CONSTITUTIVE ACTIVATION OF A SINGLE EFFECTOR PATHWAY - EVIDENCE FOR MULTIPLE ACTIVATION STATES OF A G-PROTEIN-COUPLED RECEPTOR

A cysteine-to-phenylalanine mutation in the third transmembrane domain of the alpha(1B)-adrenergic receptor constitutively activates the rec eptor, resulting in G protein coupling in the absence of agonist and a ctivation of only a single effector pathway (phospholipase C but not p hospholipase A(2)). This mutant receptor displays a higher affinity fo r the catecholamines, norepinephrine, and epinephrine, as well as for other phenethylamines, but not for imidazolines, a class of structural ly distinct alpha agonists. Dose-response studies demonstrate a higher potency and intrinsic activity of phenethylamines for polyphosphoinos itide turnover but not for arachidonic acid release. Imidazolines have wild-type potencies and intrinsic activities for both pathways. These data indicate that a single receptor subtype forms multiple conformat ions (i.e., exhibits induced conformational pleiotropy) for G protein interactions (high affinity states) that are specific for a particular G protein/ effector pathway and that multiple binding sites exist for agonists, which promote or induce these specific interactions. Pharma cological diversity may, thus, be achieved through a single receptor b y the development of compounds that induce a single activated conforme r. This has major ramifications for the eventual development of signal ing-specific therapeutics.