The surface cyclic AMP receptors, cAR1, cAR2, and cAR3, promote Ca²⁺ influx in dictyostelium discoideum by a G_α2-independent mechanism

Activation of surface folate receptors or cyclic AMP (cAMP) receptor (cAR) 1 in Dictyostelium triggers within 5-10 s an influx of extracellular Ca²⁺ that continues for 20 s. To further characterize the receptor-mediated Ca²⁺ entry, we analyzed ⁴⁵Ca²⁺ uptake in amoebas overexpressing cAR2 or cAR3, cARs present during multicellular development. Both receptors induced a cAMP-dependent Ca²⁺ uptake that had comparable kinetics, ion selectivity, and inhibitor profiles as folate- and cAR1-mediated Ca²⁺ uptake. Analysis of mutants indicated that receptor-induced Ca²⁺ entry does not require G protein α subunits G_α1, G_α2, G_α3, G_α4, G_α7, or G_α8. Overexpression of cAR1 or cAR3 in g_α2 cells did not restore certain G_α2-dependent events, such as aggregation, or cAMP-mediated activation of adenylate and guanylate cyclases, but these strains displayed a cAMP-mediated Ca²⁺ influx with kinetics comparable to wild-type aggregation-competent cells. These results suggest that a plasma membrane-associated Ca²⁺ -influx system may be activated by at least four distinct chemoreceptors during Dictyostelium development and that the response may be independent of G proteins.