Fig. 1.

Structure of the ROP1 oscillation models. The oscillation models consist of 2 major feedback loops: the F-actin–mediated positive feedback loop (indicated by “+”) and the calcium-dependent negative feedback loop (indicated by “−”). Compared with an increase in ROP1 activation rate, an increase in the rate of calcium accumulation in the tip is delayed, which is described as a time delay, τ, in the models. Calcium is assumed to participate in the negative feedback regulation of ROP1 activity by 1 of 2 mechanisms. In model I, calcium signaling inhibits ROP1 activation by promoting F-actin depolymerization through the activation of actin-binding proteins (ABPs). In model II, calcium signaling promotes ROP1 deactivation by activating ROP1-negative regulators, such as RhoGAP.