Binding of the chemoattractant folic acid by Dictyostelium discoideum cells

The binding of the chemoattractant folic acid to intact Dictyostelium discoideum cells has been studied. A few hundred thousand ligand molecules can be bound per cell with a dissociation constant in the order of 3 X 10(-7) M. Intact cells equilibrate rapidly with extracellular folic acid, suggesting that the binding sites are located on the outer cell surface. The folic-acid-binding properties do not change during early cell development induced by starvation for nutrients. A number of putative folate-binding proteins have been identified in a detergent-solubilized plasma membrane fraction by affinity chromatography on folate-Sepharose and specific elution with free folate. One of these proteins appeared to be an integral membrane protein as was deduced from its amphiphilic behaviour. It may be that among these proteins is the chemotactic receptor for folate.