Non-covalent association of IgM subunits produced by reduction and alkylation.

  • 1 December 1974
    • journal article
    • Vol. 27  (6) , 1063-71
Abstract
Purified IgM isolated from the serum of mice bearing the transplantable plasmacytoma MOPC 104E was reduced and alkylated and then analysed by sucrose density gradient centrifugation and by sodium dodecyl sulphate—polyacrylamide gel electrophoresis. On partial reduction a mixture of IgM subunits was obtained in the absence of covalently linked 19S IgM. When examined under dissociating conditions this mixture was found to consist of disulphide-linked 7S subunits (IgMs), small amounts of HL subunits and oligomeric IgM of a size intermediate between monomeric and pentameric IgM. In the absence of a dissociating agent and on sucrose density gradient, however, the mixture resolved into a 19S and a 7S peak. The 19S peak consisted primarily of oligomeric IgM and IgMs with small amounts of HL subunits. Thus alkylated IgMs and HL subunits of IgM can associate through non-covalent forces to form a molecule sedimenting at 19S, providing oligomeric forms are present. In the absence of oligomeric forms, IgMs, HL subunits and heavy and light chains sediment at about 7S. The products of partial reduction which sediment at 7S and 19S could also be isolated by preparative polyacrylamide gel electrophoresis. When this was done J chain was absent in the former and present in the latter, raising the possibility that J chain does not disulphide bond to each of the five IgMs subunits, constituting an IgM molecule. Thus, within cells secreting IgM, J chain would be expected to mediate the formation of an oligomeric form of IgM. Once the oligomeric structure has been assembled, then non-covalent forces between this and IgMs subunits will cause the formation of a 19S structure, thereby facilitating the final assembly through disulphide bonds.