SELECTIVE BINDING OF LIGANDS TO BETA-1, BETA-2 OR CHIMERIC BETA-1-BETA-2-ADRENERGIC RECEPTORS INVOLVES MULTIPLE SUBSITES

The molecular basis of ligand binding selectivity to β-adrenergic receptor subtypes was investigated by designing chimeric β1/β2-adrenergic receptors. These molecules consisted of a set of reciprocal constructions, obtained by the exchange between the wild-type receptor genes of one to three unmodified transmembrane regions, together with their extracellular flanking regions. Eight different chimeras were expressed in Escherichia coli and studied with selective β-adrenergic ligands. The evaluation of the relative effect of each chimeric exchange on ligand binding affinity was based on the analysis of ΔG values, calculated from the equilibrium binding constants, as a function of the number of substituted β2-adrenergic receptor transmembrane domains. The data showed that the contribution of each exchanged region to subtype selectivity varies with each ligand; moreover, while several regions are critical for the pharmacological selectivity of all ligands, others are involved in the selectivity of only some compounds. The selectivity displayed by β-adrenergic compounds towards β1 or β2 receptor subtypes thus results from a particular combination of interactions between each ligand and each of the subsites, variably distributed over the seven transmembrane regions of the receptor; these subsites are presumably defined by the individual structural properties of the ligands.