7 HELIX CAMP RECEPTORS STIMULATE CA2-PROTEINS IN DICTYOSTELIUM( ENTRYIN THE ABSENCE OF FUNCTIONAL G)

Surface cAMP receptors (cARs) in Dictyostelium transmit a variety of s ignals across the plasma membrane. The best characterized cAR, cAR1, c ouples to the heterotrimeric guanine nucleotide-binding protein (G pro tein) alpha-subunit G alpha 2 to mediate activation of adenylyl and gu anylyl cyclases and cell aggregation. cAR1 also elicits other cAMP-dep endent responses including receptor phosphorylation, loss of ligand bi nding (LLB), and Ca2+ influx through a G alpha 2-independent pathway t hat may not involve G proteins. Here, we have expressed cAR1 and a rel ated 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 l acks G protein activity. Both cell lines failed to aggregate, a proces s requiring the G alpha 2 and GP-subunits. In contrast, cAR1 phosphory lation in cAR1/g beta(-) cells showed a time course and cAMP dose depe ndence indistinguishable from those of cAR1/G beta(+) controls. cAMP-i nduced 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 k inetics, agonist dependence, ion specificity, and sensitivity to depol arization agents that were like those of G beta(+) controls, although they accumulated fewer Ca2+ ions per cAMP receptor than the control st rains, Together, these results suggest that the G beta-subunit is not required for the activation or attenuation of cAR1 phosphorylation, LL B, or Ca2+ influx. It may, however, serve to amplify the Ca2+ response , possibly by modulating other intracellular Ca2+ signal transduction pathways.