Cohesin regulation: fashionable ways to wear a ring.
ABSTRACT Cohesin is a multiprotein complex, conserved from yeast to humans, that mediates sister chromatid cohesion. Its ring-shaped structure first suggested that it may perform its task by embracing the sister chromatids. The interaction of cohesin with chromatin is tightly regulated throughout the cell cycle, and several proteins contribute to cohesin loading and mobilization along DNA, establishment of cohesin-mediated cohesion, and removal of cohesin during mitosis. Recent studies suggest that distinct cohesin populations exist in different chromosomal regions and have particular requirements in their dynamic interaction with chromatin. In this review, I briefly summarize these studies and discuss their implications for current and future models of cohesin behavior.
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ABSTRACT: In this paper, the evolution of the cell is investigated till the level of complexity obtained by protists. Particular attention is paid to the genomic compartment and to the question: why has the genome of prokaryotes remained so small over more than 3 billion years and more than 3 trillion generations? Constraints on their genome evolution may be attributed mainly to: 1) the fact that repetitions of nucleotide sequences longer than 12 to 15 bp are forbidden according to Thomas' principle; 2) the high cost of the control of gene expression by means of regulatory proteins: this cost increases exponentially with chromosome elongation. The formation of chromatin, i. e. the wrapping of DNA around the nucleosomes, removed these constraints and allowed the increase of the genome and especially of the redundant sequences of DNA, whose role is discussed. The transformation and growth of the genome generated a trend towards separation of the various physiological functions and of their control. The formation of a nuclear envelope may have begun with the advent of mitosis, which replaced the simple but delicate device of pushing the newly formed DNA into the daughter prokaryotic cells. An increase of the O2 concentration in waters stimulated further evolution: the new cell established symbiosis with a bacterium capable of protecting against peroxides and performing aerobic respiration. The increased O2 concentration also led to the production of sterols, which became an important component of the cell membrane. The mutual adaptation of cells belonging to different domains involved further modifications, leading to the birth of proto-eukaryotic cells and facilitating the establishment of further symbioses with photosynthetic cyanobacteria. Proto-eukaryotic cells were devoid of motility and contractility, as are the cells of red algae, fungi and Zygnematales today. Both these faculties evolved when the protist eukaryotic cell acquired flagella, cytoplasmic contractility and sensors to govern them.Tsitologiia 01/2010; 52(10):797-816.
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ABSTRACT: Antigenic variation allows Trypanosoma brucei to evade the host immune response by switching the expression of 1 out of approximately 15 telomeric variant surface glycoprotein (VSG) expression sites (ESs). VSG ES transcription is mediated by RNA polymerase I in a discrete nuclear site named the ES body (ESB). However, nothing is known about how the monoallelic VSG ES transcriptional state is maintained over generations. In this study, we show that during S and G2 phases and early mitosis, the active VSG ES locus remains associated with the single ESB and exhibits a delay in the separation of sister chromatids relative to control loci. This delay is dependent on the cohesin complex, as partial knockdown of cohesin subunits resulted in premature separation of sister chromatids of the active VSG ES. Cohesin depletion also prompted transcriptional switching from the active to previously inactive VSG ESs. Thus, in addition to maintaining sister chromatid cohesion during mitosis, the cohesin complex plays an essential role in the correct epigenetic inheritance of the active transcriptional VSG ES state.The Journal of Cell Biology 08/2009; 186(2):243-54. DOI:10.1083/jcb.200902119 · 9.69 Impact Factor
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ABSTRACT: Separase is a capase family protease that is required for the release of sister chromatid cohesion during meiosis and mitosis. Proteolytic cleavage of the alpha-kleisin subunit of the cohesin complex at the metaphase-to-anaphase transition is essential for the proper segregation of chromosomes. In addition to its highly conserved role in cleaving the alpha-kleisin subunit, separase appears to have acquired additional diverse activities in some organisms, including involvement in mitotic and meiotic anaphase spindle assembly and elongation, interphase spindle pole body positioning, and epithelial cell reorganization. Results from the characterization of Arabidopsis (Arabidopsis thaliana) separase (ESP) demonstrated that meiotic expression of ESP RNA interference blocked the proper removal of cohesin from chromosomes and resulted in the presence of a mixture of fragmented chromosomes and intact bivalents. The presence of large numbers of intact bivalents raised the possibility that separase may also have multiple roles in Arabidopsis. In this report, we show that meiotic expression of ESP RNA interference blocks the removal of cohesin during both meiosis I and II, results in alterations in nonhomologous centromere association, disrupts the radial microtubule system after telophase II, and affects the proper establishment of nuclear cytoplasmic domains, resulting in the formation of multinucleate microspores.Plant physiology 08/2009; 151(1):323-33. DOI:10.1104/pp.109.140699 · 7.39 Impact Factor