Project description:Condensin mediates chromosome condensation, which is essential for proper chromosome segregation during mitosis. Prior to anaphase of budding yeast, the ribosomal DNA (RDN) condenses to a thin loop that is distinct from the rest of the chromosomes. We provide evidence that the establishment and maintenance of this RDN condensation require the regulation of condensin by Cdc5p (polo) kinase. We show that Cdc5p is recruited to the site of condensin binding in the RDN by cohesin, a complex related to condensin. Cdc5p and cohesin prevent condensin from misfolding the RDN into an irreversibly decondensed state. From these and other observations, we propose that the spatial regulation of Cdc5p by cohesin modulates condensin activity to ensure proper RDN folding into a thin loop. This mechanism may be evolutionarily conserved, promoting the thinly condensed constrictions that occur at centromeres and RDN of mitotic chromosomes in plants and animals.
Project description:During meiosis, chromosomes undergo extensive changes in structure and intranuclear positioning. How these chromosome organization changes occur and how they influence meiosis-specific chromosome events are not fully understood. Using Hi-C, we characterized chromosome architecture throughout mouse spermatogenesis at high temporal resolution. Our study revealed an intimate link between chromosome organization features and homolog pairing and alignment. We found that the meiotic chromosomes progressively reshape from TAD-like domains into linearly arranged loop arrays during prophase I. The transcriptionally active and inactive genomic regions exhibit distinct dynamics of loop growth, resulting in alternating domains consisting of shorter and longer chromosome loops. Such a domanial organization along meiotic chromosome axes is tightly correlated with the strength and precision of inter-homolog alignment. We further showed that a significant fraction of chromosomes near chromosome ends exhibit elevated inter-chromosomal association upon entering zygotene stage, while also exhibiting a higher degree of inter-homolog alignment. Using a mouse model defective in LINC complex component SUN1, we demonstrated that the prominent alignment of chromosome ends is dependent on the association of telomeres with the mechano-transducing LINC complex, but not the tethering of telomeres to the nuclear periphery. Taken together, our results suggest the 3D chromosome organization may provide a structural framework for the regulation of meiotic chromosome processes in higher eukaryotes.
Project description:Human cell division is a highly regulated process that relies on the accurate capture and movement of chromosomes to the metaphase plate. Errors in the fidelity of chromosome congression and alignment can lead to improper chromosome segregation, which is correlated with aneuploidy and tumorigenesis. These processes are known to be regulated by extracellular signal-regulated kinase 2 (ERK2) in other species, but the role of ERK2 in mitosis in mammals remains unclear. Here, we have identified the dual-specificity phosphatase 7 (DUSP7), known to display selectivity for ERK2, as important in regulating chromosome alignment. During mitosis, DUSP7 bound to ERK2 and regulated the abundance of active phospho-ERK2 through its phosphatase activity. Overexpression of DUSP7, but not catalytically inactive mutants, led to a decrease in the levels of phospho-ERK2 and mitotic chromosome misalignment, while knockdown of DUSP7 also led to defective chromosome congression that resulted in a prolonged mitosis. Consistently, knockdown or chemical inhibition of ERK2 or chemical inhibition of the MEK kinase that phosphorylates ERK2 led to chromosome alignment defects. Our results support a model wherein MEK-mediated phosphorylation and DUSP7-mediated dephosphorylation regulate the levels of active phospho-ERK2 to promote proper cell division.
Project description:During meiosis, chromosomes undergo extensive changes in structure and intranuclear positioning. How these chromosome organization changes occur and how they influence meiosis-specific chromosome events are not fully understood. Using Hi-C, we characterized chromosome architecture throughout mouse spermatogenesis at high temporal resolution. Our study revealed an intimate link between chromosome organization features and homolog pairing and alignment. We found that the meiotic chromosomes progressively reshape from TAD-like domains into linearly arranged loop arrays during prophase I. The transcriptionally active and inactive genomic regions exhibit distinct dynamics of loop growth, resulting in alternating domains consisting of shorter and longer chromosome loops. Such a domanial organization along meiotic chromosome axes is tightly correlated with the strength and precision of inter-homolog alignment. We further showed that a significant fraction of chromosomes near chromosome ends exhibit elevated inter-chromosomal association upon entering zygotene stage, while also exhibiting a higher degree of inter-homolog alignment. Using a mouse model defective in LINC complex component SUN1, we demonstrated that the prominent alignment of chromosome ends is dependent on the association of telomeres with the mechano-transducing LINC complex, but not the tethering of telomeres to the nuclear periphery. Taken together, our results suggest the 3D chromosome organization may provide a structural framework for the regulation of meiotic chromosome processes in higher eukaryotes.
Project description:During meiosis, Structural Maintenance of Chromosome (SMC) complexes underpin two fundamental features of meiosis: homologous recombination and chromosome segregation. While meiotic functions of the cohesin and condensin complexes have been delineated, the role of the third SMC complex, Smc5/6, remains enigmatic. Diminished Smc5/6 function causes severe defects in nuclear division, but the underlying causes of these defects remain unclear. Here we identify specific, essential meiotic functions for the Smc5/6 complex in homologous recombination and regulation of cohesin. We show that Smc5/6 is enriched at centromeres and cohesin-association sites where it regulates sister-chromatid cohesion and the timely removal of cohesin from chromosomal arms, respectively. Smc5/6 also localizes to recombination hotspots, where it promotes normal formation and resolution of joint-molecule intermediates. Furthermore, we find that Smc5/6 specifically promotes resolution of joint molecules via the XPF-family endonuclease, Mus81-Mms4Eme1. We propose that Smc5/6 acts as a chaperone for M-bM-^@M-^XmitoticM-bM-^@M-^Y-like recombination processes during meiosis. ChIP-chip was used to compare Smc5 localization in wild-type and spo11 strains
Project description:Eukaryotic cell proliferation requires chromosome replication and precise segregation to ensure daughter cells have identical genomic copies. The genus Plasmodium, the causative agent of malaria, has developed remarkable characteristic of nuclear division throughout its lifecycle to meet some peculiar and unique challenges of DNA replication and chromosome segregation. The parasite undergoes atypical endomitosis and endoreduplication with an intact nuclear membrane and intranuclear mitotic spindle. To understand diverse modes of cell division in Plasmodium we have studied the behaviour and composition of a key part of the mitotic apparatus, the outer kinetochore Ndc80 complex that attaches the centromere of chromosomes to the microtubules of the mitotic spindle. Using Ndc80 live-cell imaging in Plasmodium berghei we observe dynamic changes throughout proliferative life-stages including highly unusual kinetochore arrangements in sexual stages of the parasite. Furthermore, we identify a divergent Spc24 component of Ndc80 complex, previously thought to be missing, and completing a canonical, albeit unusual, Ndc80 complex structure. Altogether our studies reveal the kinetochore as an ideal starting point towards understanding non-canonical modes of chromosome segregation and cell division in Plasmodium.
Project description:Spindle assembly checkpoint (SAC) regulators such as the Mps1 kinase not only delay anaphase onset, but also correct improper chromosome-spindle linkages that would otherwise lead to missegregation and aneuploidy. However, the substrates and mechanisms involved in this pathway of error correction remain poorly understood. Using a chemically tuned kinetochore-targeting assay, we show that Mps1 destabilizes microtubule attachments (K-fibers) epistatically to Aurora B, the other major error-correcting kinase. Through chemical genetics and quantitative proteomics, we identify both known and novel sites of Mps1- regulated phosphorylation at the outer kinetochore. Modification of these substrates was sensitive to microtubule tension and counterbalanced by the PP2A-B56 phosphatase, a positive regulator of chromosome-spindle interactions. Consistently, Mps1 inhibition rescued K-fiber stability after depleting PP2A-B56. We also identify the hinge region of the W-shaped Ska complex as a key effector of Mps1 at the kinetochore-microtubule interface, as mutations that mimic constitutive phosphorylation strongly destabilized K-fibers in vivo and inhibited the Ska complex’s conversion from lattice diffusion to end-coupled microtubule binding in vitro. Together these results provide new insights into how Mps1 modulates the microtubule-binding properties of the kinetochore to promote the selective stabilization of bipolar attachments and error-free chromosome segregation.