Structural insights into immunoglobulin M

Hefty structures of IgA and IgM complexes Immunoglobulin M (IgM) and IgA are antibody isotypes that can form higher-order secretory complexes (sIgM and sIgA), which allows them to effectively bind and neutralize antigens with low-affinity repetitive epitopes, such as those found on the surface of many bacteria and viruses. The assembly and transport of these molecules is also dependent on the joining chain (J-chain) and the polymeric immunoglobulin receptor (pIgR) secretory component (SC). The architecture of these complex, multimeric structures has remained elusive. Li et al. resolved cryo–electron microscopy structures of the sIgM-Fc pentamer in complex with the J-chain and SC. Using similar techniques, Kumar et al. visualized dimeric, tetrameric, and pentameric structures of secretory sIgA-Fc interacting with the J-chain and SC. Both groups report highly similar mechanisms wherein the J-chain serves as a template for antibody oligomerization. An unanticipated, amyloid-like assembly of the oligomerized structure is present in both cases, with the J-chain conferring asymmetry for pIgR binding and transcytosis. These studies may inform structure-based engineering of these molecules for future therapeutic purposes. Science , this issue p. 1014 , p. 1008