Family Report for: Arb2

Nucleosome remodeling and deacetylation (NuRD) complexes are co-transcriptional regulators implicated in differentiation, development, and diseases. Methyl-CpG binding domain (MBD) proteins play an essential role in recruitment of NuRD complexes to their target sites in chromatin. The related SHREC complex in fission yeast drives transcriptional gene silencing in heterochromatin through cooperation with HP1 proteins. How remodeler and histone deacetylase (HDAC) cooperate within NuRD complexes remains unresolved. We determined that in SHREC the two modules occupy distant sites on the scaffold protein Clr1 and that repressive activity of SHREC can be modulated by the expression level of the HDAC-associated Clr1 domain alone. Moreover, the crystal structure of Clr2 reveals an MBD-like domain mediating recruitment of the HDAC module to heterochromatin. Thus, SHREC bi-functionality is organized in two separate modules with separate recruitment mechanisms, which work together to elicit transcriptional silencing at heterochromatic loci.

Hda1 is the catalytic core component of the H2B- and H3- specific histone deacetylase (HDAC) complex from Saccharomyces cerevisiae, which is involved in the epigenetic repression and plays a crucial role in transcriptional regulation and developmental events. Though the N-terminal catalytic HDAC domain of Hda1 is well characterized, the function of the C-terminal ARB2 domain remains unknown. In this study, we determine the crystal structure of the ARB2 domain from S. cerevisiae Hda1 at a resolution of 2.7 A. The ARB2 domain displays an alpha/beta sandwich architecture with an arm protruding outside. Two ARB2 domain molecules form a compact homo-dimer via the arm elements, and assemble as an inverse "V" shape. The pull-down and ITC results reveal that the ARB2 domain possesses the histone binding ability, recognizing both the H2A-H2B dimer and H3-H4 tetramer. Perturbation of the dimer interface abolishes the histone binding ability of the ARB2 domain, indicating that the unique dimer architecture of the ARB2 domain coincides with the function for anchoring to histone. Collectively, our data report the first structure of the ARB2 domain and disclose its histone binding ability, which is of benefit for understanding the deacetylation reaction catalyzed by the class II Hda1 HDAC complex.

The RNA-induced transcriptional silencing (RITS) complex, containing Ago1, Chp1, Tas3 and centromeric small interfering RNAs (siRNAs), is required for heterochromatic gene silencing at centromeres. Here, we identify a second fission yeast Argonaute complex (Argonaute siRNA chaperone, ARC), which contains, in addition to Ago1, two previously uncharacterized proteins, Arb1 and Arb2, both of which are required for histone H3 Lys9 (H3-K9) methylation, heterochromatin assembly and siRNA generation. Furthermore, whereas siRNAs in the RITS complex are mostly single-stranded, siRNAs associated with ARC are mostly double-stranded, indicating that Arb1 and Arb2 inhibit the release of the siRNA passenger strand from Ago1. Consistent with this observation, purified Arb1 inhibits the slicer activity of Ago1 in vitro, and purified catalytically inactive Ago1 contains only double-stranded siRNA. Finally, we show that slicer activity is required for the siRNA-dependent association of Ago1 with chromatin and for the spreading of histone H3-K9 methylation.

CHARGE syndrome-which stands for coloboma of the eye, heart defects, atresia of choanae, retardation of growth/development, genital abnormalities, and ear anomalies-is a severe developmental disorder with wide phenotypic variability, caused mainly by mutations in CHD7 (chromodomain helicase DNA-binding protein 7), known to encode a chromatin remodeler. The genetic lesions responsible for CHD7 mutation-negative cases are unknown, at least in part because the pathogenic mechanisms underlying CHARGE syndrome remain poorly defined. Here, we report the characterization of a mouse model for CHD7 mutation-negative cases of CHARGE syndrome generated by insertional mutagenesis of Fam172a (family with sequence similarity 172, member A). We show that Fam172a plays a key role in the regulation of cotranscriptional alternative splicing, notably by interacting with Ago2 (Argonaute-2) and Chd7. Validation studies in a human cohort allow us to propose that dysregulation of cotranscriptional alternative splicing is a unifying pathogenic mechanism for both CHD7 mutation-positive and CHD7 mutation-negative cases. We also present evidence that such splicing defects can be corrected in vitro by acute rapamycin treatment.

Nucleosome remodeling and deacetylation (NuRD) complexes are co-transcriptional regulators implicated in differentiation, development, and diseases. Methyl-CpG binding domain (MBD) proteins play an essential role in recruitment of NuRD complexes to their target sites in chromatin. The related SHREC complex in fission yeast drives transcriptional gene silencing in heterochromatin through cooperation with HP1 proteins. How remodeler and histone deacetylase (HDAC) cooperate within NuRD complexes remains unresolved. We determined that in SHREC the two modules occupy distant sites on the scaffold protein Clr1 and that repressive activity of SHREC can be modulated by the expression level of the HDAC-associated Clr1 domain alone. Moreover, the crystal structure of Clr2 reveals an MBD-like domain mediating recruitment of the HDAC module to heterochromatin. Thus, SHREC bi-functionality is organized in two separate modules with separate recruitment mechanisms, which work together to elicit transcriptional silencing at heterochromatic loci.

Hda1 is the catalytic core component of the H2B- and H3- specific histone deacetylase (HDAC) complex from Saccharomyces cerevisiae, which is involved in the epigenetic repression and plays a crucial role in transcriptional regulation and developmental events. Though the N-terminal catalytic HDAC domain of Hda1 is well characterized, the function of the C-terminal ARB2 domain remains unknown. In this study, we determine the crystal structure of the ARB2 domain from S. cerevisiae Hda1 at a resolution of 2.7 A. The ARB2 domain displays an alpha/beta sandwich architecture with an arm protruding outside. Two ARB2 domain molecules form a compact homo-dimer via the arm elements, and assemble as an inverse "V" shape. The pull-down and ITC results reveal that the ARB2 domain possesses the histone binding ability, recognizing both the H2A-H2B dimer and H3-H4 tetramer. Perturbation of the dimer interface abolishes the histone binding ability of the ARB2 domain, indicating that the unique dimer architecture of the ARB2 domain coincides with the function for anchoring to histone. Collectively, our data report the first structure of the ARB2 domain and disclose its histone binding ability, which is of benefit for understanding the deacetylation reaction catalyzed by the class II Hda1 HDAC complex.

The RNA-induced transcriptional silencing (RITS) complex, containing Ago1, Chp1, Tas3 and centromeric small interfering RNAs (siRNAs), is required for heterochromatic gene silencing at centromeres. Here, we identify a second fission yeast Argonaute complex (Argonaute siRNA chaperone, ARC), which contains, in addition to Ago1, two previously uncharacterized proteins, Arb1 and Arb2, both of which are required for histone H3 Lys9 (H3-K9) methylation, heterochromatin assembly and siRNA generation. Furthermore, whereas siRNAs in the RITS complex are mostly single-stranded, siRNAs associated with ARC are mostly double-stranded, indicating that Arb1 and Arb2 inhibit the release of the siRNA passenger strand from Ago1. Consistent with this observation, purified Arb1 inhibits the slicer activity of Ago1 in vitro, and purified catalytically inactive Ago1 contains only double-stranded siRNA. Finally, we show that slicer activity is required for the siRNA-dependent association of Ago1 with chromatin and for the spreading of histone H3-K9 methylation.