A novel microtubule nucleation pathway for meiotic spindle assembly in oocytes
Oocytes lack centrosomes that are the major sites for microtubule nucleation in somatic cells.
Moreover, the microtubule nucleation pathway depending on the Ran-GTP gradient is dispensable for meiosis I spindle assembly in oocytes. Thus, a crucial yet unresolved question in meiosis is how spindle microtubules are generated without centrosomes in oocytes to ensure accurate chromosome segregation.
Using Drosophila, we uncovered a novel microtubule nucleation pathway that is: (1) active only in ooc)tes (2) essential (together with Augmin pathway) for assembling most of the spindle microtubules around the meiotic chromosomes, and (3) sufficient for triggering microtubule assembly.
We discovered that, in Drosophila oocyres, the kinesin-6 Subito (Kif20AA4Klp2) recruits the gamma-tubulin complex to assemble microtubules near the chromosomes, while Augmin fulfils this function at the spindle poles. Co-depletion of Subito and Augmin leads to a dramatic loss of microtubules to an extent that is comparable to the loss of the gamma-tubulin complex. Interestingly, over-expression of Subito lacking its non-motor N-terminus (Subito ÂN) induces ectopic microtubule nucleation away from the meiotic chromosomes, suggesting that this novel nucleation pathway is inhibited in the cytoplasm by the N-terminus of Subito. An in vitro assay showed that Subito N-terminus is preventing Subito/gamma-tubulin interaction. Subito ÂN complexes isolated from Drosophila ovaries are competent to nucleate microtubules in vitro while Subito WT complexes barely nucleate any microtubules.
Our results suggest that around the chromosomes, a signal suppresses Subito N-terminus inhibition allowing the kinesin to recruit the gamma-tubulin complex and nucleate microtubules near the cluomosomes. In contrast, away from the chromosomes, the N-terminus of Subito suppresses its binding to gamma-tubulin complexes to prevent ectopic microtubule nucleation. This study provides the first observation of a kinesin recruiting active microtubule nucleators. Moreover, it highlights an alternative pathway used by oocytes to locally restrict spindle assembly in a large volume of cytoplasm that lacks spatial cues such as centrosomes and cell boundaries.
Invité par Régis GIET
>> Friday 21 September 2018 at 11:00 '- IGDR conference room, ground floor, building 4/ Villejean Campus