OCCUPANCY OF THE DICTYOSTELIUM CAMP RECEPTOR, CAR1, INDUCES A REDUCTION IN AFFINITY WHICH DEPENDS UPON COOH-TERMINAL SERINE RESIDUES

Many G-protein coupled receptors display a rapid decrease in ligand bi nding following pretreatment with agonist, cAR1, a cAMP receptor expre ssed early in the developmental program of Dictyostelium, mediates che motaxis, activation of adenylyl cyclase, and gene expression changes t hat bring about the aggregation of 10(5) amoebae to form a multicellul ar structure. Occupancy of cAR1 by cAMP initiates multiple desensitiza tion processes, one of which is an apparent reduction in binding sites . In transformed cells expressing cAR1 constitutively, Scatchard analy ses revealed that this apparent loss of ligand binding is largely due to a significant reduction in the affinity of cAR1 for cAMP, A paralle l increase in the dose dependence of cAR1-mediated cAMP uptake was obs erved, Consistent with these findings, proteolysis of intact cells and immunofluorescence suggested that cAR1 remains on the cell-surface fo llowing cAMP treatment, Finally, agonist-induced loss of ligand bindin g is impaired in cAR1 mutants lacking a cluster of cytoplasmic serine residues, which are targets of cAMP-induced phosphorylation.