An engineered interleukin-4-binding protein (IL4-BP) representing the extracellular domain of the human interleukin-4 (IL-4) receptor alpha chain was expressed in Sf9 cells. The purified IL4-BP was immobilized via a single biotinylated SH group near the carboxyl end to a biosensor matrix and analysed in real time for interaction with IL-4 and IL-4 variants. IL-4 was bound to IL4-BP at a molar ratio of approximately 1:1. The association and dissociation at pH 7.4 and 150 mM NaCl had rate constants of 1.9 +/- 0.3 x 10(7) M-1 s-1 and 2 +/- 1 x 10(-3) s-1, respectively. Glycosylation and engineered amino acid substitutions of IL4-BP did not alter the kinetic constants as shown by a parallel analysis of IL4-BP variants produced in Escherichia coli or Chinese hamster ovary cells. The rate of association was only slightly affected in binding-deficient variants [E9Q]IL-4 and [R88Q]IL-4 and by acidic pH down to values of 4.5, but it was reduced up to fivefold at higher ionic strength. The rate of dissociation was increased 70-fold and 150-fold with the IL-4 variants and fivefold at an acidic pH of 4.5, but it was not affected by high ionic strength. Temperatures between 6 degrees C and 37 degrees C yielded similar rates of IL-4 dissociation and only a marginally reduced rate of IL-4 association at 6 degrees C. These results indicate that the high-affinity binding of IL-4 to its receptor (Kd approximately 100 pM) is mainly the result of an unusually high association rate. The IL-4/IL4-BP interaction appears to be dominated by charge effects. The exceedingly high rate of IL-4/IL4-BP association is augmented by the overall electrostatic potentials of both proteins (electrostatic steering). Localized charges and the formation of ion pairs may control the rate of complex dissociation.