Kappa-chain allotypes and isotypes in the rabbit: cDNA sequences of clones encoding b9 suggest an evolutionary pathway and possible role of the interdomain disulfide bond in quantitative allotype expression.

The constant regions of rabbit kappa light chains are unusual because the sequences of the allotypic forms can differ more from each other than do some variable regions with which they associate. We report the nucleic acid sequence of a full-length cDNA clone of b9 allotype and show comparisons to available sequences of the rabbit kappa allotypes b4, b5, and bas-N4. Our analyses suggest that the primordial rabbit kappa gene encoded a bas-like sequence. They also reveal a surprising difference in the position of the variable region cysteine that forms the interdomain disulfide bond that is unique to most rabbit kappa chains. One b9 cDNA sequence lacks the usual cysteine-80 and instead encodes cysteine-108, which in three-dimensional models appears capable of forming the interdomain disulfide bond with cysteine-171 in the constant region. A partial sequence of a second b9 clone encodes both cysteine-80 and cysteine-108; the translation product of this clone could have a free reactive sulfhydryl group that might lead to an unstable nonfunctional Ig molecule. The fact that pre-B cells with b9 kappa chains do not differentiate and expand into productive Ig-producing cells with frequencies comparable to the other allotypes may be explained if a substantial proportion of the gene products have a free sulfhydryl group. Our sequence results suggest that in cells differentiating to produce kappa light chains of b9 allotype the number and location of the cysteines influence immunoglobulin expression.