Rapsynoid/partner of inscuteable controls asymmetric division of larval neuroblasts in Drosophila

Asymmetric cell division generates daughter cells with different developmen tal fates. In Drosophila neuroblasts, asymmetric divisions are characterize d by (1) a difference in size between the two daughter cells and (2) an asy mmetric distribution of cell fate determinants, including Prospero and Numb , between the two daughter cells. In embryonic neuroblasts, the asymmetric localization of cell fate determinants is under the control of the protein Inscuteable (Insc), which is itself localized asymmetrically as an apical c rescent. Here, we describe a new Drosophila protein, Rapsynoid (Raps), whic h interacts in a two-hybrid assay with the signal transduction protein G al pha i. We show that Raps is localized asymmetrically in dividing larval neu roblasts and colocalizes with Insc. Moreover, in raps mutants, the asymmetr ic divisions of neuroblasts are altered: (1) Insc is no longer asymmetrical ly localized in the dividing neuroblast; and (2) the neuroblast division pr oduces two daughter cells of similar sizes. However, the morphologically sy mmetrical divisions of raps neuroblasts still lead to daughter cells with d ifferent fates, as shown by differences in gene expression. Our data show t hat Raps is a novel protein involved in the control of asymmetric divisions of neuroblasts.