Schematics of a sequence of models explored in this paper.
a) A basic single GTPase (“wave pinning”) module with crosstalk to the phosphoinositides (PIs). The GTPase module can only polarize on its own , but not when connected to PIs in this way. b) As before but with an additional passive Rho module: still no polarization possible with PI crosstalk. c) Mutual inhibitory Rac-Rho module: Polarization observed both with and without the PI layer. d) A more complete Cdc42-Rac-Rho module that exhibits polarization both with and without PIs. Model equations are shown in (1), (8), (16) and represents the strength of PI feedback to Rac. Arrows represent upregulation and bars represent inhibition. In all cases, proposed interactions between GTPases and PIs are taken from the literature , –.