Correlation between the exposure of aromatic chromophores at the surface of the Fc domains of immunoglobulin G and their ability to bind complement

Abstract

The recognition that certain biological effector functions associated with the Fe region of human IgG [immunoglobulin G] are mediated exclusively by the C''.gamma.2 [.gamma.2 constant region] or C''.gamma.3 domains prompted a study of some of the physical properties of the isolated domains in an attempt to correlate these functional differentiation. The degree of aromatic chromophore exposure on intact Fe and fragments corresponding to the C''.gamma.2 and C''.gamma.3 domains were determined by solvent perturbation difference spectroscopy using 20% ethylene glycol. For the monomeric C''.gamma.2 fragment 1 of the 2 tryptophans and all 4 of the tyrosines were exposed to solvent. In the pFc'' fragment, which represented a dimer of 2 intact C''.gamma.3 domains, an average of 0.4 of the 2 tryptophans and 3.3 of the 5 tyrosines per chain were exposed. Tryptophan is apparently involved in complement (C) fixation since C''.gamma.2 binds C1q [q fragment of the 1st C component] but pFc'' does not. Several fragments derived from the C''.gamma.3 region, which were previously shown to have differing environments for their aromatic side chains from circular dichroism studies, exhibited different degrees of chromophore exposure to solvent. Removal of the carboxy-terminal heptapeptide from the intact C''.gamma.3 domain resulted in a fragment showing a greater exposure of aromatic residues and having the ability to bind Clq. The structural requirements for Clq binding may be commonplace within Fc, but tertiary folding limits their expression except in C''.gamma.2 in the native molecule. The solvent perturbation observed with Fc was somewhat lower than would be expected from the results with the isolated domains, suggesting that interdomain interactions may result in burial of aromatic residues.