Activation of Arp2/3 by WASp Is Essential for the Endocytosis of Delta Only during Cytokinesis in Drosophila.

The actin nucleator Arp2/3 generates pushing forces in response to signals integrated by SCAR and WASp. In Drosophila, the activation of Arp2/3 by WASp is specifically required for Notch signaling following asymmetric cell division. How WASp and Arp2/3 regulate Notch activity and why receptor activation requires WASp and Arp2/3 only in the context of intra-lineage fate decisions are unclear. Here, we find that WASp, but not SCAR, is required for Notch activation soon after division of the sensory organ precursor cell. Conversely, SCAR, but not WASp, is required to expand the cell-cell contact between the two SOP daughters. Thus, these two activities of Arp2/3 can be uncoupled. Using a time-resolved endocytosis assay, we show that WASp and Arp2/3 are required for the endocytosis of Dl only during cytokinesis. We propose that WASp-Arp2/3 provides an extra pushing force that is specifically required for the efficient endocytosis of Dl during cytokinesis.