MKlp2 recruits and provides continuous Aurora B kinase activity to the growing furrow for furrow propagation and completion.

<p>(<b>A–D</b>) Time-lapse live-cell images. HeLa cells were transfected with expression vectors encoding mCherry-MKlp2 and GFP-UtrCH (cortical marker), and 24 h after transfection, the cells were subjected to monopolar cytokinesis. In panel <b>A</b>, arrows indicate where a typical polarized furrow was completed. Asterisks indicate the site where mCherry-MKlp2 accumulated at the cell cortex and the furrow. Arrow heads in panels <b>A</b>, <b>C</b> and <b>D</b> denote the sites of the growing furrow before adding ZM447439 or hesperadin. For panel <b>B</b>, Purv A and ZM447439 were added concurrently. For panels <b>C</b> and <b>D</b>, ZM447439 or hesperadin was added sequentially after purvalanol A treatment for 10 min. The fate of the same cell was continuously monitored. White bars represent 5 µm. (<b>E</b>) Monopolar cells (from <b>A–D</b>) were scored as cells that completed monopolar cytokinesis (polarized MPC), formed ectopic furrows without a polarized cortex (non-polarized with ectopic furrowing), no significant furrowing activity without a polarized cortex (non-polarized without furrowing), or formed polarized furrows but subsequently regressed (regressed after polarization). The average percentages based on three independent experiments (total <i>n</i>>100 per condition, +/− standard deviation) are shown. (<b>F</b>) Proposed model. (a) In monopolar cytokinesis, inhibition of Cdk1 triggers a symmetry-breaking reaction that initiates polarization of the monopolar spindles and the cell cortex. Although monopolar spindles are symmetrically positioned, they become asymmetric upon Cdk1 inhibition. Thus, the original event triggering the asymmetry of monopolar spindles remains unclear. (b) During this reaction, MKlp2 mediates Aurora B translocation from the monopolar spindles (or the spindle midzone) towards the cell cortex (or the equatorial cortex) in an interdependent manner. At the ends of monopolar spindles that contact the cell cortex, MKlp2 may act as the bridge between the asymmetrically polarizing monopolar spindles and the actomyosin filaments at the cell cortex. This event may stabilize the polarizing monopolar spindles towards the cell cortex. In turn, it facilitates a rapid accumulation of MKlp2-Aurora B to the cell cortex in a polarized manner. This cortical accumulation of MKlp2-Aurora B promotes furrow formation (or might constrict contractile ring in bipolar cytokinesis) via the kinase activity of Aurora B towards currently unidentified target(s). (c) Once the furrow is formed, MKlp2 targets Aurora B to actomyosin filaments in the gap region between the stably polarized monopolar spindles and the furrowing cortical cap. This event may continuously provide Aurora B kinase activity to the growing furrow for propagation and completion. Interestingly, the kinase activity of Aurora B at the growing furrow may also be essential for maintaining the monopolar spindles polarized towards the furrow (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064826#pone.0064826.s010" target="_blank">Movie S1</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0064826#pone.0064826.s011" target="_blank">S2</a>), indicating that positive feedback loops may exist that maintain polarization between the monopolar spindles and the growing furrow.