Site of binding of IgG2b and IgG2a by mouse macrophage Fc receptors by using cyanogen bromide fragments

Cyanogen bromide fragments of murine IgG2b and IgG2a immunoglobulins were used to localize the sequences that are bound by specific IgG2b and IgG2a Fc receptors on murine macrophages. One fragment from the C(H)2 domain of IgG2b bound to the γ2b Fc receptor. Two fragments from IgG2a - one from the C(H)2 domain, differing by only four amino acids from the homologous IgG2b fragment, and the other from the C(H)3 domain - specifically bound to the γ2a Fc receptor. In both a rosetting assay and a radioactive binding assay, these two fragments from IgG2 competed with intact IgG2a: however, they did not compete with each other. Rather, binding of the fragment from the C(H)3 domain of IgG2a augmented the binding of the fragment from the C(H)2 domain of IgG2a but not that of the homologous fragment from IgG2b. The binding of both IgG2a fragments was abolished by trypsin treatment of macrophages. These data suggest that 1) a sequence in the C(H)2 domain of IgG2b is sufficient for binding to the γ2b Fc receptor, 2) sequences from both the C(H)2 and C(H)3 domains of IgG2a bind to the γ2a Fc receptor, and 3) the binding of sequences from the C(H)3 domain of IgG2a may induce a conformational change in the γ2a Fc receptor that leads to enhanced binding of sequences from the C(H)2 domain.