Ammonium sulfate modifies adenylate cyclase and the chemotactic receptor of Dictyostelium discoideum. Evidence for a G protein effect.

Abstract

(NH4)2SO4 was found to activate adenylate cyclase in Dictyostelium discoideum membranes. The effect of (NH4)2SO4 on the enzyme was observed after pretreatment of membranes but could not be observed if the salt was added to the assay mixture. Activation was seen when membranes were pretreated with 0.16 M (NH4)2SO4 and was maximal at 0.6-1.0 M. The maximal activation of the enzyme was observed within 3 min of pretreatment and was not readily reversible. The effect was specific for the NH+4 ion since pretreatment of membranes with other NH+4 salts could activate the enzyme, whereas pretreatment with NaCl or KCl could not. Pretreatment of plasma membranes with (NH4)2SO4 eliminated the sensitivity of the enzyme to the inhibitory effect of guanine nucleotides. (NH4)2SO4 pretreatment also significantly attenuated the inhibition by guanine nucleotides of cAMP binding to its plasma membrane receptor. The effect of (NH4)2SO4 on GTP inhibition of cAMP binding to its receptor was even more dramatic when the salt was present in the binding assay. (NH4)2SO4 also increased the ADP-ribosylation by cholera toxin of a 39,000-Da membrane protein. The data support the hypothesis that (NH4)2SO4-induced changes in adenylate cyclase and the cAMP receptor are due to an alteration of a putative G protein.