The binding of soluble recombinant human Fcγ receptor I for human immunoglobulin G is conferred by its first and second extracellular domains

Human Fc gamma RI is a high affinity receptor for the Fc portion of human immunoglobulin G (IgG), and has extracellular, transmembrane and cytoplasmic regions. The extracellular region of human Fc gamma RI, which is the part that interacts with human IgG, is comprised of three immunoglobulin-like domains. Unlike low affinity Fc gamma receptors (Fc gamma RII and Fc gamma RIII), Fc gamma RI has a unique third extracellular domain (D3). This study investigated the contribution of D3 to the binding between recombinant human Fc gamma RI (rhFc gamma RI) and human IgG. The three extracellular domains and the first and second extracellular domains of human Fc gamma RI were expressed by Escherichia coli as rhFc gamma RI and rhFc gamma RI-D1D2, respectively. The binding specificity of rhFc gamma RI-D1D2 to human IgG subclasses was the same as that of rhFc gamma RI. From surface plasmon resonance analysis, the binding affinity of rhFc gamma RI-D1D2 for human IgG1/kappa was high (the equilibrium dissociation constant: K-D = 8.04 x 10(-10) M), but slightly lower than that of rhFc gamma RI (K-D = 2.59 x 10(-10) M). While the association of rhFc gamma RI-D1D2 with human IgG1/kappa was same as that of rhFc gamma RI, the dissociation of rhFc gamma RI-D1D2 was faster than that of rhFc gamma RI. From these results, D3 of rhFc gamma RI would not contribute directly to the binding specificity and association of rhFc gamma RI, but to the holding bound human IgG. (c) 2013 Elsevier Ltd. All rights reserved.