Protein-protein recognition: The neonatal Fe receptor and immunoglobulin G

[摘要] The neonatal Fe receptor (FeRn) binds the Fe portion of immunoglobulin G (IgG)at the acidic pH of endosomes or the gut and releases IgG at the alkaline pH of blood.FeRn is responsible for the maternofetal transfer of IgG and for rescuing endocytosedIgG from a default degradative pathway. We investigated how FeRn interacts with IgGby constructing a heterodimeric form of the Fe (hdFc) that contains one FeRn bindingsite. This molecule was used to characterize the interaction between one FeRn moleculeand one Fe and to determine under what conditions FeRn forms a dimer. The hdFc bindsone FeRn molecule at pH 6.0 with a K_d of 80 nM. In solution and with FeRn anchored tosolid supports, the heterodimeric Fe does not induce a dimer of FeRn molecules. FcRnhdFccomplex crystals were obtained and the complex structure was solved to 2.8 Åresolution. Analysis of this structure refined the understanding of the mechanism of thepH-dependent binding, shed light on the role played by carbohydrates in the Fe binding,and provided insights on how to design therapeutic IgG antibodies with longer serumhalf-lives. The FcRn-hdFc complex in the crystal did not contain the FeRn dimer. Tocharacterize the tendency of FeRn to form a dimer in a membrane we analyzed thetendency of the hdFc to induce cross-phosphorylation of FeRn-tyrosine kinase chimeras.We also constructed FeRn-cyan and FeRn-yellow fluorescent proteins and have analyzedthe tendency of these molecules to exhibit fluorescence resonance energy transfer. As ofnow, neither of these analyses have lead to conclusive results. In the process of acquiringthe context to appreciate the structure of the FcRn-hdFc interface, we developed a studyof 171 other nonobligate protein-protein interfaces that includes an original principalcomponent analysis of the quantifiable aspects of these interfaces.

[发布机构] University:California Institute of Technology;Department:Biology

[关键词] Biochemistry and Molecular Biophysics