The spindle orienting machinery requires activation, not just localization

Summary The orientation of the mitotic spindle at metaphase determines the placement of the daughter cells. Spindle orientation in animals typically relies on an evolutionarily conserved biological machine comprised of at least four proteins - called Partner of Inscuteable (Pins), Gαi, Mushroom body defective (Mud), and Dynein in flies - that exert a pulling force on astral microtubules and reels the spindle into alignment. The canonical model for spindle orientation holds that the direction of pulling is determined by asymmetric placement of this machinery at the cell cortex. In most cell types, this placement is thought to be mediated by Pins, and a substantial body of literature is therefore devoted to identifying polarized cues that govern localized cortical enrichment of Pins. In Drosophila neuroblasts, for example, this cue is thought to be Inscuteable, which helps recruit Pins to the apical cell surface. In this study we revisit the canonical model. We find that spindle orientation in the follicular epithelium requires not only Pins localization but also activation, which relies on direct interaction between Pins and the multifunctional protein Discs large. This mechanism is distinct from the one mediated by Inscuteable, which we find also has an activating step. Together our results show that the canonical model is incomplete. Local enrichment of Pins is not sufficient to determine spindle orientation; an activation step is also necessary.