Figure 1.

Schematic diagrams of the allosteric model for the K_ACh channel and the model for receptor-G-protein interaction. Top: Schematic representation of the allosteric model. In this scheme, each K_ACh channel with either high or low affinities to G_βγ (K_ACh/high or K_ACh/low, respectively) is assumed to be an oligomer composed of four identical subunits. Each subunit is in either the available (R, relaxed) or unavailable (T, tense) state, which is represented by a circle or a square, respectively. Each subunit in the R or T state binds to one dissociated G-protein βγ subunit (solid circles) independently of the other subunits, with the microscopic dissociation constants K_R or K_T, respectively. In this model, all subunits in the same oligomer must change their conformations simultaneously. R₄ and T₄ are in equilibrium with the allosteric constant L. Bottom: Models of receptor-G-protein interaction. The low-affinity state of m₂Rs was incorporated into our previous model (21). A, ACh; R_high and R_low, high-affinity and low-affinity m₂Rs, respectively; G, G protein.