The modulation of cell surface cAMP receptors from Dictyostelium discoideum by ammonium sulfate

Dictyostelium discoideum cells contain a heterogeneous population of cell surface cAMP receptors with components possessing different affinities (Kd between 15 and 450 nM) and different off-rates of the cAMP-receptor complex (t 1/2 between 0.7 and 150 s). The association of cAMP to the receptor and the dissociation of the cAMP-receptor complex still occur in the presence of 3.4 M ammonium sulfate. However, these processes are strongly altered. (1) Low concentrations of ammonium sulfate (approximately equal to 50 mM) induce an approx. 2-fold increase of the number of cAMP binding sites. The same effect is induced by millimolar concentrations of CaCl2. Ammonium sulfate and CaCl2 are not additive, which suggests that these salts may act via the same mechanism. (2) High concentrations of ammonium sulfate (3.4 M) induce an alteration in the proportioning of the various cAMP binding sites to the components with the highest affinity. (3) High concentrations of ammonium sulfate (3.4 M) retard the dissociation of all binding sites about 3-6-fold, thus giving rise to an increase in the affinity of all cAMP-binding components.