Identification of locally activated spindle-associated proteins in oocytes uncovers a phosphatase-driven mechanism.

The meiotic spindle forms only around the chromosomes in oocytes, despite the exceptionally large volume of the cytoplasm. This spatial restriction is likely to be governed by local activation of key microtubule regulators around the chromosomes in oocytes, but the identities of these microtubule regulators and the mechanisms remain unclear. To address this, we developed a novel assay to visualise spatial regulation of spindle-associated proteins in Drosophila oocytes by inducing ectopic microtubule clusters. This assay identified several proteins including the TPX2 homologue Mei-38, which localised more strongly to microtubules near the chromosomes than away from them. In Mei-38, we identified a microtubule-binding domain containing a region that was also highly conserved in humans. The domain itself is regulated spatially, and contains a conserved serine and a nearby PP2A-B56-docking motif. A non-phosphorylatable mutation of this serine residue allowed the domain to localise to ectopic microtubules as well as spindle microtubules, whereas mutations in the PP2A-B56-docking motif greatly reduced the spindle localisation. As this phosphatase is concentrated at the kinetochores, it might act as a novel chromosomal signal spatially regulating spindle proteins within oocytes.