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The nuclear matrix protein SAFB maintains heterochromatin architecture through RNA-dependent phase separation [Hi-C]

ABSTRACT: Eukaryotic chromosomes fold into topologically associating domains (TADs), which further gather in active (A) or inactive (B) genomic compartments. Here we show that Scaffold Attachment Factor B (SAFB), a nuclear matrix-associated protein with RNA binding functions, modulates global chromatin condensation in a dosage-dependent manner. Upon the depletion of SAFB, the genome coverage of Lamina-associated-domains (LADs) decreases from 53.33% to 45.93%, and both inter- and intra-TAD chromatin-chromatin interactions in compartment B decrease significantly. In the nucleus, SAFB favorably co-localizes with H3K9me3, a marker of heterochromatin, and the disruption of SAFB leads to a collapse of H3K9me3 foci. Furthermore, we show that SAFB proteins can form liquid-liquid phase separation in the cell and in vitro. Intriguingly, SAFB interacts with RNAs from repetitive elements enriched in heterochromatin (e.g., Major Satellites and LINE1), which can promote SAFB-mediated phase separation, depending on the density of SAFB recognizing motifs in the RNAs.

ORGANISM(S): Mus musculus

PROVIDER: GSE141080 | GEO | 2019/12/01

REPOSITORIES: GEO

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The nuclear matrix protein SAFB maintains heterochromatin architecture through RNA-dependent phase separation [HiChIP]

Project description:Eukaryotic chromosomes fold into topologically associating domains (TADs), which further gather in active (A) or inactive (B) genomic compartments. Here we show that Scaffold Attachment Factor B (SAFB), a nuclear matrix-associated protein with RNA binding functions, modulates global chromatin condensation in a dosage-dependent manner. Upon the depletion of SAFB, the genome coverage of Lamina-associated-domains (LADs) decreases from 53.33% to 45.93%, and both inter- and intra-TAD chromatin-chromatin interactions in compartment B decrease significantly. In the nucleus, SAFB favorably co-localizes with H3K9me3, a marker of heterochromatin, and the disruption of SAFB leads to a collapse of H3K9me3 foci. Furthermore, we show that SAFB proteins can form liquid-liquid phase separation in the cell and in vitro. Intriguingly, SAFB interacts with RNAs from repetitive elements enriched in heterochromatin (e.g., Major Satellites and LINE1), which can promote SAFB-mediated phase separation, depending on the density of SAFB recognizing motifs in the RNAs.

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Project description:Interphase chromatin is hierarchically organized into higher-order architectures that are essential for gene functions, yet the biomolecules that regulate these 3D architectures remain poorly understood. Here, we show that scaffold attachment factor B (SAFB), a nuclear matrix (NM)-associated protein with RNA- binding functions, modulates chromatin condensa- tion and stabilizes heterochromatin foci in mouse cells. SAFB interacts via its R/G-rich region with heterochromatin-associated repeat transcripts such as major satellite RNAs, which promote the phase separation driven by SAFB. Depletion of SAFB leads to changes in 3D genome organization, including an increase in interchromosomal interactions adjacent to pericentromeric heterochromatin and a decrease in genomic compartmentalization, which could result from the decondensation of pericentromeric heterochromatin. Collectively, we reveal the integrated roles of NM-associated proteins and repeat RNAs in the 3D organization of heterochromatin, which may shed light on the molecular mechanisms of nuclear architecture organization.

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Project description:Interphase chromatin is hierarchically organized into higher-order architectures that are essential for gene functions, yet the biomolecules that regulate these 3D architectures remain poorly understood. Here, we show that scaffold attachment factor B (SAFB), a nuclear matrix (NM)-associated protein with RNA-binding functions, modulates chromatin condensation and stabilizes heterochromatin foci in mouse cells. SAFB interacts via its R/G-rich region with heterochromatin-associated repeat transcripts such as major satellite RNAs, which promote the phase sep- aration driven by SAFB. Depletion of SAFB leads to changes in 3D genome organization, including an increase in interchromosomal interactions adjacent to pericentromeric heterochromatin and a decrease in genomic compartmentalization, which could result from the decondensation of pericentromeric heterochromatin. Collectively, we reveal the integrated roles of NM-associated proteins and repeat RNAs in the 3D organization of heterochromatin, which may shed light on the molecular mechanisms of nuclear architecture organization.

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The nuclear matrix protein SAFB maintains heterochromatin architecture through RNA-dependent phase separation [H3K9me3 ChIP-seq]

Project description:The nuclear matrix protein SAFB maintains heterochromatin architecture through RNA-dependent phase separation [H3K9me3 ChIP-seq]

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Identification of SAFB and HnrnpU interactors

Project description:Scaffold Attachment Factor B (SAFB) is a conserved RNA Binding Protein (RBP) that is essential for early mammalian development. However, the RNAs that associate with SAFB in mouse embryonic stem cells have not been characterized. Here, we addressed this unknown using RNA-seq and SAFB RNA immunoprecipitation followed by RNA-seq (RIP-seq) in wild-type mouse embryonic stem cells (ESCs) and in ESCs in which SAFB and SAFB2 were knocked out. The transcript most enriched in SAFB association was the lncRNA Malat1, which contains a series of purine-rich motifs in its 5 end. Beyond Malat1, SAFB predominantly associated with introns of protein-coding genes also through purine-rich motifs. Knockout of SAFB/2 led to down- and upregulation of genes in multiple biological pathways. The nascent transcripts of many downregulated genes associated with high levels of SAFB in wild-type cells, implying that SAFB binding promotes the expression of these genes. Reintroduction of SAFB into double-knockout cells restored gene expression towards wild-type levels, an effect that was again observable at the level of nascent transcripts. Proteomic analyses indicate an enrichment of nuclear speckle-associated, SR proteins in FLAG-tagged SAFB immunoprecipitated samples. Comparison to immunoprecipitates made from FLAG-tagging of another nuclear-enriched RNA-binding protein called HNRNPU (also known as SAF-A) identified both similarities and differences. Perhaps most notably, we observed a stronger enrichment for speckle-associated proteins in SAFB immunoprecipitations and a strong enrichment for paraspeckle-associated proteins in HNRNPU immunoprecipitations. Our findings suggest that among other potential functions in mouse embryonic stem cells, SAFB directly promotes the expression of a subset of genes through its ability to bind purine regions in nascent RNA.

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