Structural and evolutionary insights into FcαμR recognition of polymeric immunoglobulins

Cryo-EM structures show mammalian FcαμR binds polymeric IgM and IgA in 1:1 stoichiometry, with higher affinity for IgM.

• Why it matters: The molecular basis of how FcαμR recognizes both IgM and IgA has been unclear, limiting understanding of pIg receptor function and evolution.
• What they did: The study determined cryo-EM structures of mammalian FcαμR bound to IgM or IgA and compared binding interfaces with pIgR and IgM; it also tested internalization using a di-leucine–independent pathway.
• The result: FcαμR efficiently internalizes both IgM and IgA without requiring the di-leucine motif, and amphibian FcR/pIgR structures clarify evolutionary diversification of the FcR/pIgR family.

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