Biosynthesis and assembly of IgM. Free thiol groups present on the intracellular subunits

We have studied the biosynthesis of IgM and its subunits, and the role played by the thiol groups of these subunits in the assembly of the IgM molecule using mouse plasma cell tumor MOPC 104E. In addition to 19 S IgM and light chain, the tumor secretes a small amount of 7 S IgM (IgMs), whereas the cells contain light chain, an HL subunit, IgMs and only a small amount of 19 S IgM. Although the light chain monomers excreted in the urine have blocked thiol groups, the light chain monomers inside the cell and freshly secreted from it have one free thiol group per polypeptide chain. Both the intracellular IgMs and HL subunits also carry free thiol groups, the evidence indicating that there are four per intracellular IgMs subunit. Since there is some evidence that the cysteine residues involved in the formation of the intersubunit bridges may not be free, the polymerization of IgMs to 19 S IgM may be initiated by a rearrangement of thiol groups