PHOSPHORYLATION OF CHEMOATTRACTANT RECEPTORS IS NOT ESSENTIAL FOR CHEMOTAXIS OR TERMINATION OF G-PROTEIN-MEDIATED RESPONSES

In several G-protein-coupled signaling systems, ligand-induced recepto r phosphorylation by specific kinases is suggested to lead to desensit ization via mechanisms including receptor/G-protein uncoupling, recept or internalization, and receptor down-regulation. We report here that elimination of phosphorylation of a chemoattractant receptor of Dictyo stelium, either by site-directed substitution of the serines or by tru ncation of the C-terminal cytoplasmic domain, completely prevented ago nist-induced loss of ligand binding but did not impair the adaptation of several receptor-mediated responses including the activation of ade nylyl and guanylyl cyclases and actin polymerization, In addition, the phosphorylation deficient receptors were capable of mediating chemota xis, aggregation, and differentiation. We propose that for chemoattrac tant receptors agonist-induced phosphorylation regulates surface bindi ng activity but other phosphorylation-independent mechanisms mediate r esponse adaptation.