A mammalian Partner of inscuteable binds NuMA and regulates mitotic spindle organization

Asymmetric cell division requires the orientation of mitotic spindles along the cell-polarity axis. In Drosophila neuroblasts, this involves the inter action of the proteins Inscuteable (Insc) and Partner of inscuteable (Pins) . We report here that a human Pins-related protein, called LGN, is instead essential for the assembly and organization of the mitotic spindle. LGN is cytoplasmic in interphase cells, but associates with the spindle poles duri ng mitosis. Ectopic expression of LGN disrupts spindle-pole organization an d chromosome segregation. Silencing of LGN expression by RNA interference a lso disrupts spindle-pole organization and prevents normal chromosome segre gation. We found that LGN binds the nuclear mitotic apparatus protein NuMA, which tethers spindles at the poles, and that this interaction is required for the LGN phenotype. Anti-LGN antibodies and the LGN-binding domain of N uMA both trigger microtubule aster formation in mitotic Xenopus egg extract s, and the NuMA-binding domain of LGN blocks aster assembly in egg extracts treated with taxol. Thus, we have identified a mammalian Pins homologue as a key regulator of spindle organization during mitosis.