RhoGAP54D promotes cell size asymmetry and inhibits pulsatile myosin activity in Drosophila neural stem cells

The actomyosin cortex displays diverse dynamic behaviors that drive essential cellular processes. Myosin dynamics are often modulated by small GTPases and their regulators, including GAPs and GEFs, which provide spatio-temporal control for myosin activity. Drosophila neural stem cells, known as neuroblasts, polarize and divide asymmetrically to generate a large self-renewing stem cell and a smaller differentiating progenitor. During the neuroblast cell cycle, pulsatile contractions followed by a series of actomyosin flows occur at highly specific times and subcellular locations. The regulation and physiological relevance of the contractions and many of these flows remain incompletely understood. Here, we systematically analyze the localization of RhoGAPs and RhoGEFs throughout the neuroblast cell cycle and identify a dual role for the ARHGAP19 homolog RhoGAP54D in regulating myosin dynamics. First, during interphase and metaphase, cytoplasmic RhoGAP54D suppresses pulsatile cortical contractions. These contractions are transiently permitted during prophase, when RhoGAP54D is sequestered through nucleoporin-mediated nuclear import. RhoGAP54D also promotes daughter cell size asymmetry, likely through its anaphase-specific cortical recruitment at the apical pole (unique among thus far known polarized proteins in neuroblasts), where it locally depletes myosin and promotes membrane expansion. We further show that PsGEF functions in neuroblasts, likely as a scaffold rather than as a canonical GEF, to mediate anaphase-specific recruitment of RhoGAP54D. PsGEF itself is recruited downstream of the polarity proteins Par3 and Pins, linking cortical polarity to local RhoGAP54D activity and thereby promoting daughter cell size asymmetry during asymmetric neuroblast division.