Computer simulations of cyk-1(RNAi) rings suggest that altered kinetics are caused by F-actin binding site competition. (A) Number of broken networks as function of time since ring formation of control and fragilized networks in simulations of ring formation (50 simulations were analyzed per condition). (B and C) Parameter maps of myosin vs. crosslinker concentrations for control (left) and fragilized rings (middle) showing the measured constriction rates for simulations of ring constriction with binding site competition (B) or without binding site competition (C). The difference in constriction rates (in μm/s) between the control and fragilized situations is shown on the right (red means faster constriction in controls; blue means slower constriction in controls). Plastin concentrations are 5× or 1.25× higher than spectrin for control and fragilized ring simulations, respectively. Nine simulations were analyzed per condition. (D) Distribution of cytoskeletal components along the perimeter of a fragilized ring before (left) and after (right) ring rupture. Location of rupture is indicated by an arrow.