A KEY AMINO-ACID DETERMINING G3M(B) ALLOTYPIC MARKERS

A key amino acid substitution specific for allotypic Gm b markers, b0, b3, b5, were determined through sequence analyses of the pFc' fragments of IgG1 (Su) and IgG3 (Ba[Gm(g)], Bu[Gm(b1b3)], and Kam[Gm(b3st)]) myeloma proteins. The results indicate that serine at position 384 is responsible for the specificities. It is considered from crystallographic data of IgG-Fc [Deisenhofer et al. (1981) Biochemistry 20: 2361) that two residues, the serine and isoleucine specific for IgG3 subclass at position 422, cause the structural change responsible for b markers. The two residues are close to each other in the CH3 domain. The allocations of the epitopes are estimated to be on two bends (residue no. 382-392, 411-424) between the beta-strands, whose amino acid residues are present in wide contact area [Novotny et al (1987) Immunol. Today 8: 26).