Guanine nucleotides modulate cell surface cAMP-binding sites in membranes from Dictyostelium discoideum

Abstract

D. discoideum contains kinetically distinguishable cell surface cAMP binding sites. One class, S, is slowly dissociating and has high affinity for cAMP (K d = 15 nM, t 1 2 = 15 s ). A second class is fast dissociating ( t 1 2 about 1 s) and is composed of high affinity binding sites H (K d ≈ 60 nM), and low affinity binding sites L (K d = ≈ 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.