FIGURE 1.

Model for PDE activation. (a) Activated rhodopsin (R*) can be phosphorylated by rhodopsin kinase (RK) or deactivated by arrestin binding. The phosphorylation rates λ_n and arrestin binding rates μ_n depend on the state n of rhodopsin phosphorylation. Through phosphorylation, R* undergoes a transition from a state n to n − 1. (b) R* in state n activates transducin with a phosphorylation-dependent activation rate k_act(n). Activated transducin (T*) binds to PDE with a rate k₃ and forms a complex denoted by PDE*. PDE* is deactivated through RGS9 with a deactivation rate k₄.