Models of small-molecule modulators binding to Smo. (a) The two-state model shows direct competition for a single binding site between the agonist (Ag, green square) and the antagonist (Ant, pink circle). (b) The ternary complex model suggests that there are two independent sites, and that agonist and antagonists are in a dynamic equilibrium (denoted by arrows) between Smo conformers bound at one site, both sites or not bound by ligand. On the basis of experimental data, binding at either site would decrease the affinity of interaction at the other site (allosteric binding with high negative cooperativity). A hypothetical signal transduction coupler, or effector, (the blue structure labeled X) is introduced in the ternary complex model. A coupler/effector-bound form is considered to be the active signaling complex. According to the model, only single active agonist-bound species of Smowt is seen (bottom left). For Smoact (bottom right), the model predicts that the activating point mutation, W539L, results in a stable, distorted form of Smo that binds the antagonist poorly and has an increased affinity for the coupler/effector, even in the absence of agonist, thus leading to elevated basal signaling. This mutant form can nevertheless bind agonist and assume a conformation like that of the normal activated Smowt. Residue 539 is designated as either W for Smowt or as L in the Smoact mutant.