In humans, there are two subclasses of IgA, IgA1 and IgA2, with IgA2 existing as three allotypes, IgA2m(1), IgA2m(2) and IgA2(n). In IgA1, Cys133 in CH1 forms the disulfide bond to the L chain. Our previous studies indicated that in IgA2 lacking Cys133, a disulfide bond forms between the α-chain and the L chain when Cys220 is followed by Arg221, but not when Cys220 is followed by Pro221, suggesting that the Cys in CH1 might be involved in disulfide bonding to the L chain. However, here we show that covalent assembly of the H and L chains in IgA2(n) requires hinge-proximal Cys241 and Cys242 in CH2 and not Cys196 or Cys220 in CH1. Using pulse-chase experiments, we have demonstrated that wild-type IgA2(n) with Arg221 and Cys241 and Cys242 assembles through a disulfide-bonded HL intermediate. In contrast, the major intermediate for IgA2 m(1) with Pro221 assembly was H2 even though both Cys241 and Cys242 were present. Only a small fraction of IgA2 m(1) assembles through disulfide-bonded HL. Overall, our studies indicate that for IgA2 covalent assembly of the H and L chains requires the hinge-proximal cysteines in CH2 and that the structure of CH1 influences the efficiency with which this covalent bond forms.
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