MUTAGENESIS OF THE HUMAN IGA1 HEAVY-CHAIN TAILPIECE THAT PREVENTS DIMER ASSEMBLY

The structural features of the human IgA1 tailpiece required for inter action with J chain in IgA dimer assembly were investigated using a pr otein engineering approach. Wild-type and mutant forms of IgA1 were ex pressed in the mouse myeloma cell line, J558L, which endogenously expr esses 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 p enultimate residue of the tailpiece, Cys(471), with serine resulted in the secretion of IgA monomers alone. Substitution of Asn(459) with al anine to prevent attachment of N-linked carbohydrate to the tailpiece also resulted in markedly reduced dimer assembly. These results indica te the critical role played by the tailpiece, and Cys(471) in particul ar, in IgA dimerization. In addition, we found tailpiece-deleted IgA1 and the Cys to Ser(471) mutant IgA1 were secreted as mixtures of coval ently associated monomers (alpha(2)L(2)) and at half-molecules. The ta ilpiece may thus play some role in promoting the association of alpha- chains required for IgA monomer assembly.