Four α-chain allotypic specificities of the IgA-g subclass of rabbit IgA are controlled by allelic genes at the Cαg locus. The structural basis for three of these allotypic specificities, g74, g75, and g76, was investigated by amino terminal sequence analysis. α-Chain fragments were isolated from Fc2α fragments which were obtained by tryptic digestion of g74, g75, and g76 secretory IgA. Amino acid sequence data for the chains were obtained for 92 positions; comparison of these sequences with the sequence of human α1-chain revealed approximately 70% homology with the Cα2 domain. The results indicated that g74 α-chains were cleaved 60 residues N-terminal to the cleavage site in g75 and g76 α-chains.
By comparison of sequences of 24 residues of g75 and g76 α-chains, one allotype-related difference was observed (a cys/ser interchange at position 311). Based on homologies with the human α1-chain, a model for the arrangement of disulfide bonds in rabbit α-chains is proposed; several differences between rabbit and human are indicated, as are differences among the rabbit allotypes. The most striking difference is the apparent presence of the “extra” intradomain disulfide bond in the Cα2 domain of g74 molecules and its absence in g75 and g76 molecules.