Fig. 5: Relative protein numbers determine robustness of cell polarity.

Linear stability analysis for a range of density ratios [P]/[A₂] and [A₁]/[A₂]; [A₂] was kept constant. Each graph shows the range of phosphorylation ratios (k_Pa/k_Ap) and relative reactivation lengths (ℓ∕L) where the base state is linearly unstable, with δσ given by the same colour code as in Fig. 4a; fixed parameters are k_Ap = 0.4 μm s⁻¹ and D_cy = 30 μm² s⁻¹, and further parameters not varied can be found in Table 1. FEM parameter sweeps of k_Pa and λ, with fixed parameters k_Ap = 0.4 μm s⁻¹ and D_cyt = 30 μm² s⁻¹, for each density set show that the steady-state polarisation axis also depends strongly on the ratio [A₁]/[A₂]. The steady state switches from short- to long-axis polarisation at the black line in each graph, indicating ℓ^⋆.