Surface cAMP receptors (cARs) in Dictyostelium transmit a variety of signals across the plasma membrane. The best characterized cAR, cAR1, couples to the heterotrimeric guanine nucleotide-binding protein (G protein) -subunit G2 to mediate activation of adenylyl and guanylyl cyclases and cell aggregation. cAR1 also elicits other cAMP-dependent responses including receptor phosphorylation, loss of ligand binding (LLB), and Ca influx through a G2-independent pathway that may not involve G proteins. Here, we have expressed cAR1 and a related receptor, cAR3, in a g strain (Lilly, P., Wu. L., Welker, D. L., and Devreotes, P. N.(1993) Genes & Dev. 7, 986-995), which lacks G protein activity. Both cell lines failed to aggregate, a process requiring the G2 and G-subunits. In contrast, cAR1 phosphorylation in cAR1/g cells showed a time course and cAMP dose dependence indistinguishable from those of cAR1/G controls. cAMP-induced LLB was also normal in the cAR1/g cells. Finally, cAR1/g cells and cAR3/g cells showed a Ca response with kinetics, agonist dependence, ion specificity, and sensitivity to depolarization agents that were like those of G controls, although they accumulated fewer Ca ions per cAMP receptor than the control strains. Together, these results suggest that the G-subunit is not required for the activation or attenuation of cAR1 phosphorylation, LLB, or Ca influx. It may, however, serve to amplify the Ca response, possibly by modulating other intracellular Ca signal transduction pathways.

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