MUTAGENESIS AND LIGAND MODIFICATION STUDIES ON GALANIN BINDING TO ITSGTP-BINDING-PROTEIN-COUPLED RECEPTOR GALR1

In this study, a large number of receptor mutants were generated and s everal N-terminally modified galanin analogues synthesized to refine t he previously proposed binding site model for galanin to its GTP-bindi ng-protein-coupled receptor GalR1. In addition to ligand-binding studi es, the functionality of mutant receptors was evaluated by assessing t heir ability to mediate galaninergic inhibition of isoproterenol-stimu lated adenylyl cyclase activity. The His264Ala and Phe282Ala receptor mutants, although deficient in binding in the concentration range of g alanin used, remain functional albeit 20-fold less efficient than the wild-type receptor in mediating inhibition of stimulated cAMP producti on by galanin. The His267Ala mutant is, apart from being deficient in galanin binding, also severely impaired in functional coupling. While His264 and Phe282 seem to be important in forming the binding pocket f or galanin, His267 might play a role in forming or stabilizing the act ive conformation of the GalR1 receptor rather than directly participat ing in the formation of the binding pocket for galanin. N-terminal car boxylic acid analogues of galanin have low affinity to wild-type GalR1 , but substantially increased affinity to the Glu271Lys receptor mutan t. This, together with the binding that an alanine substitution of Phe 115 in TM III results in a tenfold decrease in affinity for galanin, s uggests that the N-terminus of galanin interacts with Phe115, In contr ast to the Phe282Ala mutation in TM VII, a conservative mutation of Ph e282 to tyrosine did not alter the affinity for galanin. Thus, the int eraction between Tyr9 of galanin and Phe282 is likely to be of an arom atic-aromatic nature.