Production of a noncovalently bonded pentamer of immunoglobulin M: relationship of J chain.

Human immunoglobulin M was reduced with concentrations of 2-mercaptoethylamine chosen so that approximately 40% of the immunoglobulin M was reduced to 7S subunits. Under these conditions, which selectively cleave intersubunit disulfides, J chain was released. The 7S subunits of immunoglobulin M so produced did not contain J chain. The high-molecular-weight immunoglobulin M remaining after treatment with 2-mercaptoethylamine had a sedimentation coefficient of 18.0 S and molecular weight of 1 million, and dissociated in 4 M guanidine into subunits similar in size to the 7S subunits. J chain was found in the 18.0S, noncovalently linked immunoglobulin M from which it was released only after more complete reduction with 10 mM dithiothreitol. The results are consistent with the hypothesis that J chain participates in the formation of the intersubunit linkages. However, it need not necessarily be directly involved in all of the intersubunit disulfides. It may play an important role in modulating the assembly of immunoglobulin M subunits, perhaps by inducing conformational changes that lead to noncovalent interactions between the subunits.