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On the structure of polymeric IgM.

A C Davis1, K H Roux, M J Shulman

  • 1Department of Immunology, University of Toronto, Canada.

European Journal of Immunology
|July 1, 1988
PubMed
Summary
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Researchers investigated immunoglobulin mu heavy chain assembly. They found that cysteines other than Cys575 can form disulfide bonds, enabling mutant IgM polymer formation and enhancing complement-dependent cytolysis.

Area of Science:

  • Immunology
  • Molecular Biology
  • Protein Chemistry

Background:

  • Polymeric immunoglobulin M (IgM) is crucial for the immune response.
  • The cysteine at position 575 (Cys575) in the immunoglobulin mu heavy chain was believed to be essential for forming disulfide bonds that assemble IgM monomers into polymers.
  • Understanding IgM assembly is key to elucidating its function in immunity.

Purpose of the Study:

  • To investigate the role of Cys575 in the assembly of mouse polymeric IgM.
  • To determine if other cysteine residues can mediate inter-subunit disulfide bond formation in IgM.

Main Methods:

  • Site-directed mutagenesis was used to replace Cys575 with serine in the immunoglobulin mu heavy chain.
  • Analysis of secreted mutant IgM using sucrose gradient sedimentation, SDS-PAGE, and electron microscopy.

Related Experiment Videos

  • Assessment of J chain incorporation and complement-dependent cytolysis activity.

    • Main Results:

    • Mutant IgM with serine at position 575 was still secreted as a covalently assembled polymer, indicating alternative disulfide bond formation.
    • This mutant polymeric IgM lacked J chain, exhibited higher molecular weight, and contained six subunits.
    • Wild-type IgM also exists in higher molecular weight forms (e.g., hexamers) that are J chain-deficient and significantly more potent in activating complement-dependent cytolysis.

    Conclusions:

    • Cys575 is not the sole cysteine residue responsible for inter-subunit disulfide bonds in mouse polymeric IgM.
    • Alternative disulfide linkages allow for the formation of non-canonical IgM polymers (e.g., hexamers).
    • J chain-deficient, higher molecular weight IgM forms, including hexamers, are more efficient at mediating complement-dependent cytolysis, suggesting a distinct functional role.