Guanine Nucleotides Modulate Cell Surface cAMP-Binding Sites in Membranes from Dictyostelium discoideum

D. discoideum contains kinetically distinguishable cell surface cAMP binding sites. One class, S, is slowly dissociating and has high affinity for cAMP (Kd = 15 nM, t½ = 15 s). A second class is fast dissociating (t½ about 1 s) and is composed of high affinity binding sites H (Kd ≈ 60 nM), and low affinity binding sites L (Kd ≈ 450 nM) which interconvert during the binding reaction. Guanine nucleotides affect these three binding types in membranes prepared by shearing D. discoideum cells through Nucleopore filters. The affinity of S for cAMP is reduced by guanine nucleotides from 13 nM to 25 nM, and the number of S-sites is reduced about 50%. The number of fast dissociating sites is not altered by guanine nucleotides, but these sites are mainly in the low affinity state. Half-maximal effects are obtained at about 1 µM GTP, 2 µM GDP and 10 µM Gpp(NH)p(guanyl-5'-yl-imidodiphosphate); ATP and ADP are without effect up to 1 mM. These results indicate that D. discoideum cells have a functionally active guanine nucleotide binding protein involved in the transduction of extracellular cAMP signals via cell surface cAMP receptors