Abstract
During asymmetric division, fate assignation in daughter cells is mediated by the partition of determinants from the mother. In the fly sensory organ precursor cell, Notch signalling partitions into the pIIa daughter. Notch and its ligand Delta are endocytosed into Sara endosomes in the mother cell and they are first targeted to the central spindle, where they get distributed asymmetrically to finally be dispatched to pIIa. While the processes of endosomal targeting and asymmetry are starting to be understood, the machineries implicated in the final dispatch to pIIa are unknown. We show that Sara binds the PP1c phosphatase and its regulator Sds22. Sara phosphorylation on three specific sites functions as a switch for the dispatch: if not phosphorylated, endosomes are targeted to the spindle and upon phosphorylation of Sara, endosomes detach from the spindle during pIIa targeting.
Highlights
During asymmetric division, fate assignation in daughter cells is mediated by the partition of determinants from the mother
Downstream the Par complex, Notch signalling is regulated by endocytosis and endosomal trafficking through four independent mechanisms: (1) The E3 Ubiquitin ligase Neuralized is segregated to the pIIb cell, where it induces the endocytosis and thereby the activation of the Notch ligand Delta[6]; (2) Recycling endosomes accumulate in the perinuclear region of the pIIb cell, in which they enhance the recycling and activation of Delta[7]; (3) The endocytic proteins a-adaptin and Numb are segregated to the pIIb cell, where they inhibit the Notch activator Sanpodo[8,9]; (4) During Sensory Organ Precursor cells (SOPs) mitosis, Sara endosomes transport a signalling pool of Notch and Delta to the pIIa cell, where Notch can be activated[10,11]
Asymmetric Sara endosomes have been shown to operate in the larval neural stem cells[10] as well as in the adult intestinal stem cells in flies[12], where they play a role during asymmetric Notch signalling
Summary
Fate assignation in daughter cells is mediated by the partition of determinants from the mother. We show that the Sara protein itself controls both the targeting and the final dispatch of Sara endosomes to the pIIa daughter cell.
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