Effect of interchain disulfide bond on hapten binding properties of light chain dimer of protein 315.

The hapten binding characteristics of the covalent light chain dimer, derived from the murine IgA secreted by plasmacytoma MOPC-315, to two nitroaromatic compounds, epsilon-N-(2,4-dinitrophenyl)-L-lysine and 4-(alpha-N-alanine)-m-nitrobenz-2-oxa-1,3-diazole, were investigated by differential spectroscopic titrations. The binding curves for both haptens were found to display sigmoidity similar to that reported earlier for the reduced and alkylated dimer held together by noncovalent bonds only. However, the presence of the interchain disulfide bond in the covalent dimer was found to cause marked changes in its binding properties. The data, like those obtained for the noncovalent dimer, fit the allosteric model of Monod, Wyman, and Changeux in which binding of the first hapten to the dimer causes a conversion of both sites of the protein molecule from a lower to a higher affinity conformation. However, the binding parameters show that both the affinity and the positive cooperativity in the interaction between haptens and the covalent dimer are significantly enhanced. The differences in the parameters of the binding and of the allosteric transition caused by the presence of the interchain disulfide bond demonstrate the existence of longitudinal interactions in immunoglobulin derivatives. These properties of the light chain dimer make it a potential model for the receptors present on thymus-derived lymphocytes.