Structural requirements for the interaction of human IgA with the human polymeric Ig receptor

Transport of polymeric IgA onto mucosal surfaces to become secretory IgA is mediated by the polymeric Ig receptor (pIgR). To study the interaction of human dimeric IgA (dIgA) (the predominant form of IgA polymer) with the human pIgR (hpIgR), we generated recombinant wild-type dIgA1 and dIgA2m(1) and various mutant dIgA1 and analyzed their interaction with a recombinant human secretory component and membrane-expressed hpIgR. We found that wild-type dIgA1 and dIgA2m(1) bound to recombinant human secretory component with similar affinity and were transcytosed by the hpIgR to the same extent. Mutation of the IgA Calpha2 domain residue Cys311 to Ser reduced binding to hpIgR, possibly through disruption of noncovalent interactions between the Calpha2 domain and domain 5 of the receptor. Within the Calpha3 domain of IgA1, we found that combined mutation of residues Phe411, Val413, and Thr414, which lie close to residues previously implicated in hpIgR binding, abolished interaction with the receptor. Mutation of residue Lys377, located very close to this same region, perturbed receptor interaction. In addition, 4 aa (Pro440-Phe443), which lie on a loop at the domain interface and form part of the binding site for human FcalphaRI, appear to contribute to hpIgR binding. Lastly, use of a monomeric IgA1 mutant lacking the tailpiece revealed that the tailpiece does not occlude hpIgR-binding residues in IgA1 monomers. This directed mutagenesis approach has thus identified motifs lying principally across the upper surface of the Calpha3 domain (i.e., that closest to Calpha2) critical for human pIgR binding and transcytosis.