Conformational dependency of human IgG heavy chain-associated Gm allotypes

Human IgG allotypic markers Gm(a)[Glm(1)], Gm(x)[Glm(2)]; Gm(f)[Glm(4)], Gm(b)[G3m(5) and (11)] and Gm(g)[G3m(21)] were studied after chemical modification of IgG histidines by diethylpyrocarbonate, tyrosines by N-acetylimidazole and lysines by formaldehyde and sodium borohydride. Degrees of substitution were estimated by trinitrobenzenesulfonic acid assay. IgG of known Gm phenotype isolated from serum of hyperimmune anti-tetanus toxoid donors was studied. Histidyl modification resulted in virtually complete loss of Gm(a) and Gm(g) antigenicity but preservation of Gm(x), Gm(b) and Gm(f). Reconstitution of the histidyl residues using hydroxylamine resulted in virtually complete restoration of Gm(a) and Gm(g) antigenicity. Histidine modification resulted in no significant decrease in ELISA anti-tetanus antibody activity. Alteration of tyrosyl residues using N-acetylimidazole considerably diminished Gm(a) and Gm(f) expression. This effect was reversed by hydroxylamine treatment. Moreover, chemical alteration of tyrosyl residues produced a complete loss of Gm(g) antigenicity which was only partially restored after deacylation. A urinary H chain fragment containing the VH region directly linked to C gamma 3 which contained the Gm(a) specific and Gm(x) specific amino acid residues was positive for Gm(a) but negative for Gm(x). Another urinary H chain fragment containing only the C gamma 3 domain was negative for both Gm(a) and (x). These findings indicate that Gm allotypic markers may depend on conformational determinants in which strongest expression for Gm(a) and (x) depends on structures expressed by C gamma 3 linked to C gamma 2 domains. Although RFs react with the region encompassing the C gamma 2-C gamma 3 interface, Gm-specificities of such reactions are affected allosterically through single or double amino acid substitutions at a relatively distant site.