RESIDUES CRITICAL FOR H-L DISULFIDE BOND FORMATION IN HUMAN IGA1 AND IGA2

There are two subclasses of human IgA, IgA1 and IgA2, lgA2 exists as t wo known allotypes, lgA2 m(1) and IgA2 m(2) with a recently reported n ovel lgA2 (IgA2(n)) possibly representing a third allotype. The varian ts of human IgA differ in their H and L chain disulfide-bonding patter n; in IgA1, IgA2(n), and IgA2 m(2), a disulfide bond connects a cystei ne residue in C(H)1 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 cha ins. Here we have used site-directed mutagenesis to demonstrate that C ys133 is essential for the formation of the H-L disulfide bond in IgA1 , However, lgA2 m(2) and the IgA2(n) but not lgA2 m(1) form an H-L dis ulfide in the absence of Cys133, lgA2 m(1) differs from IgA2 m(2) and the IgA2(n) at two positions in C(H)1; 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 lgA2 m(1) that inter feres with the H-L disulfide in the absence of Cys133, Contrary to wha t has been previously reported, protein purified from culture supernat ants of IgA2 m(1) show some HL, H(2)L(2), and H(4)L(4)J, suggesting th at IgA2 m(1) can exist either as a form lacking H-L disulfide bonds or as a form with H-L disulfides.