Mutagenesis of the human IgA1 heavy chain tailpiece that prevents dimer assembly

The structural features of the human IgA1 tailpiece required for interaction with J chain in IgA dimer assembly were investigated using a protein engineering approach. Wild-type and mutant forms of IgA1 were expressed in the mouse myeloma cell line, J558L, which endogenously expresses J chain. Wild-type IgA1 was secreted as a mixture of dimers and monomers. Deletion of the entire tailpiece by stop codon introduction completely prevented dimer formation. Similarly, substitution of the penultimate residue of the tailpiece, Cys⁴⁷¹, with serine resulted in the secretion of IgA monomers alone. Substitution of Asn⁴⁵⁹ with alanine to prevent attachment of N-linked carbohydrate to the tailpiece also resulted in markedly reduced dimer assembly. These results indicate the critical role played by the tailpiece, and Cys⁴⁷¹ in particular, in IgA dimerization. In addition, we found tailpiece-deleted IgA1 and the Cys to Ser⁴⁷¹ mutant IgA1 were secreted as mixtures of covalently associated monomers (α₂L₂) and αL half-molecules. The tailpiece may thus play some role in promoting the association of α-chains required for IgA monomer assembly.