Convergent solutions to binding at a protein-protein interface.

Area of Science:

• Immunology
• Structural Biology
• Biochemistry

Background:

• The Fc fragment of human immunoglobulin G (IgG) interacts with various natural protein scaffolds.
• These interactions occur at a common binding site located between the C(H2) and C(H3) domains.

Purpose of the Study:

• To investigate the binding preferences of the Fc fragment's common site.
• To characterize the structural basis of peptide binding to the Fc fragment.

Main Methods:

• Bacteriophage display for in vitro selection of high-affinity peptides.
• X-ray crystallography to determine the structure of an Fc-peptide complex at 2.7 angstrom resolution.
• Comparative structural analysis of multiple Fc-protein and Fc-peptide complexes.

Main Results:

• A consensus binding site on the Fc fragment was identified, preferred for its intrinsic physiochemical properties.
• A selected 13-amino acid peptide exhibited high affinity (Kd ~25 nM) for this site.
• The peptide adopted a unique compact structure, yet its Fc binding interactions mimicked those of natural proteins.
• Structural analysis revealed the binding surface to be accessible, adaptive, hydrophobic, and with limited polar interactions.

Conclusions:

• The Fc fragment's binding site preference is driven by inherent physicochemical characteristics, not solely biological function.
• Engineered peptides can effectively mimic natural protein interactions at the Fc fragment.
• The properties of the Fc binding site may facilitate cross-reactive binding, relevant to protein-protein and hormone-receptor interactions.