The work described here is an investigation of the molecular mechanism of pHdependent immunoglobulin G (IgG) binding by the neonatal Fc receptor (FcRn). FcRnbinds IgG at acidic, but not alkaline pHs, in two important physiological processes. Theseprocesses are the acquisition of passive immunity by the fetus or newborn and protectingIgG from a default degradative pathway.
A biosensor assay is used to characterize the interaction of a soluble form of FcRnwith IgG. Immobilization of FcRn on the biosensor surface reproduces the high affinityIgG binding observed for membrane bound FcRn, whereas immobilization of IgG resultsin lower affinity binding similar to that of the FcRn/IgG interaction in solution. Thestatistical method of cross-validation is used to show that there are two classes of noninteracting binding sites. The IgG binding interaction is characterized for several mutant FcRns with designed amino acid substitutions. These mutations map the functional IgGbinding site on FcRn.
The structure of FcRn at an alkaline pH is described. This structure determinationreveals an extensive carbohydrate mediated interaction between the dimer related FeRnmolecules. The physiological relevance of this interaction is discussed in the context of the FcRn dimerization literature. A further refined structure of FcRn at an acidic pH isdescribed that includes additional carbohydrate structure. These structures are comparedwith specific attention to the pH dependence of FcRn stability and IgG affinity. Finally, amechanism for pH dependent antibody binding to FcRn is proposed based on these structures and the body of structure/function literature concerning this interaction.
