Evidence for a long-range conformational change induced by antigen binding to IgM antibody.

The effects of antigen binding on IgM antibody conformation were investigated by measuring the immunological reactivity of the Fc-bound J polypeptide. For such measurements anti-azophenyl-beta-D-lactoside and anti-azobenzenearsonate IgM antibodies were examined in a J chain radioimmunoassay before and after complexing with various hapten-conjugates. The assays showed that (i) the accessibility of J chain determinants is very limited in uncomplexed IgM and (ii) their accessibility is significantly enhanced in the presence of an excess of specific antigen. In both antibody systems, enhanced J chain exposure was achieved with the homologous multi-hapten-substituted antigen (1.9-fold), with a small multivalent antigen in which three to four hapten groups were coupled to a heterologous carrier (1.55-fold), and with monohapten-substituted antigen (1.4-fold). Because the J chain is located in the terminal CH4 Fc domains, these data provide direct evidence that a change in Fc conformation is induced by the binding of antigen to the distant Fab combining sites. Moreover, the data indicate that the changes in J chain exposure do not depend on crosslinking by antigen, but can be induced by the interaction of antigen at individual IgM combining sites.