Structural evolution of Fcα/μR-mediated antibody recognition

Duck Fc alpha-mu receptor (Fc/R) binds duck IgM and IgA with cryo-EM structures at 3.27 Å for pentameric IgM and 3.57 Å for tetrameric IgA.

• Why it matters: Fc/R is an ancient Fc-receptor linked by ancestry to pIgR, but how it evolved and recognizes IgM/IgA across vertebrates is unclear.
• What they did: The study identified FCAMR expression in duck lymphoid tissues, tested recombinant apFc/R ectodomain binding to apIgM/apIgA, and solved cryo-EM complexes plus a mmFc/R–mmIgM structure.
• The result: apFc/R affinity was ~10-fold higher for apIgM than apIgA and ~10-fold higher than mouse Fc/R, and both complexes show three Ig-like domains (D1–D3) binding asymmetrically with 1:1 stoichiometry.

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