SCAR and the Arp2/3 complex polarise the actomyosin cortex and plasma membrane organization in asymmetrically dividing neuroblasts

While the Formin-nucleated actomyosin cortex has been shown to drive the changes in cell shape that accompany cell division in both symmetric and asymmetric cell divisions, it is not clear whether or not Arp2/3-nucleated branched actin filament networks also play a role. In order to look for mitotic roles of the Arp2/3 complex, here we use Drosophila neural stem cells as a model system. These cells are unusual in that they divide asymmetrically to produce a large and small daughter cell with different fates. Our analysis identifies a pool of Arp2/3-dependent actin-based membrane protrusions that form at the apical cortex of these cells as they enter mitosis. Strikingly, at metaphase, these protrusions co-localise with components of the SCAR complex. By perturbing Arp2/3 complex activity we show that this apical pool of actin likely functions to limit the accumulation of apical Myosin in metaphase. Following the onset of anaphase, the loss of these SCAR and Arp2/3 dependent structures then leads to a delay in the clearance of apical Myosin and to cortical instability at cytokinesis. These data point to a role for a polarised branched actin filament network in fine tuning the apical actomyosin cortex to enable the precise control of cell shape during asymmetric cell division.