Synthesis and binding properties of hybrid immunoglobulin light chain dimers.

Bence-Jones proteins are dimers of identical immunoglobulin light chains held together by noncovalent interactions and a single interchain disulfide bond. They are structurally and functionally similar to the antigen-binding regions of antibodies and thus provide a simplified model system for studying the molecular basis of antigen binding specificity. A wealth of structural information is available for light chains including many primary sequences and several complete X-ray structures. As an approach to identifying the structural determinants of antibody specificity, a method was devised for the construction of hybrid light chain dimers. Initial attempts confirmed the earlier results of other investigators which demonstrated the reluctance of light chains to form heterodimers. When mixtures of two different noncovalent light chain dimers were denatured in propionic acid and then renatured, hybrid molecules were formed in yields of only about 10%. An alternative approach considered the formation of hybrids as a problem in the synthesis of a mixed disulfide. Accordingly, the interchain disulfide bond of one parental species was converted to thiols by reduction. The equivalent bond of a second species was converted to S-sulfo derivatives by reaction with sodium sulfite in the presence of catalytic quantities of 5,5'-dithiobis-(2-nitrobenzoic acid). After removal of excess reagents by gel filtration the two species were mixed under nondissociating conditions in the absence of atomospheric oxygen. When heterodimers formed by subunit interchange, the S-sulfo and thiol groups reacted to form a hybrid interchain disulfide bond. By this method twelve different hybrid dimers have been produced. In the twelve cases tested no light chains failed to hybridize and all hybrids were produced in yields approaching 100%. The hybridization reaction displays second order kinetics with rate constants from 0.4 to 18 M('-1) sec('-1) for different pairs of light chains. Equilibrium dialysis experiments with a hapten-like molecule (bis(DNP)lysine) confirmed the presence of a functional binding site in the hybrids. Several molecules showed the capacity to adopt two alternative binding conformations adding to available evidence for conformational flexibility in immunoglobulins. Several hybrid molecules have been crystallized and X-ray diffraction analyses are in progress.