Figure 4.

Ordered model of phosphatase regulatory adaptor subunits. (A) Diagram of the regulatory adaptor model. The previously described 2-Kinase/1-Phosphatase model (Fig. 1A) was expanded to include two regulatory adaptor subunits, A₁ and A₂, that target the phosphatase to substrates S₁ and S₂, respectively. The phosphatase must bind an adaptor domain to bind and dephosphorylate either substrate. (B) The fraction S₁^∗ as a function of the initial concentration of each adaptor subunit with low levels of K₁ activity (r₁ = 0.05) and high levels of K₂ activity (r₂ = 2). The concentrations of both adaptor subunits are equal and are varied simultaneously. Inset: Diagram of the ordered model. The phosphatase must first bind to an adaptor subunit. This complex can then bind to and dephosphorylate a phosphorylated substrate. (C) The fraction S₁^∗ as a function of r₁ at r₂ = 0 (black) and r₂ = 2 (red) with [A_x]₀ = 100 nM. In this model, the adaptor subunits do not eliminate the potential for phosphatase-mediated crosstalk even though they are perfectly specific for their respective substrates. To see this figure in color, go online.