Fc-III IgG-Binding Peptide, Disulfide-Cyclized, Amidated

Research background

Fc-III is a 13-residue peptide that binds the Fc region of human immunoglobulin G. DeLano and colleagues isolated it by phage display and solved its complex with human IgG1 Fc, deposited as PDB entry 1DN2. The structure showed the peptide engaging a hinge-proximal site on the Fc domain that is also used by several unrelated natural Fc ligands, an observation the authors framed as convergence on a limited number of favourable binding surfaces at that protein-protein interface. The published molecule is cyclized through a Cys2-Cys12 disulfide and carries a C-terminal amide; both features are specified in this product.

The affinity is notable for a molecule of this size. In a comparative review of Fc-binding ligands, Choe and colleagues reported a dissociation constant near 25 nM by competitive assay, placing Fc-III only about twofold weaker than Staphylococcal Protein A despite being a small fraction of its mass, and noted that the peptide does not compete with Protein A or Protein G for binding. Peptidomimetic derivatives built on the same scaffold have reached considerably tighter binding, with a reported Ki of 0.4 nM for FcBP-2. Immobilized Fc-III has been used as a chromatographic ligand, with antibody recovered at roughly 95% purity, elution under comparatively mild acidic conditions and column reuse over dozens of cycles.

The peptide has also been adapted as a genetically encoded affinity tag. In a study of the Fc-III tag system, fusion proteins were shown by mass spectrometry to contain correctly cyclized peptide, and carbonic anhydrase and creatine kinase were purified on immobilized IgG-Fc with yields in the range of 88 to 92 percent. The same tag allowed Fc-III-tagged CD38 to be visualized at the membrane of HuH-7 cells using fluorescently labelled IgG-Fc. Critically, reduction and alkylation of the cysteines abolished binding, confirming that the disulfide is not optional. More recent work has exploited the peptide's defined binding site for site-specific antibody modification, including photochemical crosslinking approaches that generate homogeneous conjugates from unmodified antibodies.

Because binding depends on both the disulfide and the C-terminal amide, published affinity and performance data for Fc-III should not be extended to reduced, linear or free-acid analogues.

Research applications

• Affinity ligand for immunoglobulin G purification after immobilization on chromatographic supports
• Site-specific antibody labelling and conjugation, including photochemical crosslinking strategies for homogeneous conjugates
• Competition and epitope-mapping assays to test whether a binder engages the hinge-proximal consensus site on Fc
• Surface plasmon resonance and other biophysical measurement of peptide-Fc binding kinetics
• Detection and capture of Fc-III-tagged recombinant proteins using labelled or immobilized IgG-Fc
• Biosensor and immunoassay capture surfaces requiring oriented antibody immobilization
• Structural and peptidomimetic studies building on the 1DN2 complex, including scaffold optimization

References

1. DeLano WL, Ultsch MH, de Vos AM, Wells JA. Convergent solutions to binding at a protein-protein interface. Science (2000) 287:1279-1283. doi:10.1126/science.287.5456.1279
2. Fc fragment of human IgG1 in complex with the engineered 13-residue peptide DCAWHLGELVWCT-NH2. PDB 1DN2
3. Development of the Fc-III tagged protein expression system for protein purification and detection. PLOS ONE (2012) 7:e44208. doi:10.1371/journal.pone.0044208
4. Choe W, Durgannavar TA, Chung SJ. Fc-binding ligands of immunoglobulin G: an overview of high affinity proteins and peptides. Materials (2016) 9:994. doi:10.3390/ma9120994
5. Affinity-based methods for site-specific conjugation of antibodies. Bioconjugate Chemistry (2021). doi:10.1021/acs.bioconjchem.1c00313
6. Photoconjugation of an Fc-specific peptide enables efficient DAR 2 antibody-drug conjugate formation. Organic Letters (2020). doi:10.1021/acs.orglett.0c03049
7. Kinetics-based structural requirements of human immunoglobulin G binding peptides. ACS Omega (2019). doi:10.1021/acsomega.9b01104

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For research use only. Not for use in diagnostic or therapeutic procedures, or for human or veterinary use.