Role of the second extracellular loop of adenosine receptors in agonist and antagonist binding. Analysis of chimeric A1/A3 adenosine receptors.

Adenosine receptor (AR) agonists and antagonists are approximately 100-fold and 100,000-fold, respectively, more potent at the bovine A1AR as compared to the rat A3AR. To determine regions of ARs involved in ligand recognition, chimeric receptors composed of bovine A1AR and rat A3AR sequence were constructed and their ligand binding properties examined following expression in COS-7 cells. Substitutions of the second extracellular loop or a region encompassing transmembrane domains 6 and 7 of the A1AR into the A3AR resulted in enhanced affinities of both agonists and antagonists compared to wild-type A3AR. The region of the second extracellular loop of the A1AR responsible for this effect was identified as the distal eleven amino acids of the loop. Replacement of this segment of the A3AR with that of the A1AR in combination with the regions encompassing transmembrane domains 6 and 7 resulted in a 50,000-fold increase in the Kd for antagonist radioligand, [3H]1,3-dipropyl-8- cyclopentylxanthine. Agonist affinity at this chimeric was over 100-fold greater than that displayed by wild-type A3AR. Thus, multiple regions of ARs including a segment of the second extracellular loop are involved in ligand recognition, and considerable overlap exists in structural features required for agonist and antagonist binding.