Activation of adenylate cyclase by divalent cations and polyamines in saponin-treated Dictyostelium discoideum cells.

Binding of an intrinsic agonist (cyclic AMP) to specific receptors on the cell surface induces transmembrane signals for the activation of adenylate cyclase in the cellular slime mold Dictyostelium discoideum. We found that stimulation by CaCl2, MgSO4 or polyamines having two to four positive charges induced the activation of adenylate cyclase in cells treated with saponin. The activation was roughly identical to the stimulation of the intrinsic agonist both in amount and in time course. The intact cells (saponin-untreated cells) responded to neither divalent cations nor polyamines. While saponin is known to have a detergent-like effect and to make the plasma membrane permeable, low molecular weight dyes did not penetrate the plasma membrane under our conditions for the saponin-treatment. Caffeine is known to inhibit the cAMP-induced activation of adenylate cyclase by blocking signal transduction, but not by acting directly on the enzyme [Brenner, M. and Thoms, S.D. (1984) Dev. Biol. 101, 136-146]. We found that caffeine inhibited the cation-induced activation. These results suggest that these divalent and polyvalent cations do not act directly on adenylate cyclase but that they mimic or induce the transmembrane activation signal for adenylate cyclase in the saponin-treated cells.