Expression of cohesin and condensin genes during zebrafish development supports a non-proliferative role for cohesin

Cohesin and condensin are similar, but distinct multi-subunit protein complexes that have well-described roles in sister chromatid cohesion and chromosome condensation, respectively. Recently it has emerged that cohesin, and proteins that regulate cohesin function have additional developmental roles. To further understand the role of cohesin in development, we analyzed the expression of genes encoding cohesin and condensin subunits in developing zebrafish embryos and juvenile brain. We found that cohesin subunits are expressed in a pattern that is similar (but not quite identical) to the expression of condensin subunits. Cohesin genes smc1a, rad21, pds5b and smc3 were expressed in the forebrain ventricular zone, the tectum, the mid-hindbrain boundary, the fourth ventricle, branchial arches, the otic vesicle, the eye and faintly in the developing pectoral fins. Condensin genes smc2 and smc4 were expressed in the forebrain ventricular zone, the tectum, the mid-hindbrain boundary, the fourth ventricle, branchial arches, eye and pectoral fins. Condensin genes were additionally expressed in the hindbrain proliferative zone, an area in which cohesin genes were not detected. A comparison with pcna expression and BrdU incorporation revealed that the expression of cohesins and condensins closely overlap with zones of proliferation. Interestingly, cohesin genes were expressed in non-proliferating cells flanking rhombomere boundaries in the developing brain. In mature brain and eye, cohesin was expressed in both proliferating cells and in broad zones of post-mitotic cells. The distribution of cohesin and condensin mRNAs supports existing evidence for a non-cell cycle role for cohesin in the developing brain.     

Cohesin dysfunction results in cell wall defects in budding yeast

Cohesin is a conserved chromatin-binding multisubunit protein complex involved in diverse chromosomal transactions such as sister-chromatid cohesion, chromosome condensation, regulation of gene expression, DNA replication, and repair. While working with a budding yeast temperature-sensitive mutant, mcd1-1, defective in a cohesin subunit, we observed that it was resistant to zymolyase, indicating an altered cell wall organization. The budding yeast cell wall is a strong but elastic structure essential for maintenance of cell shape and protection from extreme environmental challenges. Here, we show that the cohesin complex plays an important role in cell wall maintenance. Cohesin mutants showed high chitin content in the cell wall and sensitivity to multiple cell wall stress-inducing agents. Interestingly, temperature-dependent lethality of cohesin mutants was osmoremedial, in a HOG1-MAPK pathway-dependent manner, suggesting that the temperature sensitivity of these mutants may arise partially from cell wall defects. Moreover, Mpk1 hyper-phosphorylation indicated activation of the cell wall integrity (CWI) signaling pathway in cohesin mutants. Genetic interaction analysis revealed that the CWI pathway is essential for survival of mcd1-1 upon additional cell wall stress. The cell wall defect was independent of the cohesion function and accompanied by misregulation of expression of several genes having cell wall-related functions. Our findings reveal a requirement of cohesin in maintenance of CWI that is independent of the CWI pathway, and that may arise from cohesin’s role in regulating the expression of multiple genes encoding proteins involved in cell wall organization and biosynthesis.     

Interactions among the bHLH domains of the proteins encoded by theEnhancer of split andachaete-scute gene complexes ofDrosophila

TheEnhancer of split andachaete-scute gene complexes [E(spl)-C and AS-C] encode helix-loop-helix proteins required for neurogenesis inDrosophila. Using a heterologous bacterial system, we show that (i) the bHLH domains of the proteins encoded by the two gene complexes differ in their ability to form homo- and/or heterodimers; (ii) the bHLH domains of the E(spl)-C proteins m5, m7 and m8 interact with the bHLH domains of the Ac and Sc proteins. These bHLH domains form an interaction network which may represent the molecular mechanism whereby the competent state of the proneural cells is maintained until the terminal determination to neuroblast occurs. Also, the pattern of interactions of the bHLH domains of certain proteins encoded by the two gene complexes may explain their functional redundancy.     

The Cohesin loading factor NIPBL recruits histone deacetylases to mediate local chromatin modifications

Cornelia de Lange Syndrome (CdLS) is a rare congenital malformation disorder. About half of the patients with CdLS carry mutations in the NIPBL gene encoding the NIPBL protein, a subunit of the Cohesin loading complex. Recent studies show association of Cohesin with chromatin-remodeling complexes, either by establishing cohesion or by recruiting Cohesin to specific chromosome locations. In yeast two-hybrid assays, we identified an interaction of NIPBL with the histone deacetylases -1 and -3. These interactions were confirmed in mammalian cells by coimmunoprecipitation and a critical region for interaction was defined to a stretch of 163 amino acids of a highly conserved region of NIPBL, which is mutated in patients with CdLS. Utilizing reporter gene assays, we could show that NIPBL fused to the GAL4-DNA-binding domain (GAL4-DBD) represses promoter activity via the recruitment of histone deacetylases. Interestingly, this effect is dramatically reduced by both NIPBL missense mutations identified in CdLS and by chemical inhibition of the histone deacetylases. Our data are the first to indicate a molecular and functional connection of NIPBL with chromatin-remodeling processes via the direct interaction with histone deacetylases.     

黏着素与基因表达调控

黏着素(cohesin)是一种多亚基蛋白复合体,在进化上相当保守。在真核生物细胞中,黏着素主要功能是将复制产生的姐妹染色单体连接在一起,直到细胞分裂的后期,黏着素亚基Scc1水解最终导致染色单体的分离。但是最近研究表明,黏着素在基因表达、染色质结构变化和发育调节等方面也起着非常重要的作用,并且发现黏着素对基因的调节作用与其对染色体的黏着功能无关。在酵母中,黏着素最初定位于其装载蛋白Scc2的DNA结合位点上,但是在细胞周期的G2期,黏着素聚集于转录汇集区之间进而调控转录终止。在果蝇染色体上,黏着素与装载蛋白Scc2的同源物Nipped-B共定位,其作用是阻抑增强子和启动子的远距离接触。而在哺乳动物中,黏着素与CTCF隔离子蛋白共定位,并以依赖于CTCF的方式调控转录。本文概述了黏着素在不同真核生物染色体上的定位与分布,并对其在基因表达调控中的功能机制及其研究现状进行了重点阐述。     

Cohesin acetylation and Wapl‐Pds5 oppositely regulate translocation of cohesin along DNA

Cohesin is a ring‐shaped protein complex that plays a crucial role in sister chromatid cohesion and gene expression. The dynamic association of cohesin with chromatin is essential for these functions. However, the exact nature of cohesin dynamics, particularly cohesin translocation, remains unclear. We evaluated the dynamics of individual cohesin molecules on DNA and found that the cohesin core complex possesses an intrinsic ability to traverse DNA in an adenosine triphosphatase (ATPase)‐dependent manner. Translocation ability is suppressed in the presence of Wapl‐Pds5 and Sororin; this suppression is alleviated by the acetylation of cohesin and the action of mitotic kinases. In Xenopus laevis egg extracts, cohesin is translocated on unreplicated DNA in an ATPase‐ and Smc3 acetylation‐dependent manner. Cohesin movement changes from bidirectional to unidirectional when cohesin faces DNA replication; otherwise, it is incorporated into replicating DNA without being translocated or is dissociated from replicating DNA. This study provides insight into the nature of individual cohesin dynamics and the mechanisms by which cohesin achieves cohesion in different chromatin contexts.