title: Interaction with both domain I and III of albumin is required for optimal pH-dependent binding to the neonatal Fc receptor (FcRn).
abstract: Albumin is an abundant blood protein that acts as a transporter of a plethora of small molecules like fatty acids, hormones, toxins, and drugs. In addition, it has an unusual long serum half-life in humans of nearly 3 weeks, which is attributed to its interaction with the neonatal Fc receptor (FcRn). FcRn protects albumin from intracellular degradation via a pH-dependent cellular recycling mechanism. To understand how FcRn impacts the role of albumin as a distributor, it is of importance to unravel the structural mechanism that determines pH-dependent binding. Here, we show that although the C-terminal domain III (DIII) of human serum albumin (HSA) contains the principal binding site, the N-terminal domain I (DI) is important for optimal FcRn binding. Specifically, structural inspection of human FcRn (hFcRn) in complex with HSA revealed that two exposed loops of DI were in proximity with the receptor. To investigate to what extent these contacts affected hFcRn binding, we targeted selected amino acid residues of the loops by mutagenesis. Screening by in vitro interaction assays revealed that several of the engineered HSA variants showed decreased binding to hFcRn, which was also the case for two missense variants with mutations within these loops. In addition, four of the variants showed improved binding. Our findings demonstrate that both DI and DIII are required for optimal binding to FcRn, which has implications for our understanding of the FcRn-albumin relationship and how albumin acts as a distributor. Such knowledge may inspire development of novel HSA-based diagnostics and therapeutics.
authors.affiliation: From the Centre for Immune Regulation (CIR) and Department of Biosciences, University of Oslo, N-0316 Oslo, Norway, CIR and Department of Immunology, Oslo University Hospital Rikshospitalet and University of Oslo, Norway, P. O. Box 4950, N-0424 Oslo, Norway.
authors.forename: Kine Marita Knudsen
authors.initials: KM
authors.lastname: Sand
authors.affiliation: From the Centre for Immune Regulation (CIR) and Department of Biosciences, University of Oslo, N-0316 Oslo, Norway, CIR and Department of Immunology, Oslo University Hospital Rikshospitalet and University of Oslo, Norway, P. O. Box 4950, N-0424 Oslo, Norway.
authors.forename: Malin
authors.initials: M
authors.lastname: Bern
authors.affiliation: CIR and Department of Immunology, Oslo University Hospital Rikshospitalet and University of Oslo, Norway, P. O. Box 4950, N-0424 Oslo, Norway, Institute of Clinical Medicine, University of Oslo, N-0424 Oslo, Norway.
authors.forename: Jeannette
authors.initials: J
authors.lastname: Nilsen
authors.affiliation: Department for Microbiology, Oslo University Hospital Rikshospitalet and University of Oslo, P. O. Box 4950, Nydalen, N-0424 Oslo, Norway, Department of Medical Biochemistry, Oslo University Hospital Rikshospitalet and University of Oslo, P. O. Box 4950, Nydalen, N-0424 Oslo, Norway, and Novozymes Biopharma UK Ltd., Castle Court, 59 Castle Boulevard, NG7 1FD Nottingham, United Kingdom.
authors.forename: Bjørn
authors.initials: B
authors.lastname: Dalhus
authors.affiliation: CIR and Department of Immunology, Oslo University Hospital Rikshospitalet and University of Oslo, Norway, P. O. Box 4950, N-0424 Oslo, Norway.
authors.forename: Kristin Støen
authors.initials: KS
authors.lastname: Gunnarsen
authors.affiliation: Novozymes Biopharma UK Ltd., Castle Court, 59 Castle Boulevard, NG7 1FD Nottingham, United Kingdom.
authors.forename: Jason
authors.initials: J
authors.lastname: Cameron
authors.affiliation: From the Centre for Immune Regulation (CIR) and Department of Biosciences, University of Oslo, N-0316 Oslo, Norway, CIR and Department of Immunology, Oslo University Hospital Rikshospitalet and University of Oslo, Norway, P. O. Box 4950, N-0424 Oslo, Norway.
authors.forename: Algirdas
authors.initials: A
authors.lastname: Grevys
authors.affiliation: Institute of Clinical Medicine, University of Oslo, N-0424 Oslo, Norway.
authors.forename: Karen
authors.initials: K
authors.lastname: Bunting
authors.affiliation: From the Centre for Immune Regulation (CIR) and Department of Biosciences, University of Oslo, N-0316 Oslo, Norway, CIR and Department of Immunology, Oslo University Hospital Rikshospitalet and University of Oslo, Norway, P. O. Box 4950, N-0424 Oslo, Norway.
authors.forename: Inger
authors.initials: I
authors.lastname: Sandlie
authors.affiliation: CIR and Department of Immunology, Oslo University Hospital Rikshospitalet and University of Oslo, Norway, P. O. Box 4950, N-0424 Oslo, Norway, j.t.andersen@medisin.uio.no.
authors.forename: Jan Terje
authors.initials: JT
authors.lastname: Andersen
mesh_terms: D019943:Amino Acid Substitution; D001667:Binding, Competitive; D015395:Histocompatibility Antigens Class I / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008958:Models, Molecular; D011485:Protein Binding; D055550:Protein Stability; D017434:Protein Structure, Tertiary; D011961:Receptors, Fc / Q000378:metabolism*; D012709:Serum Albumin / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D015395:Histocompatibility Antigens Class I; D011961:Receptors, Fc; D012709:Serum Albumin; C100688:Fc receptor, neonatal
keywords: Albumin; Biodegradation; Bioengineering; Fc Receptor; Pharmacokinetics; pH Regulation