**a.** Scheme of the mathematical model. (Inset) The F-actin and Ca2+ dependencies of the three vesicle state transitions, Capture, Passage and Fusion. Vcortex and V_{pm} are the respective concentrations of vesicles bound to actin cortex and plasma membrane. **b.** Simulation output of the model. The upper panel shows the model input: Ca2+ and F-actin oscillations. The middle panel shows the changes in the concentration of actin bound (blue) and plasma membrane bound (green) vesicles. The lower panel shows the output, the secretion rate. **c.** Comparison of model predictions of secretion rates for oscillatory and constant of F-actin and Ca2+ input levels. Two levels of constant Ca2+ and F-actin are shown: average, where the F-actin and Ca2+ levels are the mean of the oscillatory inputs and optimal where the levels where chose to maximize total secretion. **d**. Monitoring antigen-triggered recruitment of myosin Va, a motor protein that binds vesicles to F-actin and transports them. Left image panels show that recruitment of myosin Va occurs in the center where fusion occurs (maximum projection before and 2 minutes after antigen addition; scale-bar 5 μm). Right bar graph shows that, during F-actin oscillation cycles, the recruitment preferentially occurs when F-actin levels are high. As a control average intensity of a GFP membrane marker (Lyn) is compared to GFP-Myosin Va (MyoVa) during low actin (light green) and high actin (dark green) phases during oscillations. Errorbars are s.e.m, n = 83, p-value < 0.001 student t-test.

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