MUTATION OF THE 3RD INTRACELLULAR LOOP OF THE CAMP RECEPTOR, CAR1, OFDICTYOSTELIUM YIELDS MUTANTS IMPAIRED IN MULTIPLE SIGNALING PATHWAYS

Seven-membrane span receptors transduce a wide range of signals across the plasma membrane. One member of this family, the cAMP receptor, cA R1, of Dictyostelium, mediates some responses (eg. adenylyl cyclase ac tivation, multicellular aggregation) which require G-proteins and othe rs (e.g. Ca2+ influx, loss of ligand binding, cAR1 phosphorylation) wh ich appear to be G-protein-independent. In this study, we randomly mut agenized the NH2-terminal eight amino acids of the third intracellular loop of cAR1 and examined the ability of these mutants to exhibit the three G-protein-independent responses listed above. Most mutants (cla sses I, II) exhibited wild-type or mildly defective responses. Several mutants (class III), however, were severely impaired in all three pro cesses but not in cAMP binding. Furthermore, these mutants failed to c ouple productively with G-proteins and could not replace cAR1 in a car 1- cell. For these reasons, we propose that class III mutations interf ere with the formation of an ''active'' conformation of the receptor.