Reversible Binding of the Chemoattractant Folic Acid to Cells of Dictyostelium discoideum

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Abstract
The chemoattractant folic acid binds reversibly to receptors at the surface of Dictyostelium cells. In undifferentiated cells (t0.5) 6 .times. 104 binding sites per cell with a K0.5 value of 1.5 .times. 10-7 M were determined. The number of folic acid receptors per cell decreases slightly during cell development. In differentiated cells (t10) 3.3 .times. 104 binding sites per cell were estimated. The folic acid binding sites appear to be specific for folic acid and its derivatives. 2-Deamino-2-hydroxyfolic acid, 4-aminofolic acid (aminopterin), and 4-amino-10-methylfolic acid (amethopterin) compete for the folic-acid-binding sites. Nor do adenosine 3'',5''-phosphate and guanosine 3'',5''-phosphate affect binding of folic acid. The folic acid receptors appear to be distinct from the catalytic sites of the membrane-bound folic acid deaminase.