Two distinct types of cell surface folic acid-binding proteins in Dictyostelium discoideum

Abstract

Folic acid is degraded too fast by Dictyostelium discoideum to study binding of this ligand to cell surface binding proteins. Folate deaminase activity was inhibited in the presence of 3.3 × 10 −4 M 8-azaguanine. This inhibitor enabled us to detect two folate binding proteins. One type bound folic acid and deamino-folic acid with the same affinity ( K 0.5 = 3–6 × 10 −7 M) and apparently negative cooperativity. Binding to only this type was observed if 8-azaguanine was omitted. The second type bound folic acid noncooperatively with K d = 7 × 10 −7 M. Deamino-folic acid did not compete even at a 1000-fold excess. This type may correspond to the chemotactic receptor.