Ohi Ryoma

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Ohi
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Ryoma
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  • Article
    Bipolarization and poleward flux correlate during xenopus extract spindle assembly
    (American Society for Cell Biology, 2004-09-22) Mitchison, Timothy J. ; Maddox, P. ; Groen, Aaron C. ; Cameron, Lisa ; Perlman, Z. ; Ohi, Ryoma ; Desai, Ankur R. ; Salmon, Edward D. ; Kapoor, Tarun M.
    We investigated the mechanism by which meiotic spindles become bipolar and the correlation between bipolarity and poleward flux, using Xenopus egg extracts. By speckle microscopy and computational alignment, we find that monopolar sperm asters do not show evidence for flux, partially contradicting previous work. We account for the discrepancy by describing spontaneous bipolarization of sperm asters that was missed previously. During spontaneous bipolarization, onset of flux correlated with onset of bipolarity, implying that antiparallel microtubule organization may be required for flux. Using a probe for TPX2 in addition to tubulin, we describe two pathways that lead to spontaneous bipolarization, new pole assembly near chromatin, and pole splitting. By inhibiting the Ran pathway with excess importin-alpha, we establish a role for chromatin-derived, antiparallel overlap bundles in generating the sliding force for flux, and we examine these bundles by electron microscopy. Our results highlight the importance of two processes, chromatin-initiated microtubule nucleation, and sliding forces generated between antiparallel microtubules, in self-organization of spindle bipolarity and poleward flux.
  • Preprint
    Spindle assembly in the absence of a RanGTP gradient requires localized CPC activity
    ( 2009-05) Maresca, Thomas J. ; Groen, Aaron C. ; Gatlin, Jesse C. ; Ohi, Ryoma ; Mitchison, Timothy J. ; Salmon, Edward D.
    During animal cell division, a gradient of GTP-bound Ran is generated around mitotic chromatin. It is generally accepted that this RanGTP gradient is essential for organizing the spindle since it locally activates critical spindle assembly factors. Here, we show in Xenopus egg extract, where the gradient is best characterized, that spindles can assemble in the absence of a RanGTP gradient. Gradient-free spindle assembly occurred around sperm nuclei but not around chromatin-coated beads and required the chromosomal passenger complex (CPC). Artificial enrichment of CPC activity within hybrid bead arrays containing both immobilized chromatin and the CPC supported local microtubule assembly even in the absence of a RanGTP gradient. We conclude that RanGTP and the CPC constitute the two major molecular signals that spatially promote microtubule polymerization around chromatin. Furthermore, we hypothesize that the two signals mainly originate from discreet physical sites on the chromosomes to localize microtubule assembly around chromatin: a RanGTP signal from any chromatin, and a CPC-dependent signal predominantly generated from centromeric chromatin.