Signal transduction in Dictyostelium fgd A mutants with a defective interaction between surface cAMP receptors and a GTP-binding regulatory protein

Abstract. Transmembrane signal transduction was in-vestigated in four Dictyostelium discoideum utants that belong to the fgd A complementation group. The results show the following. (a) Cell surface cAMP receptors are present in fgd A mutants, but cAMP does not induce any of the intracellular responses, including the activation of adenylate or guanylate cyclase and chemotaxis. (b) cAMP induces down-regulation a d the covalent modification (presumably phosphoryla-tion) of the cAMP receptor. (c) The inhibitory effects of GTP~S and GDPI3S on cAMP binding are reduced; the stimulatory effect of cAMP on GTPTS binding is lost in fgd A mutants. (d) Basal high-affinity GTPase activity is reduced 40 % and the stimulatory effect of cAMP is decreased from 40 % in wild type to 30 % i