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Biochemistry. 1997 Aug 5;36(31):9374-80.

High-affinity binding of the neonatal Fc receptor to its IgG ligand requires receptor immobilization.

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Division of Biology 156-29 and Howard Hughes Medical Institute, California Institute of Technology, Pasadena, California 91125, USA.


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.

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