Structure and function of immunoglobulin domains. III. Isolation and characterization of a fragment corresponding to the Cgamma2 homology region of human immunoglobin G1.

Exposure of Fc fragments derived from human IgG1 myeloma proteins to acid pH rendered the region between the Cgamma2 and Cgamma3 domains transiently susceptible to cleavage by trypsin upon return to neutral pH. Trypsin covalently linked to Sepharose was used and two fragments derived from the Cgamma2 region and one from the Cgamma3 region were purified by column chromatography. On the basis of amino acid analysis, primary sequency data, antigenic properties, and m.w., one of the Cgamma2 fragments was shown to consist of two polypeptide chains of unequal mass joined by the inter-heavy chain disulfide bonds. The larger chain corresponded to a stretch of gamma-chain between Thr 223 and Lys 338 (Eu numbering) and the shorter chain to the section between Thr 223 and Lys 248. The other Cgamma2-fragment was a disulfide-linked dimer of the Thr 223 to Lys 338 sections of the paired gamma-chains. When this latter fragment was reduced under mild conditions it dissociated into monomers indicating that there was little or no noncovalent interactions between the Cgamma2 domains. The Cgamma3-fragment was shown to be a noncovalent dimer composed of the Glu 345 to Lys 349 sections of the two gamma-chains although some heterogeneity was apparent at the amino-terminus. Circular dichroism was used to probe the conformational relationships between the isolated domains and the parent Fc. The spectral properties of Fc could not be fully accounted for on the basis of the spectra observed for the isolated domains which suggested that inter-domain interaction might be significant in Fc.