Cell Polarity: Rac-Rho Excitable Waves

Antagonistic GTPase dynamics — traveling fronts and cell polarity

■ Rac (protrusion) ■ Rho (contraction) ■ Cell boundary

Rac activation a 1.00 Rho activation b 1.00

Rac diffusion D_R 0.30 Rho diffusion D_ρ 0.10

Mutual inhibit γ 2.00 Global signal S 0.50

Rac-Rho wave model: Mutual inhibition creates an excitable system on the cell membrane.
∂R/∂t = D_R ∇²R + a·S/(1+γρ²) − R (Rac)
∂ρ/∂t = D_ρ ∇²ρ + b·S/(1+γR²) − ρ (Rho)
Traveling waves: leading edge Rac-high (lamellipodia), trailing Rho-high (stress fibers).