A novel microtubule nucleation pathway for meiotic spindle assembly in oocytes

Abstract

1The meiotic spindle in oocytes is assembled in the absence of centrosomes, the major 2 microtubule nucleation sites in mitotic and male meiotic cells. A crucial, yet unresolved 3 question in meiosis is how spindle microtubules are generated without centrosomes and 4 only around chromosomes in the large volume of oocytes. Here we report a novel oocyte-5 specific microtubule nucleation pathway that is essential for assembling most spindle 6 microtubules complementarily with the Augmin pathway, and sufficient for triggering 7 microtubule assembly in oocytes. This pathway is mediated by the kinesin-6 Subito/MKlp2, 8 which recruits the γ-tubulin complex to the spindle equator to nucleate microtubules in 9 Drosophila oocytes. Away from chromosomes, Subito interaction with the γ-tubulin complex 10 is suppressed by its N-terminal region to prevent ectopic microtubule assembly in oocytes. 11We further demonstrate that the Subito complex from ovaries can nucleate microtubules 12 from pure tubulin dimers in vitro. Taken together, microtubule nucleation regulated by 13Subito drives spatially restricted spindle assembly in oocytes.14 15 not peer-reviewed)