High-affinity binding of the neonatal Fc receptor to its IgG ligand requires receptor immobilization.
The neonatal Fc receptor (FcRn) binds maternal immunoglobulin G (IgG) during the acquisition of passive immunity by the fetus or newborn. In adult mammals, FcRn also binds IgG and returns it to the bloodstream, thus protecting IgG from a default degradative pathway. Biosensor assays have been used to characterize the interaction of a soluble form of FcRn with IgG. We use the statistical method of cross-validation to show that there are two classes of noninteracting binding sites, and these are sufficient to account for previously observed nonlinear Scatchard plots of FcRn/IgG binding data. We demonstrate that immobilization of FcRn on the biosensor surface reproduces the high-affinity IgG binding observed for membrane-bound FcRn, whereas immobilization of IgG results in lower affinity binding similar to that of the FcRn/IgG interaction in solution. The dependence of FcRn/IgG binding affinity on the coupled molecule provides further evidence in support of the previously hypothesized model that an FcRn dimer forms the high-affinity IgG binding site.
Vaughn
D E
DE
Division of Biology 156-29 and Howard Hughes Medical Institute, California Institute of Technology, Pasadena, California 91125, USA.
Bjorkman
P J
PJ
D000818:Animals; D000831:Animals, Newborn; D001665:Binding Sites; D015374:Biosensing Techniques; D016466:CHO Cells; D006224:Cricetinae; D007074:Immunoglobulin G / Q000378:metabolism*; D007700:Kinetics; D011485:Protein Binding; D051381:Rats; D011961:Receptors, Fc / Q000378:metabolism*; D011994:Recombinant Proteins / Q000378:metabolism
D007074:Immunoglobulin G; D011961:Receptors, Fc; D011994:Recombinant Proteins