The MHC class I-related neonatal Fc receptor (FcRn) serves in the homeostatic regulation of IgG and albumin by increasing their half-lives. FcRn may bind IgG and albumin simultaneously, and in a pH-dependent manner, with ligand binding at pH 6.0-6.5 and release at pH 7.0-7.4. The FcRn-IgG interaction has been extensively characterized at the amino acid level and shown to depend on conserved histidine residues in the IgG-Fc part that interact with negatively charged residues in the a-2 domain of FcRn. The recently discovered FcRn-albumin interaction remains to be elucidated. Guided by the pH dependence of the FcRn-albumin interaction, we compared the sequence of the FcRn a-2 domain from eleven different species, and identified histidine residues that were conserved in all (H166) or seven (H161) of these. Both residues are located directly opposite to the IgG interaction site in the folded molecule. We did in vitro mutagenesis (H161A or H166A) in combination with interaction studies (ELISA and surface plasmon resonance) with recombinant, soluble, purified receptors and IgG and albumin to investigate the role of the two histidine residues. Our results show clear evidence that the conserved H166 is a key player in the FcRn-albumin interaction.

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