Studies on the structural and biological functions of the Cmu4 domains of IgM.

The development of methods for the production of intact Cμ3 and Cμ4 domains of IgM have made possible the assessment of some of their structural and biological functions. Antiserum against Fcμ fragment detected both domains and illustrated their complete antigenic non-identity. Circular dichroism spectroscopy and the retention of antigenicity indicated that both domains had retained most of their native structure. No interaction of the type Cμ3—Cμ3, Cμ4—Cμ4 or Cμ3—Cμ4 could be detected under non-dissociating conditions by analytical ultracentrifugation or molecular exclusion chromatography experiments. These results lead us to believe that the transmission of effector messages between the Fab and Fc parts of IgM takes place through structural changes at the quaternary level. C[unk]1-fixation experiments with IgM and several of its fragments and domains show that (a) the Cμ4 domain contains the C[unk]1-fixing site; (b) the high C[unk]1-fixing capacity of IgM or Fcμ cannot be explained on the basis of a simple accumulative model of complement fixing domains; (c) the C[unk]1-fixing site is independent of the native structure of the Cμ4 domain; (d) the C[unk]1-fixing site does not contain carbohydrate. Examination of the IgM receptor on the surface of human T lymphocytes show that (a) Cμ4 domain is primarily responsible for the reaction and Cμ3 domain has very little affinity; (b) native structure is essential for the reaction because reduction and alkylation of the Cμ4 domain destroyed both its original conformation and affinity for this receptor; (c) IgM and Fcμ had a much greater affinity for the receptor than monomeric subunits: (d) carbohydrate on Cμ4 domain is not involved in the affinity reaction.