Figure 6. Simulation results for G-protein activation as a function of time at (A) 2% and (B) 30% raft coverage for changing values of r, the relative G-protein density in lipid rafts, (C) Simulation results for maximal G-protein activity as a function of scaled ligand concentration for different values of raft coverage.

Lines marked “no lipid raft” show the predicted level of G-protein activation in the absence of lipid rafts assuming random distribution of G-proteins on the cell membrane. Although G-protein signaling is attenuated at 30% raft coverage, the rate of termination of the response is smaller compared with no lipid raft condition. This occurs due to reduced rate of (G_βγ-dependent) diffusion-limited phosphorylation (and thus desensitization) of receptors. Magnitudes of the commonly measured pharmacological parameters maximal effect, E_max, and half maximal effective concentration, EC₅₀, are marked in (C). Parameter values are as listed in Table 1 with k_hyd = 10 s⁻¹ and [L] = K_d in (A) and (B).