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Polymerization of IgA and IgM: Roles of Cys³⁰⁹/Cys⁴¹⁴ and the Secretory Tailpiece¹

Vigdis Sørensen^*, Vibeke Sundvold, Terje E. Michaelsen and Inger Sandlie^2,*

^* Department of Molecular Cell Biology, Institute of Biology, University of Oslo, Olso, Norway; Institute of Immunology and Rheumatology, The National Hospital, Oslo, Norway; and Department of Vaccinology, National Institute of Public Health, Oslo, Norway

We have investigated how the secretory tailpiece (tp), Cys⁴¹⁴ and the amino acids flanking Cys⁴¹⁴ or Cys³⁰⁹ are involved in regulating the different polymerization of IgM and IgA to pentamers and dimers/monomers, respectively. Whereas changing the tp of IgM to that of IgA has little effect on IgM polymerization, introducing the µtp to IgA leads to the formation of larger than wild-type IgA polymers, including pentamers and hexamer. This shows that the secretory tp can differentially regulate polymerization depending on the heavy chain context. Cys⁴¹⁴, which is engaged in intermonomeric disulfide bonds in IgM, is not crucial for the difference in IgM and IgA polymerization; IgM with a C414S mutation forms more large polymers than IgA. Also, IgA with IgM-like mutations in the five amino acids flanking Cys³⁰⁹, which is homologous to Cys⁴¹⁴, oligomerize similarly as IgA wild type. Thus, IgA appears to have an inherent tendency to form monomers and dimers that is partially regulated by the tp, while the Cys³⁰⁹ region has only a minor effect. We also show that complement activation by IgM is sensitive to alterations in the polymeric structure, while IgA is inactive in classical complement activation even for polymers such as pentamers and hexamers.

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