THE PROSPERO TRANSCRIPTION FACTOR IS ASYMMETRICALLY LOCALIZED TO THE CELL CORTEX DURING NEUROBLAST MITOSIS IN DROSOPHILA

Both intrinsic and extrinsic factors are known to regulate sibling cel l fate. Here we describe a novel mechanism for the asymmetric localiza tion of a transcription factor to one daughter cell at mitosis. The Dr osophila CNS develops from asymmetrically dividing neuroblasts, which give rise to a large neuroblast and a smaller ganglion mother cell (GM C), The prospero gene encodes a transcription factor necessary for pro per GMC gene expression. We show that the prospero protein is synthesi zed in the neuroblast where it is localized to the F-actin cell cortex . At mitosis, prospero is asymmetrically localized to the budding GMC and excluded from the neuroblast. After cytokinesis, prospero is trans located from the GMC cortex into the nucleus. Asymmetric cortical loca lization of prospero in neuroblasts requires entry into mitosis; it do es not depend on numb function. prospero is also observed in cortical crescents in dividing precursors of the peripheral nervous system and adult midgut. The asymmetric cortical localization of prospero at mito sis is a mechanism for rapidly establishing distinct sibling cell fate s in the CNS and possibly other tissues.