Aurora A Kinase Amplifies a Midzone Phosphorylation Gradient to Promote High-Fidelity Cytokinesis

During cytokinesis, aurora B kinase (ABK) relocalizes from centromeres to the spindle midzone, where it is thought to provide a spatial cue for cytokinesis. While global ABK inhibition in Drosophila S2 cells results in macro- and multi-nucleated large cells, mislocalization of midzone ABK (mABK) by depletion of Subito (Drosophila MKLP2) does not cause notable cytokinesis defects. Subito depletion was, therefore, used to investigate the contribution of other molecules and redundant pathways to cytokinesis in the absence of mABK. Inhibiting potential polar relaxation pathways via removal of centrosomes (CNN RNAi) or a kinetochore-based phosphatase gradient (Sds22 RNAi) did not result in cytokinesis defects on their own or in combination with loss of mABK. Disruption of aurora A kinase (AAK) activity resulted in midzone assembly defects, but did not significantly affect contractile ring positioning or cytokinesis. Live-cell imaging of a Förster resonance energy transfer (FRET)-based aurora kinase phosphorylation sensor revealed that midzone substrates were less phosphorylated in AAK-inhibited cells, despite the fact that midzone levels of active phosphorylated ABK (pABK) were normal. Interestingly, in the absence of mABK, an increased number of binucleated cells were observed following AAK inhibition. The data suggest that equatorial stimulation rather than polar relaxation mechanisms is the major determinant of contractile ring positioning and high-fidelity cytokinesis in Drosophila S2 cells. Furthermore, we propose that equatorial stimulation is mediated primarily by the delivery of factors to the cortex by noncentrosomal microtubules (MTs), as well as a midzone-derived phosphorylation gradient that is amplified by the concerted activities of mABK and a soluble pool of AAK.