7 HELIX CAMP RECEPTORS STIMULATE CA2+ ENTRY IN THE ABSENCE OF FUNCTIONAL G-PROTEINS IN DICTYOSTELIUM

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) alpha-subunit G alpha 2 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 Ca2+ influx through a G alpha 2-independent pathway that may not involve G proteins. Here, we have expressed cAR1 and a related receptor, cAR3, in a g beta(-) 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 G alpha 2 and GP-subunits. In contrast, cAR1 phosphorylation in cAR1/g beta(-) cells showed a time course and cAMP dose dependence indistinguishable from those of cAR1/G beta(+) controls. cAMP-induced LLB was also normal in the cAR1/g beta(-) cells. Finally, cAR1/g beta(-) cells and eAR3/g beta(-) cells showed a Ca2+ response with kinetics, agonist dependence, ion specificity, and sensitivity to depolarization agents that were like those of G beta(+) controls, although they accumulated fewer Ca2+ ions per cAMP receptor than the control strains, Together, these results suggest that the G beta-subunit is not required for the activation or attenuation of cAR1 phosphorylation, LLB, or Ca2+ influx. It may, however, serve to amplify the Ca2+ response, possibly by modulating other intracellular Ca2+ signal transduction pathways.