Table 3. Parameters used to perform simulations described in the main text.

Description	Parameter	For purely 2D simulations	For quasi-3D simulations	Ref.
BemGEFc → BemGEFm	k1a	10 s-1	10 s-1	[52]
BemGEFm → BemGEFc	k1b	40 s-1	10 s-1	[52]
Cdc42Dm + BemGEFm → Cdc42T	k2a	Target 0.032 μm2s-1 Fitted 0.040 μm2s-1	Target 0.16 μM-1s-1 Fitted 0.16 μM-1s-1	[52]
	λ2a	5.3 s-1	5.3 s-1	—
Cdc42T → Cdc42Dm	k2b	0.35 s-1	0.32 s-1	[35]
Cdc42Dm + BemGEF42 → Cdc42T	k3	Target 0.280 μm2s-1 Fitted 0.184 μm2s-1	Target 0.35 μM-1s-1 Fitted 0.16 μM-1s-1	[52]
	λ3	180 s-1	15.7 s-1	—
BemGEFm + Cdc42T → BemGEF42	k4a	Target 0.050 μm2s-1 Fitted 0.054 μm2s-1	Target 10 μM-1s-1 Fitted 0.79 μM-1s-1	[52]
	λ4a	9.6 s-1	8250 s-1	—
BemGEF42 → BemGEFm + Cdc42T	k4b	Target 40 s-1 Fitted 31.4 s-1	Target 10 s-1 Fitted 0.37 s-1	[52]
Cdc42Dc → Cdc42Dm	k5a	36 s-1	36 s-1	[52]
Cdc42Dm → Cdc42Dc	k5b	13 s-1	0.65 s-1	[52]
BemGEFc + Cdc42T → BemGEF42	k7	2.0014 μm2s-1	10 μM-1s-1	[52]
	λ7	256 s-1	256 s-1	—
Diffusion coefficient in cytoplasm	Dcyto	15 μm2s-1	15 μm2s-1	—
Diffusion coefficient on membrane	Dmemb	0.0025 μm2s-1	0.0025 μm2s-1	[52]
Membrane to cytoplasm volume ratio	η	1	0.01006	[52]
Membrane surface area	A	0.21–10.5π	25π	[52]
Molecular interaction radii	$\overline{\sigma },\overline{\varrho }$	0.050 μm	0.050 μm	—
Total Cdc42		14.5–145.5 particles/μm2	0.05–0.30 μM	—
Total BemGEF		0.87–8.8 particles/μm2	0.06 μM	[41]

References are for the quasi-3D parameters. “Target” and “fitted” values for k_2a, k₃, k_4a, and k_4b exist because of the empirical fitting described in the main text. The target was used as the input to 2D $\lambda -\overline{\varrho }$ theory as the starting point, producing the corresponding microscopic rates λ_2a, λ₃, λ_4a, and λ₇. Fitted macroscopic rates were obtained after fitting as described earlier (see Fig 3). For particle simulations, we used Δt = 0.1 ms, and for RDE simulations, we used Δt = 1 ms. For quasi-3D simulations, we assumed a cell volume corresponding to a 5 μm diameter sphere.