X-ray Crystal Structures of Monomeric and Dimeric Peptide Inhibitors in Complex with the Human Neonatal Fc Receptor, FcRn

Adam R Mezo,Vandana Sridhar,John Badger,Paul Sakorafas,Vicki Nienaber

doi:10.1074/jbc.m110.120667

Adam R Mezo, Vandana Sridhar + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m110.120667

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Abstract

The neonatal Fc receptor, FcRn, is responsible for the long half-life of IgG molecules in vivo and is a potential therapeutic target for the treatment of autoimmune diseases. A family of peptides comprising the consensus motif GHFGGXY, where X is preferably a hydrophobic amino acid, was shown previously to inhibit the human IgG:human FcRn protein-protein interaction (Mezo, A. R., McDonnell, K. A., Tan Hehir, C. A., Low, S. C., Palombella, V. J., Stattel, J. M., Kamphaus, G. D., Fraley, C., Zhang, Y., Dumont, J. A., and Bitonti, A. J. (2008) Proc. Natl. Acad. Sci. U.S.A., 105, 2337-2342). Herein, the x-ray crystal structure of a representative monomeric peptide in complex with human FcRn was solved to 2.6 A resolution. The structure shows that the peptide binds to human FcRn at the same general binding site as does the Fc domain of IgG. The data correlate well with structure-activity relationship data relating to how the peptide family binds to human FcRn. In addition, the x-ray crystal structure of a representative dimeric peptide in complex with human FcRn shows how the bivalent ligand can bridge two FcRn molecules, which may be relevant to the mechanism by which the dimeric peptides inhibit FcRn and increase IgG catabolism in vivo. Modeling of the peptide:FcRn structure as compared with available structural data on Fc and FcRn suggest that the His-6 and Phe-7 (peptide) partially mimic the interaction of His-310 and Ile-253 (Fc) in binding to FcRn, but using a different backbone topology.

Highlights

The atomic coordinates and structure factors have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ
The binding affinity of peptide 1 and Fc to both shFcRn and deg-shFcRn were determined by surface plasmon resonance (SPR), and were unchanged in comparison to the glycosylated form of FcRn. (Table 1)
Since there is no published structure of human FcRn in complex with human Fc, we modeled the hFcRn:hFc complex by overlaying a crystal structure of the

Summary

The abbreviations used are

FcRn, neonatal Fc receptor; deg-shFcRn, deglycosylated soluble human FcRn; SPR, surface plasmon resonance; Pen, L-penicillamine amino acid; Sar, sarcosine; NMeLeu, N-methylleucine; SAR, structure-activity relationships; PDB, Protein Data Bank. The x-ray crystal structure of the soluble extracellular domain of rat FcRn ( referred to as srFcRn) was first reported in 1994, and its structure possessed a MHC-class I-like fold where the peptide groove is closed and unable to bind peptides at that site [13]. In 2001, a high resolution structure of the srFcRn:rFc complex was solved using a heterodimeric Fc that precluded the formation of the oligomeric ribbons [15] This structure revealed the details of the specific interaction between the two molecules including the pH-sensitive histidine salt bridges. We obtained the x-ray co-crystal structure of degshFcRn in complex with dimeric peptide 3, a more soluble analog of peptide dimer 2 (Fig. 1) These data shed light on the peptide mechanism of action, and provide detailed insights into how the peptide is able to bind FcRn with high affinity, and block the IgG:FcRn protein-protein interaction

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION