Residues critical for H-L disulfide bond formation in human IgA1 and IgA2

There are two subclasses of human IgA, IgA1 and IgA2. IgA2 exists as two known allotypes, IgA2 m(1) and IgA2 m(2) with a recently reported novel IgA2 (IgA2(n)) possibly representing a third allotype. The variants of human IgA differ in their H and L chain disulfide-bonding pattern; in IgA1, IgA2(n), and IgA2 m(2), a disulfide bond connects a cysteine residue in CH1 of the H chain with the L chains while human IgA2 m(1) has been reported to lack a covalent bond between the H and L chains. Here we have used site-directed mutagenesis to demonstrate that Cys133 is essential for the formation of the H-L disulfide bond in IgA1. However, IgA2 m(2) and the IgA2(n) but not IgA2 m(1) form an H-L disulfide in the absence of Cys133. IgA2 m(1) differs from IgA2 m(2) and the IgA2(n) at two positions in CH1; IgA2 m(1) has Pro212 and Pro221 whereas IgA2 m(2) and the IgA2(n) have Ser212 and Arg221. Our studies demonstrate that it is the presence of Pro221 in IgA2 m(1) that interferes with the H-L disulfide in the absence of Cys133. Contrary to what has been previously reported, protein purified from culture supernatants of IgA2 m(1) show some HL, H2L2, and H4L4J, suggesting that IgA2 m(1) can exist either as a form lacking H-L disulfide bonds or as a form with H-L disulfides.