The Cγ3 Homology Region in Human IgG Subclasses and Allotypes

Amino acid composition, end-group analysis, and antigenic analysis have been performed on isolated pFc' fragments (Cγ3 homology region) from the major allotypic variants of human IgG. A comparison of the amino acid composition of Gm(a+x+) and Gm(a+x−) pFc' fragments showed less serine and more glycine in the Gm(a+x+) proteins. Two IgG2 pFc' fragments were shown to resemble IgG1 Gm(f) pFc' fragments in their amino acid compositions, except that there was more threonine and less alanine in the IgG2 fragments. This was also observed in two IgG3 Gm(g) proteins and may be the basis for the expression of a non b antigen. IgG3 fragments of both Gm(g) and Gm(b) allotypes showed six amino acid differences from the IgG1 subclass. There was more arginine, serine, and isoleucine and less histidine, lysine, and valine in both allotypes. In addition, analysis of a single Gm(b) pFc' fragment showed differences from the Gm(g) proteins; there was more phenylalanine, alanine, and proline and less tyrosine, threonine, and, possibly, aspartic acid.