An extra disulfide bridge in the constant domain of Rana catesbeiana immunoglobulin light chains

The immunoglobulins of the bullfrog Rana catesbeiana are unusual in that, in all classes, the light chains are not disulfide bonded to heavy chains or to other light chains. Moreover, the light chains contain six, rather than the usual five, residues of half-cystine. As none of these half-cystines is in the sulfhydryl form or is alkylated after mild reduction, we suggested that the light chains probably contain three intrachain disulfide bridges. We have now carried out experiments to confirm the existence of an extra intrachain disulfide bridge in Rana catesbeiana light chains and to determine its location. Disulfide bridge assignments were based on 1) isolation and sequence analysis of S-(carboxymethyl)cysteine-containing peptides and 2) isolation, from unreduced light chains, of peptides containing a disulfide bridge. Half-cystine residues were found at positions 134 and 194, and these were shown to be joined in the conserved intradomain disulfide bridge. In addition, we found that a residue of half-cystine, located at the third position from the carboxy-terminus, forms a disulfide bridge with a half-cystine at position 119, near the amino-terminus of the domain, the latter residue replacing a proline that has been found at this position in all other light chains. An intrachain disulfide bridge has not been found at this location in any other light chain.