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Kinesin-1 mediates translocation of the meiotic spindle to the oocyte cortex through KCA-1, a novel cargo adapter.

Yang HY, Mains PE, McNally FJ - J. Cell Biol. (2005)

Bottom Line: Depletion of any of these subunits by RNA interference resulted in meiosis I metaphase spindles that remained stationary at a position several micrometers from the cell cortex during the time when wild-type spindles translocated to the cortex.After this prolonged stationary period, unc-116(RNAi) spindles moved to the cortex through a partially redundant mechanism that is dependent on the anaphase-promoting complex.This study thus reveals two sequential mechanisms for translocating anastral spindles to the oocyte cortex.

View Article: PubMed Central - PubMed

Affiliation: Section of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA.

ABSTRACT
In animals, female meiotic spindles are attached to the egg cortex in a perpendicular orientation at anaphase to allow the selective disposal of three haploid chromosome sets into polar bodies. We have identified a complex of interacting Caenorhabditis elegans proteins that are involved in the earliest step in asymmetric positioning of anastral meiotic spindles, translocation to the cortex. This complex is composed of the kinesin-1 heavy chain orthologue, UNC-116, the kinesin light chain orthologues, KLC-1 and -2, and a novel cargo adaptor, KCA-1. Depletion of any of these subunits by RNA interference resulted in meiosis I metaphase spindles that remained stationary at a position several micrometers from the cell cortex during the time when wild-type spindles translocated to the cortex. After this prolonged stationary period, unc-116(RNAi) spindles moved to the cortex through a partially redundant mechanism that is dependent on the anaphase-promoting complex. This study thus reveals two sequential mechanisms for translocating anastral spindles to the oocyte cortex.

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Model for the sequential action of the early, UNC-116–dependent spindle translocation pathway and the late, APC-dependent pathway. In wild-type worms, UNC-116 moves the spindle on cytoplasmic microtubules toward the cortex. At the APC-dependent metaphase–anaphase transition, astral microtubules extend from one pole and mediate pulling forces. In wild-type embryos, this APC-dependent cortical pulling results in rotation. In unc-116(RNAi) embryos, the same force generates translocation with one pole leading.
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fig7: Model for the sequential action of the early, UNC-116–dependent spindle translocation pathway and the late, APC-dependent pathway. In wild-type worms, UNC-116 moves the spindle on cytoplasmic microtubules toward the cortex. At the APC-dependent metaphase–anaphase transition, astral microtubules extend from one pole and mediate pulling forces. In wild-type embryos, this APC-dependent cortical pulling results in rotation. In unc-116(RNAi) embryos, the same force generates translocation with one pole leading.

Mentions: Our results indicate that in C. elegans female meiotic spindles are positioned at the cell cortex by two sequential mechanisms, an early, kinesin-1–dependent pathway that moves spindles in a sideways orientation and a late, APC-dependent pathway that pulls the spindle toward the cortex with one pole leading (Fig. 7). Wild-type meiotic spindles begin kinesin-1–dependent movement toward the cortex as soon as they have assembled into bipolar structures, while the embryo is still inside the spermatheca. In the absence of kinesin-1, the meiosis I spindle is stationary in the cytoplasm during the 7-min period before spindle shortening begins. The spindle then suddenly moves to the cortex with one pole leading just after spindle shortening initiates. We propose that the sudden movement of unc-116(RNAi) spindles to the cortex is mediated by the same mechanism as wild-type spindle rotation and that it is mediated by microtubules extending from one spindle pole to the cortex. In wild-type spindles, pulling a spindle pole for only 2 μm would generate a rotation, whereas in unc-116(RNAi) spindles, the same pulling force would generate a pole-first translocation over a distance of 7 μm (Fig. 7). This model is supported by the finding that both wild-type spindle rotation and UNC-116–independent translocation are blocked when the APC is inhibited. The model is also supported by the finding that both wild-type rotation and UNC-116–independent translocation occur just after initiation of APC-dependent spindle shortening. Finally, UNC-116–dependent translocation seen in wild-type worms occurs with the spindle oriented parallel to the cortex, whereas UNC-116–independent translocation occurs with one pole leading. Direct visualization of microtubules extending from one pole to the cortex has thus far been stymied by the presence of a microtubule meshwork that fills the entire meiotic embryo. Two successive and partially redundant pathways also mediate mitotic spindle positioning in budding yeast (Schuyler and Pellman, 2001).


Kinesin-1 mediates translocation of the meiotic spindle to the oocyte cortex through KCA-1, a novel cargo adapter.

Yang HY, Mains PE, McNally FJ - J. Cell Biol. (2005)

Model for the sequential action of the early, UNC-116–dependent spindle translocation pathway and the late, APC-dependent pathway. In wild-type worms, UNC-116 moves the spindle on cytoplasmic microtubules toward the cortex. At the APC-dependent metaphase–anaphase transition, astral microtubules extend from one pole and mediate pulling forces. In wild-type embryos, this APC-dependent cortical pulling results in rotation. In unc-116(RNAi) embryos, the same force generates translocation with one pole leading.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2171918&req=5

fig7: Model for the sequential action of the early, UNC-116–dependent spindle translocation pathway and the late, APC-dependent pathway. In wild-type worms, UNC-116 moves the spindle on cytoplasmic microtubules toward the cortex. At the APC-dependent metaphase–anaphase transition, astral microtubules extend from one pole and mediate pulling forces. In wild-type embryos, this APC-dependent cortical pulling results in rotation. In unc-116(RNAi) embryos, the same force generates translocation with one pole leading.
Mentions: Our results indicate that in C. elegans female meiotic spindles are positioned at the cell cortex by two sequential mechanisms, an early, kinesin-1–dependent pathway that moves spindles in a sideways orientation and a late, APC-dependent pathway that pulls the spindle toward the cortex with one pole leading (Fig. 7). Wild-type meiotic spindles begin kinesin-1–dependent movement toward the cortex as soon as they have assembled into bipolar structures, while the embryo is still inside the spermatheca. In the absence of kinesin-1, the meiosis I spindle is stationary in the cytoplasm during the 7-min period before spindle shortening begins. The spindle then suddenly moves to the cortex with one pole leading just after spindle shortening initiates. We propose that the sudden movement of unc-116(RNAi) spindles to the cortex is mediated by the same mechanism as wild-type spindle rotation and that it is mediated by microtubules extending from one spindle pole to the cortex. In wild-type spindles, pulling a spindle pole for only 2 μm would generate a rotation, whereas in unc-116(RNAi) spindles, the same pulling force would generate a pole-first translocation over a distance of 7 μm (Fig. 7). This model is supported by the finding that both wild-type spindle rotation and UNC-116–independent translocation are blocked when the APC is inhibited. The model is also supported by the finding that both wild-type rotation and UNC-116–independent translocation occur just after initiation of APC-dependent spindle shortening. Finally, UNC-116–dependent translocation seen in wild-type worms occurs with the spindle oriented parallel to the cortex, whereas UNC-116–independent translocation occurs with one pole leading. Direct visualization of microtubules extending from one pole to the cortex has thus far been stymied by the presence of a microtubule meshwork that fills the entire meiotic embryo. Two successive and partially redundant pathways also mediate mitotic spindle positioning in budding yeast (Schuyler and Pellman, 2001).

Bottom Line: Depletion of any of these subunits by RNA interference resulted in meiosis I metaphase spindles that remained stationary at a position several micrometers from the cell cortex during the time when wild-type spindles translocated to the cortex.After this prolonged stationary period, unc-116(RNAi) spindles moved to the cortex through a partially redundant mechanism that is dependent on the anaphase-promoting complex.This study thus reveals two sequential mechanisms for translocating anastral spindles to the oocyte cortex.

View Article: PubMed Central - PubMed

Affiliation: Section of Molecular and Cellular Biology, University of California, Davis, Davis, CA 95616, USA.

ABSTRACT
In animals, female meiotic spindles are attached to the egg cortex in a perpendicular orientation at anaphase to allow the selective disposal of three haploid chromosome sets into polar bodies. We have identified a complex of interacting Caenorhabditis elegans proteins that are involved in the earliest step in asymmetric positioning of anastral meiotic spindles, translocation to the cortex. This complex is composed of the kinesin-1 heavy chain orthologue, UNC-116, the kinesin light chain orthologues, KLC-1 and -2, and a novel cargo adaptor, KCA-1. Depletion of any of these subunits by RNA interference resulted in meiosis I metaphase spindles that remained stationary at a position several micrometers from the cell cortex during the time when wild-type spindles translocated to the cortex. After this prolonged stationary period, unc-116(RNAi) spindles moved to the cortex through a partially redundant mechanism that is dependent on the anaphase-promoting complex. This study thus reveals two sequential mechanisms for translocating anastral spindles to the oocyte cortex.

Show MeSH