Fig. 4
From: Optogenetic dissection of Rac1 and Cdc42 gradient shaping
A minimal model for Cdc42 and Rac1 gradient formation. a Full model based on our experimental findings. Activation rates are denoted by α and deactivation rates by β. b A minimal model that recapitulates the formation of Cdc42 and Rac1 gradients. c Gradient shaping of Cdc42 and Rac1. Since the absolute amplitude of Rho GTPases are unknown, we assigned the following arbitrary values to the rates: αC=αR=1; βC=βR=0.5; and βb=1. Left: Cdc42 (blue) is set by an exponentially distributed GEF (green) with a characteristic length λ=10 µm and uniform GAP (black). Middle: Rac1 (red) requires an additional GAP, β2-chimaerin (dashed black, characteristic length γ=5 µm), that is localized more sharply at the cell edge than Rac1 GEF (green). The overall GAP activity (plain black) is the sum of β2-chimaerin and a uniform GAP (dashed black). As a result, the putative Rac1 gradient without β2-chimaerin (dashed red) is chopped off at the cell edge resulting in a bell-shaped gradient (plain red). Right: once normalized to 1, Cdc42 and Rac1 gradients present distributions that are similar to the ones measured in cells. d Effect of the relative ratio r=βR/βb between uniformly distributed GAPs and the localized gradient of β2-chimaerin on the position of the Rac1 bump. Left: Rac1 gradients obtained with decreasing values of r (r = 2, 1, 0.6, 0.3, 0.2, 0.1 respectively from dark to light red). Right: exponentially distributed β2-chimaerin (dashed line) and uniformly distributed GAPs (βR=2, 1, 0.6, 0.3, 0.2, 0.1 from dark to light gray, solid lines) corresponding to the values used for the left plot. e Effects of Cdc42 or β2-chimaerin inhibition in silico on the Rac1 gradient (\(\alpha _{{\mathrm C}_{\mathrm b}} = 0.4\), and \(\alpha _{{\mathrm R}_{\mathrm b}} = 0.3\)). The profiles are normalized (by the same factor) to match the FRET signal values measured experimentally (Fig. 2c, d)
