Condensin and cohesin: more than chromosome compactor and glue

Cdc14 and condensin control the dissolution of cohesin-independent chromosome linkages at repeated DNA

Chromosome segregation is triggered by the cleavage of cohesins by separase. Here we show that in budding yeast separation of the ribosomal DNA (rDNA) and telomeres also requires Cdc14, a protein phosphatase known for its role in mitotic exit. Cdc14 shares this role with the FEAR network, which activates Cdc14 during early anaphase, but not the mitotic exit network, which promotes Cdc14 activity during late anaphase. We further show that CDC14 is necessary and sufficient to promote condensin enrichment at the rDNA locus and to trigger rDNA segregation in a condensin-dependent manner. We propose that Cdc14 released by the FEAR network mediates the partitioning of rDNA by facilitating the localization of condensin thereto. This dual role of the FEAR network in initiating mitotic exit and promoting chromosome segregation ensures that exit from mitosis is coupled to the completion of chromosome segregation.

Spatial and temporal regulation of Condensins I and II in mitotic chromosome assembly in human cells

Two different condensin complexes make distinct contributions to metaphase chromosome architecture in vertebrate cells. We show here that the spatial and temporal distributions of condensins I and II are differentially regulated during the cell cycle in HeLa cells. Condensin II is predominantly nuclear during interphase and contributes to early stages of chromosome assembly in prophase. In contrast, condensin I is sequestered in the cytoplasm from interphase through prophase and gains access to chromosomes only after the nuclear envelope breaks down in prometaphase. The two complexes alternate along the axis of metaphase chromatids, but they are arranged into a unique geometry at the centromere/kinetochore region, with condensin II enriched near the inner kinetochore plate. This region-specific distribution of condensins I and II is severely disrupted upon depletion of Aurora B, although their association with the chromosome arm is not. Depletion of condensin subunits causes defects in kinetochore structure and function, leading to aberrant chromosome alignment and segregation. Our results suggest that the two condensin complexes act sequentially to initiate the assembly of mitotic chromosomes and that their specialized distribution at the centromere/kinetochore region may play a crucial role in placing sister kinetochores into the back-to-back orientation.

BRU1, a novel link between responses to DNA damage and epigenetic gene silencing in Arabidopsis

DNA repair associated with DNA replication is important for the conservation of genomic sequence information, whereas reconstitution of chromatin after replication sustains epigenetic information. We have isolated and characterized mutations in the BRU1 gene of Arabidopsis that suggest a novel link between these underlying maintenance mechanisms. Bru1 plants are highly sensitive to genotoxic stress and show stochastic release of transcriptional gene silencing. They also show increased intrachromosomal homologous recombination and constitutively activated expression of poly (ADP-ribose) polymerase-2 (AtPARP-2), the induction of which is associated with elevated DNA damage. Bru1 mutations affect the stability of heterochromatin organization but do not interfere with genome-wide DNA methylation. BRU1 encodes a novel nuclear protein with two predicted protein-protein interaction domains. The developmental abnormalities characteristic of bru1 mutant plants resemble those triggered by mutations in genes encoding subunits of chromatin assembly factor (CAF-1), the condensin complex, or MRE11. Comparison of bru1 with these mutants indicates cooperative roles in the replication and stabilization of chromatin structure, providing a novel link between chromatin replication, epigenetic inheritance, S-phase DNA damage checkpoints, and the regulation of meristem development.

Drosophila nipped-B protein supports sister chromatid cohesion and opposes the stromalin/Scc3 cohesion factor to facilitate long-range activation of the cut gene

The Drosophila melanogaster Nipped-B protein facilitates transcriptional activation of the cut and Ultrabithorax genes by remote enhancers. Sequence homologues of Nipped-B, Scc2 of Saccharomyces cerevisiae, and Mis4 of Schizosaccharomyces pombe are required for sister chromatid cohesion during mitosis. The evolutionarily conserved Cohesin protein complex mediates sister chromatid cohesion, and Scc2 and Mis4 are needed for Cohesin to associate with chromosomes. Here, we show that Nipped-B is also required for sister chromatid cohesion but that, opposite to the effect of Nipped-B, the stromalin/Scc3 component of Cohesin inhibits long-range activation of cut. To explain these findings, we propose a model based on the chromatin domain boundary activities of Cohesin in which Nipped-B facilitates cut activation by alleviating Cohesin-mediated blocking of enhancer-promoter communication.

Insights into regulation of the mammalian cell cycle from studies on spermatogenesis using genetic approaches in animal models

The genetic hierarchy controlling mitosis and especially meiosis during gamete formation is not well understood, even in less complicated systems such as the yeasts. Meiotic divisions are obviously restricted to germ line cells and as such likely require mechanisms of cell cycle control that do not function and may not exist in somatic cells. While male and female germ cells have stages of cell cycle regulation in common, the timing of these events and the stage of development at which these events occur differ in the two sexes. Understanding the genetic program controlling the mitotic and meiotic divisions of the germ line represents a unique opportunity for providing insight into cell cycle control in vivo. Elucidating the key control points and proteins may also enhance our understanding of the etiology of infertility and provide new directions for contraception.

Evidence of a transcriptional co-activator function of cohesin STAG/SA/Scc3

Cohesins hold sister chromatids together from DNA replication until they are segregated. Although cohesins Smc1, Smc3, and Scc1/Rad21 are involved in chromatid cohesion and other cellular processes, little is known about the other mitotic cohesin subunit, Scc3/STAG. Here we describe STAG/Scc3, which may act as a transcriptional co-activator. STAG2 is able to enhance the activity of the tumor necrosis factor alpha, the CD69, and the human immunodeficiency virus long terminal repeat promoters in a NF-kappaB-dependent manner. In addition, STAG2 interacts with the viral transactivator Tat and enhances the Tat-mediated activation of the human immunodeficiency virus long terminal repeat promoter. Moreover, STAG2 co-activates a multimeric NF-kappaB reporter construct and enhances the activity of the transactivation domain of p65/RelA in a Gal4 system. This function is dependent on one of the LXXLL co-activation motives present in this cohesin and is substantiated by the interaction of STAG2 with the p65 subunit of NF-kappaB. These results describe a novel activity for cohesins, suggesting a role for STAG/Scc3 in transcriptional regulation.

Imaging genome abnormalities in cancer research

Increasing attention is focusing on chromosomal and genome structure in cancer research due to the fact that genomic instability plays a principal role in cancer initiation, progression and response to chemotherapeutic agents. The integrity of the genome (including structural, behavioral and functional aspects) of normal and cancer cells can be monitored with direct visualization by using a variety of cutting edge molecular cytogenetic technologies that are now available in the field of cancer research. Examples are presented in this review by grouping these methodologies into four categories visualizing different yet closely related major levels of genome structures. An integrated discussion is also presented on several ongoing projects involving the illustration of mitotic and meiotic chromatin loops; the identification of defective mitotic figures (DMF), a new type of chromosomal aberration capable of monitoring condensation defects in cancer; the establishment of a method that uses Non-Clonal Chromosomal Aberrations (NCCAs) as an index to monitor genomic instability; and the characterization of apoptosis related chromosomal fragmentations caused by drug treatments.

Differential contributions of condensin I and condensin II to mitotic chromosome architecture in vertebrate cells

The canonical condensin complex (henceforth condensin I) plays an essential role in mitotic chromosome assembly and segregation from yeast to humans. We report here the identification of a second condensin complex (condensin II) from vertebrate cells. Condensins I and II share the same pair of structural maintenance of chromosomes (SMC) subunits but contain different sets of non-SMC subunits. siRNA-mediated depletion of condensin I- or condensin II-specific subunits in HeLa cells produces a distinct, highly characteristic defect in chromosome morphology. Simultaneous depletion of both complexes causes the severest defect. In Xenopus egg extracts, condensin I function is predominant, but lack of condensin II results in the formation of irregularly shaped chromosomes. Condensins I and II show different distributions along the axis of chromosomes assembled in vivo and in vitro. We propose that the two condensin complexes make distinct mechanistic contributions to mitotic chromosome architecture in vertebrate cells.

Cell cycle-dependent phosphorylation, nuclear localization, and activation of human condensin

Condensin, one of the most abundant components of mitotic chromosomes, is a conserved protein complex composed of two structural maintenance of chromosomes (SMC) subunits (SMC2- and SMC4-type) and three non-SMC subunits, and it plays an essential role in mitotic chromosome condensation. Purified condensin reconfigures DNA structure using energy provided by ATP hydrolysis. To know the regulation of condensin in somatic cells, the expression level, subcellular localization, and phosphorylation status of human condensin were examined during the cell cycle. The levels of condensin subunits were almost constant throughout the cell cycle, and the three non-SMC subunits were phosphorylated at specific sites in mitosis and dephosphorylated upon the completion of mitosis. Subcellular fractionation studies revealed that a proportion of condensin was tightly bound to mitotic chromosomes and that this form was phosphorylated at specific sites. Condensin purified from mitotic cells had much stronger supercoiling activity than that purified from interphase cells. These results suggest that condensin functions in somatic cells are regulated by phosphorylation in two ways during the cell cycle; the phosphorylation of specific sites correlates with the chromosomal targeting of condensin, and its biochemical activity is stimulated by phosphorylation.