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 (Ca2+)-C-45 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 alpha subunits G(alpha)1, G(alpha)2, G(alpha)3,G(alpha)4,G(alpha)7, or G(alpha)8. Overexpression of cAR1 or cAR3 in g(alpha)2- cells did not restore certain G(alpha)2-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.
