Ligand-induced phosphorylation of the cAMP receptor from Dictyostelium discoideum

The cell surface cAMP receptor of Dictyostelium discoideum exists as a doublet of low (D) and high (R) electrophoretic mobility forms, both of which are phosphorylated in vivo. The R form is phosphorylated in a ligand-independent manner, while conversion of the R to D forms, induced by the chemoattractant, is accompanied by at least a 4-fold increase in the level of phosphorylation. When cells are stimulated with saturating levels of cAMP, increased phosphorylation is detectable within 5 s and reaches maximum levels by 5 min with a t1/2 of 45 s. Dephosphorylation of receptor, initiated by removal of the stimulus, is detectable within 30 s, has a half-time of 2 min, and reaches a plateau by 20 min. At half-maximal occupancy, phosphorylation occurred more slowly than at saturation, t1/2 = 1.5 min, and remained at intermediate levels until the cAMP concentration was increased. Accompanying electrophoretic mobility shifts occurred in all cases with similar, though not identical, kinetics. Both phosphorylation and mobility shift were half-maximal at 5 nM cAMP and saturated at 100 nM. Estimation of the specific activity of each receptor form indicates that not all sites are phosphorylated during the R to D transition; at least half of the sites are phosphorylated after the transition is completed. The rate of incorporation of phosphates into the receptor, held in the D form by cAMP, was less than one-third the rate of ligand-induced incorporation starting with the R form and was approximately twice the basal rate of incorporation. These results are compatible with ligand-induced receptor phosphorylation being an early event in the adaptation of other cAMP-induced responses.