Figure 2

Model framework and model simulations. (a) Schematic of the semi-phenomenological four-variable (4V) model of Eqs. (3) in the absence of diffusion. Rac cycles between inactive (GDP-bound) and active (GTP-bound) forms. Rac-GTP (denoted by R) exerts an indirect positive auto-feedback (via Rho) on itself through RacGEF. It also simultaneously exerts positive and negative (blue pathway) feedback on paxillin phosphorylation at a fast and a slow timescale, respectively. Phosphorylated paxillin, in turn, indirectly upregulates Rac activation through RacGEF. Blue pathway: refinement of the original model²¹; red pathway: the reaction directly influenced by the half-maximum phosphorylation of paxillin (\(L_K\)). (b) Time courses of active Rac (R), phosphorylation rate (B) and recovery rate (\(k_B\)) in the absence of diffusion, showing the presence of mixed-model oscillations (MMOs) in R.