Random chimeragenesis of G-protein-coupled receptors: Mapping the affinity of the cAMP chemoattractant receptors in Dictyostelium

G-protein-coupled receptors mediate a wide variety of responses to extracellular stimuli in eucaryotic cells. Binding of the ligand to these receptors is thought to involve contacts within a pocket formed by the seven transmembrane domains inferred from the sequences of these genes. A family of four surface cAMP receptors that mediate responses to secreted cAMP coordinates the developmental program of Dictyostelium. A large difference in affinity for cAMP exists between cAR1 (25 and 230 nM) and cAR2 (>5 μM). To understand the basis for this affinity difference, we generated an extensive series of cAR1/cAR2 and cAR2/cAR1 chimeras using a technique designated "random chimeragenesis." When a linearized plasmid was transformed into Escherichia coli, tandemly positioned cAK1 and cAR2 genes crossed over at homologous regions. The cAMP binding properties and EC₅₀ values for agonist-induced phosphorylation of each of the chimeras were characterized in order to map the domains that determine the affinity. These studies implicated a domain in the second extracellular loop in which only 5 residues differ between the two receptors as the major determinant of affinity. A secondary domain including residues 110-147 (11 residue differences) was identified as a minor determinant of affinity.