Identification of a lymphocyte enzyme that catalyzes pentamer immunoglobulin M assembly.

A protein with immunoglobulin M-polymerizing activity was isolated from the membrane fraction of mouse plasmacytoma cells secreting pentamer IgM. The isolation was achieved by taking advantage of the solubility of the protein in 50% ammonium sulfate, its relatively high net negative charge, and its sedimentation at 4.2 S. Analyses of the purified preparations showed that the polymerizing protein catalyzes the assembly of pentamer IgM in vitro; less than 1 mol of enzyme/10 mol of monomer IgM and 2 ml of J chain were found to promote 50% polymerization. Evidence that the enzyme also plays an essential role in the in vivo assembly process was obtained from the reaction rates of the polymerization catalyzed in vitro, the similarity between the pentamer IgM molecules synthesized in vitro and in vivo, and the finding that polymerizing enzyme is a specific product of B lymphocytes. Analyses of the mechanism of polymerization suggested that polymerizing enzyme is a sulfhydryl oxidase; it was found to be inactivated by chelating agents and to resemble Cu2+ in catalyzing the formation of IgM intersubunit disulfide bonds. These results raise the possibility that the assembly of pentamer IgM does not involve disulfide interchange is previously thought, but proceeds by the direct oxidation of monomer IgM and J chain sulfhydryls.