Localization of the human complement component C3 binding site on the IgG heavy chain

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• The complement system is crucial for innate and adaptive immunity.
• Understanding complement component C3 interactions with IgG is vital for immune response research.

Purpose of the Study:

• To pinpoint the exact covalent binding site of C3 on the IgG heavy chain.

Main Methods:

• Peptide generation via cyanogen bromide digestion of C3-IgG adducts.
• Amino-terminal sequencing of CNBr peptides.
• Radiolabeling of C3b's free thiol group with iodo[1-14C]acetamide.
• Hydroxylamine treatment to analyze ester linkage.

Main Results:

• Identified a 40-kDa dipeptide containing the covalent bond between C3 and IgG.
• Determined the binding site on C3 (residue 938) and IgG (residue 84 in the variable region).
• The adduct involves a 22-kDa C3 fragment and an 18-kDa IgG fragment.

Conclusions:

• C3 covalently binds to the IgG heavy chain within a specific region.
• This region spans the last 20 residues of the variable region to the first 20 residues of the CH2 domain.