p115 RhoGEF is required for apical extrusion. (a) p115 RhoGEF (red; separate middle panel) form filaments that point toward the basolateral surface of the extruding ring during apical extrusion, whereas RhoA (green; bottom panel) is present through the apical and basal regions of the ring. Corresponding colored arrows indicate sites of interaction with the actin/myosin ring. Bar, 10 µm. (b) siRNA-mediated knockdown of p115 RhoGEF (above, lanes from one immunoblot) disrupts apical extrusion, whereas scrambled siRNA does not. The graph shows the percentage of apical versus basal extrusion from >1,000 cells from four siRNAs experiments, where P < 0.0001. (c) p115 RhoGEF localizes to the base of the extruding ring during apical extrusion, whereas it does not when microtubules are disrupted with nocodazole or taxol, or in control live junctions, where P-values are <0.0001 for control versus each treatment; n = 10 extrusions per treatment. (d) Model for how microtubules might control extrusion direction. An early apoptotic cell sends signals to its live neighboring epithelial cells, which react by reorienting microtubules toward the basolateral surfaces of the cells that contact the dying cell. p115 RhoGEF, by association with microtubules, assembles and activates Rho-mediated actin and myosin II contraction along the basolateral surface, extruding the cell apically (top). Basal extrusion occurs if the microtubules do not reorient or if p115 RhoGEF is absent (bottom).