Rogers, G. C., Rogers, S. L. and Sharp, D. J. (2005). Spindle microtubules in flux. .
We apologise for an error that occurred in the print version of this article. In all figures some elements were missing. The online version is unaffected.
Poleward MT flux in a metaphase-stage mitotic spindle. Flux occurs on both kMTs (red lines) and non-kMTs (blue lines) in the spindle. Tubulin subunits are incorporated into polymer at MT plus-ends and removed at their minus-ends focused at the spindle poles. Arrows within the red and blue lines indicate the direction of continuous ATP-dependent polymer movement. kMT plus-end assembly stops at the transition to anaphase (although there are exceptions to this rule; Chen and Zhang, 2004; LaFountain et al., 2004; see text). Astral MTs (green lines), whose minus-ends are embedded in the centrosomes, do not flux. Orange arrows above the spindle indicate the poleward direction of force exerted by flux on each sister kinetochore. Likewise, opposing blue arrows indicate the metaphase plateward direction of force exerted by flux on each spindle pole. Importantly, the major source of MT assembly dynamics in the spindle is plus-end dynamic instability, which is not shown here for simplicity.
Poleward MT flux in a metaphase-stage mitotic spindle. Flux occurs on both kMTs (red lines) and non-kMTs (blue lines) in the spindle. Tubulin subunits are incorporated into polymer at MT plus-ends and removed at their minus-ends focused at the spindle poles. Arrows within the red and blue lines indicate the direction of continuous ATP-dependent polymer movement. kMT plus-end assembly stops at the transition to anaphase (although there are exceptions to this rule; Chen and Zhang, 2004; LaFountain et al., 2004; see text). Astral MTs (green lines), whose minus-ends are embedded in the centrosomes, do not flux. Orange arrows above the spindle indicate the poleward direction of force exerted by flux on each sister kinetochore. Likewise, opposing blue arrows indicate the metaphase plateward direction of force exerted by flux on each spindle pole. Importantly, the major source of MT assembly dynamics in the spindle is plus-end dynamic instability, which is not shown here for simplicity.
A model for flux. (A) MT minus-end release from centrosomes could occur either by separation from the γ-tubulin ring complex (γ-TuRC) at or near the centrosome or from the MT-severing activity of centrosome-associated katanin (McNally et al., 1996). (B) A Kin I kinesin is targeted and tethered to an insoluble spindle pole matrix that anchors the spindle pole to the centrosome. Kin I actively drives flux (depicted as blue lines with arrows) by disassembling MT minus-ends. (C) This activity produces a polymer-free gap between the centrosome and the spindle pole that is observed in both live (top panel) and fixed (bottom panel) Drosophila syncytial blastoderm-stage embryos. The top panel shows rhodamine-labeled MTs (red) and GFP-histones (green). Indirect immunofluorescence in the bottom panel shows MTs (red) and KLP10A (green), which localizes within the gap and on centrosomes. (D) Poleward MT flux is driven in the metaphase half-spindle by a spindle-pole-associated Kin I kinesin. kMT length is maintained by the activity of the kinetochore-associated CLASP protein that induces plus-end polymerization.
A model for flux. (A) MT minus-end release from centrosomes could occur either by separation from the γ-tubulin ring complex (γ-TuRC) at or near the centrosome or from the MT-severing activity of centrosome-associated katanin (McNally et al., 1996). (B) A Kin I kinesin is targeted and tethered to an insoluble spindle pole matrix that anchors the spindle pole to the centrosome. Kin I actively drives flux (depicted as blue lines with arrows) by disassembling MT minus-ends. (C) This activity produces a polymer-free gap between the centrosome and the spindle pole that is observed in both live (top panel) and fixed (bottom panel) Drosophila syncytial blastoderm-stage embryos. The top panel shows rhodamine-labeled MTs (red) and GFP-histones (green). Indirect immunofluorescence in the bottom panel shows MTs (red) and KLP10A (green), which localizes within the gap and on centrosomes. (D) Poleward MT flux is driven in the metaphase half-spindle by a spindle-pole-associated Kin I kinesin. kMT length is maintained by the activity of the kinetochore-associated CLASP protein that induces plus-end polymerization.
Mechanistic models for bivalent positioning and congression in meiotic spindles of grasshopper spermatocytes, using the force from flux and plus-end kMT assembly. (A) Bivalent positioning. A bivalent maintains an equilibrium position at the spindle equator owing to an equivalent amount of flux (blue arrows) on homologous kinetochores attached to an equal number of kMTs. Moderate tension on the kinetochores induces kMT plus-end assembly (red arrows). Laser irradiation (green line) partially destroys a kinetochore, which reduces the number of kMTs to which it can bind. This sudden imbalance in kMT number increases the stress on the remaining kMTs of the irradiated kinetochore, inducing a greater rate of kMT plus-end assembly (red arrow). Compressive force on the opposing kMTs inhibits or decreases the rate of plus-end assembly, and, consequently, the bivalent moves (orange arrow) towards the pole that has the greater number of kMT attachments and the larger flux-generated force (larger blue arrow). As chromosome arms bind to an increasing density of spindle MTs that resist their poleward movement (yellow arrows), the increased tension on the unirradiated kinetochore induces plus-end polymerization. A new equilibrium position is reached when tension-induced plus-end assembly on the unirradiated kinetochore equals the rate of disassembly at the pole. (B) Congression. Initially, a bivalent close to one spindle pole becomes mono-oriented and is pulled poleward by flux (blue arrow). Chromosome arms bind to an increasing density of spindle MTs and the resulting resistance (yellow arrows) increases kinetochore tension to induce plus-end polymerization (red arrows). Poleward movement stops, facilitating the capture of the homologous kinetochore by the opposite pole (blue arrows in the spindle). Capture of the unattached kinetochore produces an even greater amount of tension and polymerization (red arrow) at the opposite kinetochore, allowing the bi-oriented bivalent to move to the spindle equator (orange arrow), even though kMT number and flux-generated force (blue arrows) are greater at the lagging homologous kinetochore. Finally, an equilibrium position is established at the spindle equator when the leading kinetochore is captured by an equal number of kMTs. Poleward force from flux (blue arrows) is equivalent in each half-spindle and is balanced by an equal rate of kMT plus-end assembly (red arrows) induced by moderate tension.
Mechanistic models for bivalent positioning and congression in meiotic spindles of grasshopper spermatocytes, using the force from flux and plus-end kMT assembly. (A) Bivalent positioning. A bivalent maintains an equilibrium position at the spindle equator owing to an equivalent amount of flux (blue arrows) on homologous kinetochores attached to an equal number of kMTs. Moderate tension on the kinetochores induces kMT plus-end assembly (red arrows). Laser irradiation (green line) partially destroys a kinetochore, which reduces the number of kMTs to which it can bind. This sudden imbalance in kMT number increases the stress on the remaining kMTs of the irradiated kinetochore, inducing a greater rate of kMT plus-end assembly (red arrow). Compressive force on the opposing kMTs inhibits or decreases the rate of plus-end assembly, and, consequently, the bivalent moves (orange arrow) towards the pole that has the greater number of kMT attachments and the larger flux-generated force (larger blue arrow). As chromosome arms bind to an increasing density of spindle MTs that resist their poleward movement (yellow arrows), the increased tension on the unirradiated kinetochore induces plus-end polymerization. A new equilibrium position is reached when tension-induced plus-end assembly on the unirradiated kinetochore equals the rate of disassembly at the pole. (B) Congression. Initially, a bivalent close to one spindle pole becomes mono-oriented and is pulled poleward by flux (blue arrow). Chromosome arms bind to an increasing density of spindle MTs and the resulting resistance (yellow arrows) increases kinetochore tension to induce plus-end polymerization (red arrows). Poleward movement stops, facilitating the capture of the homologous kinetochore by the opposite pole (blue arrows in the spindle). Capture of the unattached kinetochore produces an even greater amount of tension and polymerization (red arrow) at the opposite kinetochore, allowing the bi-oriented bivalent to move to the spindle equator (orange arrow), even though kMT number and flux-generated force (blue arrows) are greater at the lagging homologous kinetochore. Finally, an equilibrium position is established at the spindle equator when the leading kinetochore is captured by an equal number of kMTs. Poleward force from flux (blue arrows) is equivalent in each half-spindle and is balanced by an equal rate of kMT plus-end assembly (red arrows) induced by moderate tension.
