Cis-unsaturated fatty acids modulate the function of the cell-surface cAMP receptor of Dictyostelium discoideum

Abstract

The binding of cAMP to the chemotactic cAMP receptor in intact Dictyostelium discoideum cells and isolated membranes is strongly inhibited by unsaturated fatty acids. In isolated membranes, cis-unsaturated fatty acids decreased the number of accessible cAMP binding sites, without significantly altering their affinity. Most potent were C 18 and C 20 cis-poly unsaturated fatty acids, like arachidonic acid, linoleic acid and linolenic acid. Trans-unsaturated fatty acid was less potent than its cis isomer, while saturated fatty acids did not affect the binding of cAMP to receptors at all. Oxidation reactions were not important for the effect of unsaturated fatty acids. When membranes were preincubated with millimolar concentrations of Ca 2+, the effect of unsaturated fatty acids was strongly diminished. Mg 2+ was ineffective. Ca 2+, if presented after the incubation of membranes with unsaturated fatty acids, did not reverse the inhibitory effect. The specificity of the fatty acid effect, and the interference with Ca 2+, but not Mg 2+, suggest that the properties of the cAMP receptor are changed as a result of alterations in the lipid bilayer structure of the membrane.