Crystal Structure of an HSA/FcRn Complex Reveals Recycling by Competitive Mimicry of HSA Ligands at a pH-Dependent Hydrophobic Interface

The long circulating half-life of serum albumin, the most abundant protein in mammalian plasma, derives from pH-dependent endosomal salvage from degradation, mediated by the neonatal Fc receptor (FcRn). Using yeast display, we identified human serum albumin (HSA) variants with increased affinity for human FcRn at endosomal pH, enabling us to solve the crystal structure of a variant HSA/FcRn complex. We find an extensive, primarily hydrophobic interface stabilized by hydrogen-bonding networks involving protonated histidines internal to each protein. The interface features two key FcRn tryptophan side chains inserting into deep hydrophobic pockets on HSA that overlap albumin ligand binding sites. We find that fatty acids (FAs) compete with FcRn, revealing a clash between ligand binding and recycling, and that our high-affinity HSA variants have significantly increased circulating half-lives in mice and monkeys. These observations open the way for the creation of biotherapeutics with significantly improved pharmacokinetics. [Display omitted] •The crystal structure of the HSA/FcRn complex reveals a large hydrophobic interface•Intramolecular histidine-mediated conformational changes drive pH-dependent binding•FcRn and long-chain FAs compete for binding to albumin•High-affinity HSA variants have longer circulating half-lives in rodents and primates Serum albumin owes its long serum half-life to pH-dependent recycling by FcRn. Schmidt et al. report the crystal structure and analysis of the HSA/hFcRn complex, revealing that hFcRn binds HSA by competitive mimicry of lipid ligands of HAS.