Forms of the Chemotactic Adenosine 3’,5’-Cyclic Phosphate Receptor in Isolated Dictyostelium discoideum Membranes and Interconversions Induced by Guanine Nucleotides

Aggregating Dictyostelium discoideum cells possess receptors for the chemoattractant cAMP on their cell surface. Membranes enriched in these receptors were isolated. Kinetic studies indicated the same receptor heterogeneity in membranes as found for intact cells. Dissociation kinetics revealed at least three receptor forms: one form, called SS, with k-1 = 0.9 × 10-3 s-1 and KD = 6.5 nM; one form, called S, with k-1 = 1.3 × 10-2 s-1 and KD = ~6 nM; and one or more forms, called F, with k-1 > 0.1 s-1. The contribution of the SS form to the dissociation process was lower in the presence of millimolar concentrations of cAMP compared to dissociation induced by dilution only. Guanosine di- and triphosphates decreased the affinity of membranes for cAMP by increasing the dissociation rate of the cAMP-receptor complex. This was shown to result from a reduction in the number of sites of the slowly dissociating, high-affinity receptor form SS and probably also the high-affinity form S. Because the total number of cAMP binding sites was not changed by guanine nucleotides, it is inferred that the SS and S receptor forms are converted to other more rapidly dissociating receptor forms with lower affinities than SS and S. We propose that cAMP receptors in Dictyostelium membranes interact with G protein which binds guanosine di- and triphosphates. The different complexes between receptor and occupied or unoccupied G protein explain the different receptor forms and their interconversions.