Both the inner and outer positive feedback loops are necessary for noise-tolerant polarization. (A) Time-course simulations of wild-type, bem1-299Δ, and bni1Δ models. After approaching steady-state, the models were run for 10 minutes of simulated time (L
= 10 nM, L
= 0, σ = 3). For a sample simulation, active Cdc42 is plotted against the angle θ for t = 0 (solid black) and t = 10 min (dashed gray). (B) Time-lapse imaging of wild-type, bem1-299Δ, and bni1Δ cells containing Ste20-GFP, a marker of active Cdc42. Cells were treated with uniform 20 nM α-factor for 1 hour and imaged every 2 min for 10 minutes. 20 cells of each genotype were observed and a typical cell is shown. All wild-type cells showed a constant region of polarization, whereas all bem1-299Δ cells did not polarize with the Ste20-GFP mainly cytoplasmic. In the majority bni1Δ cells (14/20), there were multiple regions of polarization (indicated by white arrows) that shifted position over the 10 minute interval; some cells exhibited only cytoplasmic Ste20-GFP. Scale bar = 5 μm.