Positive Feedback Between Contractile Ring Myosin and Ring-Directed Cortical Flow Drives Cytokinesis

During cytokinesis, an equatorial actomyosin contractile ring rapidly transforms cell shape by constricting at a relatively constant rate despite its progressively decreasing size. The closure rate per unit length of the ring must accelerate as the ring gets smaller to maintain the overall constant rate of closure. Here, by generating a 4D map of cortical flow in conjunction with monitoring of ring component dynamics, we show that the acceleration in ring closure rate arises due to positive feedback between the ring and the surrounding cortex. Ring myosin pulls on the adjacent cortex generating ring-directed cortical flow that, in turn, accelerates constriction by delivering cortical myosin into the ring. We derive an analytical mathematical formulation that captures the positive feedback-dependent evolution of the contractile ring and use this formulation to provide a non-intuitive explanation for why reducing myosin activation by rho kinase inhibition slows contractile ring closure.