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The emergence of adaptive immunity in jawed vertebrates depended on the appearance of variable immune receptors, BCRs and TCRs, which exhibit variable-J-constant (V J -C)-type Ig superfamily folds. Hitherto, however, the structures of IgV-J-IgC-type molecules had never been characterized in invertebrates, leaving the origin of BCR/TCR-type molecules unknown. Using x-ray crystallography, the structure of a V J -C2 molecule, named AmpIgV J -C2, was determined in amphioxus ( Branchiostoma floridae ). The first domain shows typical V folding, including the hydrophobic core, CDR analogs, and eight conserved residues. The second domain is a C2-type Ig superfamily domain, as defined by its short length and the absence of β-strand D- and C1-typical motifs. AmpIgV J -C2 molecules form homodimers, using "three-layer packing dimerization," as described for TCRs and BCRs. The AmpIgV J -C2 V domain harbors a diglycine motif in β-strand G and forms a β-bulge structure participating in V-V intermolecular interaction. By immunohistochemistry, AmpIgV J -C2 molecules were primarily found in mucosal tissues, whereas PCR and sequence analysis indicated considerable genetic variation at the single-gene level; these findings would be consistent with an immune function and a basic ability to adapt to binding different immune targets. Our results show a BCR/TCR-ancestral like molecule in amphioxus and help us to understand the evolution of the adaptive immune system.

Discovery and Analysis of Invertebrate IgVJ-C2 Structure from Amphioxus Provides Insight into the Evolution of the Ig Superfamily