Studies on IgM polymerization: reassociation to non-covalently and covalently linked Fc5mu fragments.

Fc5mu fragments were purified from a trypsin digest of native IgM by gel filtration and isoelectric focusing. Polyacrylamide gel electrophoresis of Fc5mu fragments in sodium dodecyl sulphate disclosed a major and a minor band with molecules of 320,000 and 285,000 daltons, respectively. The mu chain fragments showed a molecular weight of 34,500. After reduction of the Fc5mu fragments to free mu chain fragments and J chain removal of the reducing agent by dialysis for 24 h under nitrogen in the presence of Zn ions gave non-covalently linked Fc5mu fragments. This shows that the non-covalent interactions operating between the mu chains of non-covalently linked native IgM are present in the C-terminal part of the mu chains. Additional dialysis in the presence of Zn and Cu ions resulted in the formation of covalently linked Fc5mu fragments.