Fig. 1.

Total protein gradient. (a) When the diffusion coefficients of the phosphorylated form and the dephosphorylated form are equal (D_A=D_B), then a kinase at the left boundary and a phosphatase uniformly distributed in the cytoplasm will establish a phosphostate gradient (dashed light gray = phosphorylated, dotted dark gray = dephosphorylated), as previously noted, but no gradient in the total protein concentration (black). (b) When the diffusion coefficients of the two forms differ (D_A≠D_B), then there will be a gradient in not only the phosphostate, but also the total protein concentration. (c) and (d) Further disparity in the diffusion coefficients further increases the total protein concentration gradient. Kinase rate constant k_k=10 μm/s, phosphatase rate constant k_p=1 s⁻¹, overall protein concentration 1 μM in all panels.