Ewa Paluch
 

Max Planck Institute of Molecular Cell Biology and Genetics, Pfotenhauerstraße 108, 01307 Dresden, Germany and International Institute of Molecular and Cell Biology, Warsaw, Poland

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Cytokinesis relies on tight regulation of the mechanical properties of the cell cortex, a thin acto-myosin network lying under the plasma membrane. At anaphase onset, the cortex accumulates into an equatorial ring that drives furrow ingression. Although most studies of cytokinetic mechanics focus on force generation at the constriction ring, a contractile acto-myosin cortex remains at the poles of dividing cells throughout cytokinesis. Whether polar forces influence cytokinetic cell shape and furrow positioning is poorly understood. Using a combination of cell biology and biophysics, we demonstrate that the polar cortex makes cytokinesis an inherently unstable process, where any imbalance in contractile forces between the two poles compromises the accurate positioning of the constriction ring. We show that limited asymmetric polar contractions occur during normal cytokinesis, and that perturbing the polar cortex leads to cell shape oscillations, resulting in furrow displacement and division failure. A theoretical model based on a competition between cortex turnover and contraction dynamics accurately accounts for the oscillations. We further propose that blebs, membrane protrusions that commonly form at the poles of dividing cells and whose role in cytokinesis has long been enigmatic, stabilise the position of the cleavage furrow by acting as valves releasing cortical contractility. Taken together, our findings reveal an inherent instability in the shape of the dividing cell, indicating that polar cortex contractility must be tightly controlled to ensure successful cytokinesis.