326 research outputs found

    Activation of Ras-ERK pathway by Fgf8 and its downregulation by Sprouty2 for the isthmus organizing activity

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    In the previous studies, we showed that strong Fgf8 signaling activates the Ras-ERK pathway to induce cerebellum. Here, we show importance of negative regulation of this pathway. 'Prolonged' activation of ERK by misexpression of _Fgf8b_ and dominant-negative _Sprouty2_ (_dnSprouty2_) did not change the fate of the mesencephalic alar plate. Downregulation of ERK activity using a MEK inhibitor, U0126, or by tetracycline dependent Tet-off system after co-expression of _Fgf8b_ and _dnSprouty2_, forced the mesencephalic alar plate to differentiate into cerebellum. We then paid attention to Mkp3. After misexpression of _dnMkp3_ and _Fgf8b_, slight downregulation of ERK activity occurred, which may be due to Sprouty2, and the mesencephalon transformed to the isthmus-like structure. The results indicate that ERK must be once upregulated and then be downregulated for cerebellar differentiation, and that differential ERK activity level established by negative regulators receiving Fgf8 signal may determine regional specificity of mesencephalon and metencephalon

    A role of topoisomerase II in linking DNA replication to chromosome condensation

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    The condensin complex and topoisomerase II (topo II) have different biochemical activities in vitro, and both are required for mitotic chromosome condensation. We have used Xenopus egg extracts to investigate the functional interplay between condensin and topo II in chromosome condensation. When unreplicated chromatin is directly converted into chromosomes with single chromatids, the two proteins must function together, although they are independently targeted to chromosomes. In contrast, the requirement for topo II is temporarily separable from that of condensin when chromosome assembly is induced after DNA replication. This experimental setting allows us to find that, in the absence of condensin, topo II becomes enriched in an axial structure within uncondensed chromatin. Subsequent addition of condensin converts this structure into mitotic chromosomes in an ATP hydrolysis–dependent manner. Strikingly, preventing DNA replication by the addition of geminin or aphidicolin disturbs the formation of topo II–containing axes and alters the binding property of topo II with chromatin. Our results suggest that topo II plays an important role in an early stage of chromosome condensation, and that this function of topo II is tightly coupled with prior DNA replication

    Chromosome dynamics: Fuzzy sequences, specific attachments?

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    AbstractThe assembly of condensed chromosomes in a cell-free system is inhibited by the addition of proteins that bind AT-rich DNA. Does this implicate the AT-rich scaffold attachment regions (SARs) in the formation of chromosomes

    HEAT repeats – versatile arrays of amphiphilic helices working in crowded environments?

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    Cellular proteins do not work alone in diluted conditions. They often function as part of large macromolecular complexes, which are transported and concentrated into specific cellular compartments and function in their highly crowded environments. A central theme of modern cell biology is to understand how cellular proteins might achieve these challenging tasks efficiently and faithfully. In this Opinion article, we will focus on HEAT repeats, flexible arrays of amphiphilic helices found in many eukaryotic proteins such as karyopherins and condensins, and discuss how this uniquely designed helical repeats might underlie dynamic protein-protein interactions and support cellular functions in crowded environments. We will make bold speculations on functional similarities between HEAT repeats and intrinsically disordered regions (IDRs) in macromolecular phase separation. Potential contributions of HEAT-HEAT interactions, as well as cooperation between HEATs and IDRs, to mesoscale organelle assembly will be discussed

    FACTORS ASSOCIATED WITH DECELERATION OF RUNNING VELOCITY IN THE LAST PHASE OF THE 400-M SPRINT BASED ON KINETICS CHANGES

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    The purpose of this study was to investiThe purpose of this study was to investigate the factors associated with deceleration of running velocity during the 400-m sprint (at the 350-m point) based on kinetics changes. Fourteen male collegiate sprinters performed the 400-m sprint (50.26 ± 2.27 s) at a subjective effort level of 100%. The ground reaction force (1000 Hz) was measured 350 m from the start point, and running movements were recorded from the side by a high-speed camera (300 Hz). The results were as follows: 1) High running velocity was associated with a high stride length. 2) A statistically significant positive correlation was observed between the stride length and the angular impulse of hip flexion in the second half of the support phase. These results suggest that the angular impulse of hip flexion is higher, the leg which has been supported is being swung out forward greatly after ground release

    Condensin and cohesin display different arm conformations with characteristic hinge angles

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    Structural maintenance of chromosomes (SMC) proteins play central roles in higher-order chromosome dynamics from bacteria to humans. In eukaryotes, two different SMC protein complexes, condensin and cohesin, regulate chromosome condensation and sister chromatid cohesion, respectively. Each of the complexes consists of a heterodimeric pair of SMC subunits and two or three non-SMC subunits. Previous studies have shown that a bacterial SMC homodimer has a symmetrical structure in which two long coiled-coil arms are connected by a flexible hinge. A catalytic domain with DNA- and ATP-binding activities is located at the distal end of each arm. We report here the visualization of vertebrate condensin and cohesin by electron microscopy. Both complexes display the two-armed structure characteristic of SMC proteins, but their conformations are remarkably different. The hinge of condensin is closed and the coiled-coil arms are placed close together. In contrast, the hinge of cohesin is wide open and the coiled-coils are spread apart from each other. The non-SMC subunits of both condensin and cohesin form a globular complex bound to the catalytic domains of the SMC heterodimers. We propose that the “closed” conformation of condensin and the “open” conformation of cohesin are important structural properties that contribute to their specialized biochemical and physiological functions

    Visualization of early chromosome condensation: a hierarchical folding, axial glue model of chromosome structure

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    Current models of mitotic chromosome structure are based largely on the examination of maximally condensed metaphase chromosomes. Here, we test these models by correlating the distribution of two scaffold components with the appearance of prophase chromosome folding intermediates. We confirm an axial distribution of topoisomerase IIα and the condensin subunit, structural maintenance of chromosomes 2 (SMC2), in unextracted metaphase chromosomes, with SMC2 localizing to a 150–200-nm-diameter central core. In contrast to predictions of radial loop/scaffold models, this axial distribution does not appear until late prophase, after formation of uniformly condensed middle prophase chromosomes. Instead, SMC2 associates throughout early and middle prophase chromatids, frequently forming foci over the chromosome exterior. Early prophase condensation occurs through folding of large-scale chromatin fibers into condensed masses. These resolve into linear, 200–300-nm-diameter middle prophase chromatids that double in diameter by late prophase. We propose a unified model of chromosome structure in which hierarchical levels of chromatin folding are stabilized late in mitosis by an axial “glue.

    Cell cycle-specific phase separation regulated by protein charge blockiness

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    Dynamic morphological changes of intracellular organelles are often regulated by protein phosphorylation or dephosphorylation1-6. Phosphorylation modulates stereospecific interactions among structured proteins, but how it controls molecular interactions among unstructured proteins and regulates their macroscopic behaviours remains unknown. Here we determined the cell cycle-specific behaviour of Ki-67, which localizes to the nucleoli during interphase and relocates to the chromosome periphery during mitosis. Mitotic hyperphosphorylation of disordered repeat domains of Ki-67 generates alternating charge blocks in these domains and increases their propensity for liquid–liquid phase separation (LLPS). A phosphomimetic sequence and the sequences with enhanced charge blockiness underwent strong LLPS in vitro and induced chromosome periphery formation in vivo. Conversely, mitotic hyperphosphorylation of NPM1 diminished a charge block and suppressed LLPS, resulting in nucleolar dissolution. Cell cycle-specific phase separation can be modulated via phosphorylation by enhancing or reducing the charge blockiness of disordered regions, rather than by attaching phosphate groups to specific sites

    General Solution of 7D Octonionic Top Equation

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    The general solution of a 7D analogue of the 3D Euler top equation is shown to be given by an integration over a Riemann surface with genus 9. The 7D model is derived from the 8D Spin(7)Spin(7) invariant self-dual Yang-Mills equation depending only upon one variable and is regarded as a model describing self-dual membrane instantons. Several integrable reductions of the 7D top to lower target space dimensions are discussed and one of them gives 6, 5, 4D descendants and the 3D Euler top associated with Riemann surfaces with genus 6, 5, 2 and 1, respectively.Comment: 13 pages, Latex, 3 eps.files. Minor changes, eq.(4) adde
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