High-resolution structures of the IgM Fc domains reveal principles of its hexamer formation

Abstract

IgM is the first antibody produced during the humoral immune response. Despite its fundamental role in the immune system, IgM is structurally only poorly described. In this work we used X-ray crystallography and NMR spectroscopy to determine the atomic structures of the constant IgM Fc domains (Cμ2, Cμ3, and Cμ4) and to address their roles in IgM oligomerization. Although the isolated domains share the typical Ig fold, they differ substantially in dimerization properties and quaternary contacts. Unexpectedly, the Cμ4 domain and its C-terminal tail piece are responsible and sufficient for the specific polymerization of Cμ4 dimers into covalently linked hexamers of dimers. Based on small angle X-ray scattering data, we present a model of the ring-shaped Cμ4 structure, which reveals the principles of IgM oligomerization.