The interaction between wheat germ agglutinin and membrane incorporated glycophorin A. An optical binding study

A novel integrated optical technique is used to monitor the kinetics of incorporation of glycophorin A (GPA) from solution into a planar dimyristoylphosphatidylcholine-cholesterol bilayer membrane, and the subsequent binding of wheat germ agglutinin (WGA) to the membrane-incorporated GPA. The technique significantly improves the attainable accuracy of kinetic measurements. The number of bound molecules can be determined to a precision of ca +/- 80 mol microns-2. Our results show that GPA incorporates spontaneously into the bilayer. Binding of WGA to GPA is optimal in the presence of human serum albumin, and can be reversed by N-acetyl-D-glucosamine. The kinetics of the binding are consistent with the presence of two classes of kinetically distinguishable binding sites with association rates of 2.0 x 10(4) and 9.6 x 10(2) M-1 s-1, and dissociation rates of 2.7 x 10(-3) s-1 and < 10(-5) s-1, respectively. A stoichiometry of 4 WGA monomers per GPA monomer was determined as characteristic of the overall binding interaction.