Role of cortical tumour-suppressor proteins in asymmetric division of Drosophila neuroblast.

Cellular diversity during development arises in part from asymmetric divisions, which generate two distinct cells by transmitting localized determinants from a progenitor cell into one daughter cell. In Drosophila, neuroblasts undergo typical asymmetric divisions to produce another neuroblast and a ganglion mother cell. At mitosis, neural fate determinants, including Prospero and Numb, localize to the basal cortex, from which the ganglion mother cell buds off; Inscuteable and Bazooka, which regulate spindle orientation, localize apically. Here we show that a tumour-suppressor protein, Lethal giant larvae (Lgl), is essential for asymmetric cortical localization of all basal determinants in mitotic neuroblasts, and is therefore indispensable for neural fate decisions. Lgl, which itself is uniformly cortical, interacts with several types of Myosin to localize the determinants. Another tumour-suppressor protein, Lethal discs large (Dlg), participates in this process by regulating the localization of Lgl. The localization of the apical components is unaffected in lgl or dlg mutants. Thus, Lgl and Dlg act in a common process that differentially mediates cortical protein targeting in mitotic neuroblasts, and that creates intrinsic differences between daughter cells.