10 μM). Finally, in silico experiments provided additional information on the possible structure of the binding complex, especially on the binding ARM-PKP1 hot-spot. Our findings suggest that RYBP might be a rescuer of the high expression of PKP1 in tumors, where it could decrease the epithelial-mesenchymal transition in some cancer cells.

The intrinsically disordered, epigenetic factor RYBP binds to the citrullinating enzyme PADI4 in cancer cells

RYBP (Ring1 and YY 1 binding protein) is a multifunctional, intrinsically disordered protein (IDP), best described as a transcriptional regulator. It exhibits a ubiquitin-binding functionality, binds to other transcription factors, and has a key role during embryonic development. RYBP, which folds upon binding to DNA, has a Zn-finger domain at its N-terminal region. By contrast, PADI4 is a well-folded protein and it is one the human isoforms of a family of enzymes implicated in the conversion of arginine to citrulline. As both proteins intervene in signaling pathways related to cancer development and are found in the same localizations within the cell, we hypothesized they may interact. We observed their association in the nucleus and cytosol in several cancer cell lines, by using immunofluorescence (IF) and proximity ligation assays (PLAs). Binding also occurred in vitro, as measured by isothermal titration calorimetry (ITC) and fluorescence, with a low micromolar affinity (~1 μM). AlphaFold2-multimer (AF2) results indicate that PADI4's catalytic domain interacts with the Arg53 of RYBP docking into its active site. As RYBP sensitizes cells to PARP (Poly (ADP-ribose) polymerase) inhibitors, we applied them in combination with an enzymatic inhibitor of PADI4 observing a change in cell proliferation, and the hampering of the interaction of both proteins. This study unveils for the first time the possible citrullination of an IDP, and suggests that this new interaction, whether it involves or not citrullination of RYBP, might have implications in cancer development and progression.

A Toll-dependent Bre1/Rad6-cact feedback loop in controlling host innate immune response

The Toll signaling pathway was initially identified for its involvement in the control of early embryogenesis. It was later shown to be also part of a major innate immune pathway controlling the expression of anti-microbial peptides in many eukaryotes including humans; cactus, the essential negative regulator of this pathway in flies, was found to be induced in parallel to the Toll-dependent activation process during immune defenses. We were interested in the mechanisms of this dual effect and provide here evidence that upon pathogenic stimuli, dorsal, one of the transcription factors of the fly Toll pathway, can induce the expression of the E3 ligase Bre1. We further show that Bre1 complexes with the E2 Rad6 to mono-ubiquitinate histone H2B and to promote the transcription of cactus to achieve homeostasis of the Toll immune response. Our studies characterize a Toll signal-dependent regulatory machinery in governing the Toll pathway in Drosophila.

Variant Polycomb complexes in Drosophila consistent with ancient functional diversity

Polycomb group (PcG) mutants were first identified in Drosophila on the basis of their failure to maintain proper Hox gene repression during development. The proteins encoded by the corresponding fly genes mainly assemble into one of two discrete Polycomb repressive complexes: PRC1 or PRC2. However, biochemical analyses in mammals have revealed alternative forms of PRC2 and multiple distinct types of noncanonical or variant PRC1. Through a series of proteomic analyses, we identify analogous PRC2 and variant PRC1 complexes in Drosophila, as well as a broader repertoire of interactions implicated in early development. Our data provide strong support for the ancient diversity of PcG complexes and a framework for future analysis in a longstanding and versatile genetic system.

CTCF organizes inter-A compartment interactions through RYBP-dependent phase separation

Chromatin is spatially organized into three-dimensional structures at different levels including A/B compartments, topologically associating domains and loops. The canonical CTCF-mediated loop extrusion model can explain the formation of loops. However, the organization mechanisms underlying long-range chromatin interactions such as interactions between A-A compartments are still poorly understood. Here we show that different from the canonical loop extrusion model, RYBP-mediated phase separation of CTCF organizes inter-A compartment interactions. Based on this model, we designed and verified an induced CTCF phase separation system in embryonic stem cells (ESCs), which facilitated inter-A compartment interactions, improved self-renewal of ESCs and inhibited their differentiation toward neural progenitor cells. These findings support a novel and non-canonical role of CTCF in organizing long-range chromatin interactions via phase separation.

Comparative interactome analysis of the PRE DNA-binding factors: purification of the Combgap-, Zeste-, Psq-, and Adf1-associated proteins

The Polycomb group (PcG) and Trithorax group (TrxG) proteins are key epigenetic regulators controlling the silenced and active states of genes in multicellular organisms, respectively. In Drosophila, PcG/TrxG proteins are recruited to the chromatin via binding to specific DNA sequences termed polycomb response elements (PREs). While precise mechanisms of the PcG/TrxG protein recruitment remain unknown, the important role is suggested to belong to sequence-specific DNA-binding factors. At the same time, it was demonstrated that the PRE DNA-binding proteins are not exclusively localized to PREs but can bind other DNA regulatory elements, including enhancers, promoters, and boundaries. To gain an insight into the PRE DNA-binding protein regulatory network, here, using ChIP-seq and immuno-affinity purification coupled to the high-throughput mass spectrometry, we searched for differences in abundance of the Combgap, Zeste, Psq, and Adf1 PRE DNA-binding proteins. While there were no conspicuous differences in co-localization of these proteins with other functional transcription factors, we show that Combgap and Zeste are more tightly associated with the Polycomb repressive complex 1 (PRC1), while Psq interacts strongly with the TrxG proteins, including the BAP SWI/SNF complex. The Adf1 interactome contained Mediator subunits as the top interactors. In addition, Combgap efficiently interacted with AGO2, NELF, and TFIID. Combgap, Psq, and Adf1 have architectural proteins in their networks. We further investigated the existence of direct interactions between different PRE DNA-binding proteins and demonstrated that Combgap-Adf1, Psq-Dsp1, and Pho-Spps can interact in the yeast two-hybrid assay. Overall, our data suggest that Combgap, Psq, Zeste, and Adf1 are associated with the protein complexes implicated in different regulatory activities and indicate their potential multifunctional role in the regulation of transcription.

Roles of Polycomb complexes in regulating gene expression and chromatin structure in plants

The evolutionary conserved Polycomb Group (PcG) repressive system comprises two central protein complexes, PcG repressive complex 1 (PRC1) and PRC2. These complexes, through the incorporation of histone modifications on chromatin, have an essential role in the normal development of eukaryotes. In recent years, a significant effort has been made to characterize these complexes in the different kingdoms, and despite there being remarkable functional and mechanistic conservation, some key molecular principles have diverged. In this review, we discuss current views on the function of plant PcG complexes. We compare the composition of PcG complexes between animals and plants, highlight the role of recently identified plant PcG accessory proteins, and discuss newly revealed roles of known PcG partners. We also examine the mechanisms by which the repression is achieved and how these complexes are recruited to target genes. Finally, we consider the possible role of some plant PcG proteins in mediating local and long-range chromatin interactions and, thus, shaping chromatin 3D architecture.

Polycomb and Trithorax Group Proteins: The Long Road from Mutations in Drosophila to Use in Medicine

Polycomb group (PcG) and Trithorax group (TrxG) proteins are evolutionarily conserved factors responsible for the repression and activation of the transcription of multiple genes in Drosophila and mammals. Disruption of the PcG/TrxG expression is associated with many pathological conditions, including cancer, which makes them suitable targets for diagnosis and therapy in medicine. In this review, we focus on the major PcG and TrxG complexes, the mechanisms of PcG/TrxG action, and their recruitment to chromatin. We discuss the alterations associated with the dysfunction of a number of factors of these groups in oncology and the current strategies used to develop drugs based on small-molecule inhibitors.

Evolving Role of RING1 and YY1 Binding Protein in the Regulation of Germ-Cell-Specific Transcription

Separation of germline cells from somatic lineages is one of the earliest decisions of embryogenesis. Genes expressed in germline cells include apoptotic and meiotic factors, which are not transcribed in the soma normally, but a number of testis-specific genes are active in numerous cancer types. During germ cell development, germ-cell-specific genes can be regulated by specific transcription factors, retinoic acid signaling and multimeric protein complexes. Non-canonical polycomb repressive complexes, like ncPRC1.6, play a critical role in the regulation of the activity of germ-cell-specific genes. RING1 and YY1 binding protein (RYBP) is one of the core members of the ncPRC1.6. Surprisingly, the role of Rybp in germ cell differentiation has not been defined yet. This review is focusing on the possible role of Rybp in this process. By analyzing whole-genome transcriptome alterations of the Rybp^-/- embryonic stem (ES) cells and correlating this data with experimentally identified binding sites of ncPRC1.6 subunits and retinoic acid receptors in ES cells, we propose a model how germ-cell-specific transcription can be governed by an RYBP centered regulatory network, underlining the possible role of RYBP in germ cell differentiation and tumorigenesis.

RYBP Is a K63-Ubiquitin-Chain-Binding Protein that Inhibits Homologous Recombination Repair

Ring1-YY1-binding protein (RYBP) is a member of the non-canonical polycomb repressive complex 1 (PRC1), and like other PRC1 members, it is best described as a transcriptional regulator. However, several PRC1 members were recently shown to function in DNA repair. Here, we report that RYBP preferentially binds K63-ubiquitin chains via its Npl4 zinc finger (NZF) domain. Since K63-linked ubiquitin chains are assembled at DNA double-strand breaks (DSBs), we examined the contribution of RYBP to DSB repair. Surprisingly, we find that RYBP is K48 polyubiquitylated by RNF8 and rapidly removed from chromatin upon DNA damage by the VCP/p97 segregase. High expression of RYBP competitively inhibits recruitment of BRCA1 repair complex to DSBs, reducing DNA end resection and homologous recombination (HR) repair. Moreover, breast cancer cell lines expressing high endogenous RYBP levels show increased sensitivity to DNA-damaging agents and poly ADP-ribose polymerase (PARP) inhibition. These data suggest that RYBP negatively regulates HR repair by competing for K63-ubiquitin chain binding.

Dangerous liaisons: interplay between SWI/SNF, NuRD, and Polycomb in chromatin regulation and cancer

In this review, Bracken et al. discuss the functional organization and biochemical activities of remodelers and Polycomb and explore how they work together to control cell differentiation and the maintenance of cell identity. They also discuss how mutations in the genes encoding these various chromatin regulators contribute to oncogenesis by disrupting the chromatin equilibrium.

Changes in chromatin structure mediated by ATP-dependent nucleosome remodelers and histone modifying enzymes are integral to the process of gene regulation. Here, we review the roles of the SWI/SNF (switch/sucrose nonfermenting) and NuRD (nucleosome remodeling and deacetylase) and the Polycomb system in chromatin regulation and cancer. First, we discuss the basic molecular mechanism of nucleosome remodeling, and how this controls gene transcription. Next, we provide an overview of the functional organization and biochemical activities of SWI/SNF, NuRD, and Polycomb complexes. We describe how, in metazoans, the balance of these activities is central to the proper regulation of gene expression and cellular identity during development. Whereas SWI/SNF counteracts Polycomb, NuRD facilitates Polycomb repression on chromatin. Finally, we discuss how disruptions of this regulatory equilibrium contribute to oncogenesis, and how new insights into the biological functions of remodelers and Polycombs are opening avenues for therapeutic interventions on a broad range of cancer types.

Deletion of 3p13-14 locus spanning FOXP1 to SHQ1 cooperates with PTEN loss in prostate oncogenesis

A multigenic locus at 3p13-14, spanning FOXP1 to SHQ1, is commonly deleted in prostate cancer and lost broadly in a range of cancers but has unknown significance to oncogenesis or prognosis. Here, we report that FOXP1-SHQ1 deletion cooperates with PTEN loss to accelerate prostate oncogenesis and that loss of component genes correlates with prostate, breast, and head and neck cancer recurrence. We demonstrate that Foxp1-Shq1 deletion accelerates prostate tumorigenesis in mice in combination with Pten loss, consistent with the association of FOXP1-SHQ1 and PTEN loss observed in human cancers. Tumors with combined Foxp1-Shq1 and Pten deletion show increased proliferation and anaplastic dedifferentiation, as well as mTORC1 hyperactivation with reduced Akt phosphorylation. Foxp1-Shq1 deletion restores expression of AR target genes repressed in tumors with Pten loss, circumventing PI3K-mediated repression of the androgen axis. Moreover, FOXP1-SHQ1 deletion has prognostic relevance, with cancer recurrence associated with combined loss of PTEN and FOXP1-SHQ1 genes.

The GAGA factor regulatory network: Identification of GAGA factor associated proteins

The Drosophila GAGA factor (GAF) has an extraordinarily diverse set of functions that include the activation and silencing of gene expression, nucleosome organization and remodeling, higher order chromosome architecture and mitosis. One hypothesis that could account for these diverse activities is that GAF is able to interact with partners that have specific and dedicated functions. To test this possibility we used affinity purification coupled with high throughput mass spectrometry to identify GAF associated partners. Consistent with this hypothesis the GAF interacting network includes a large collection of factors and complexes that have been implicated in many different aspects of gene activity, chromosome structure and function. Moreover, we show that GAF interactions with a small subset of partners is direct; however for many others the interactions could be indirect, and depend upon intermediates that serve to diversify the functional capabilities of the GAF protein.

RYBP stimulates PRC1 to shape chromatin-based communication between Polycomb repressive complexes

Polycomb group (PcG) proteins function as chromatin-based transcriptional repressors that are essential for normal gene regulation during development. However, how these systems function to achieve transcriptional regulation remains very poorly understood. Here, we discover that the histone H2AK119 E3 ubiquitin ligase activity of Polycomb repressive complex 1 (PRC1) is defined by the composition of its catalytic subunits and is highly regulated by RYBP/YAF2-dependent stimulation. In mouse embryonic stem cells, RYBP plays a central role in shaping H2AK119 mono-ubiquitylation at PcG targets and underpins an activity-based communication between PRC1 and Polycomb repressive complex 2 (PRC2) which is required for normal histone H3 lysine 27 trimethylation (H3K27me3). Without normal histone modification-dependent communication between PRC1 and PRC2, repressive Polycomb chromatin domains can erode, rendering target genes susceptible to inappropriate gene expression signals. This suggests that activity-based communication and histone modification-dependent thresholds create a localized form of epigenetic memory required for normal PcG chromatin domain function in gene regulation.

DOI:http://dx.doi.org/10.7554/eLife.18591.001