Biosynthesis and assembly of IgM free thiol groups present on cytoplasmic and membrane associated IgM

The free thiol groups present on various intracellular precursors of polymeric IgM have been studied in two mouse tumours; McPC 1748 which resembles a 10-25h LPS-stimulated lymphocyte with surface IgM, and Y5781 which resembles a 80-110h LPS stimulated lymphocyte with no surface IgM and a large pool of intracellular 19S IgM. The intracellular IgMs* of both cell lines possesses four free thiol groups per molecule, two of which are probably involved in linking the subunits during polymerisation. In conjunction with earlier reports this shows that independent IgM-synthesizing tumours with widely differing characteristics contain IgMs with four free thiol groups, indicating that this is the major precursor form preceding polymerisation. Although the secreted 19S IgM had no detectable free thiol groups, some of the H chains of the intracellular 19S IgM of Y5781 had at least one which may be involved in J chain binding. The IgMs of slow turnover associated with McPC 1748 cells was shown to be exposed on the cell surface and was identical with the cytoplasmic IgMs in pI and number of free thiol groups. These thiol groups become blocked before release into the medium.