Interconversion of conformational isomers of light chains in the Mcg immunoglobulins

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Abstract
Previous crystallographic studies in this laboratory demonstrated that immunoglobulin [Ig] L chains with the same amino acid sequence can have at least 2 and probably 3 or more conformations, depending on whether the 2nd member of an interacting pair is a L or H chain. If a heavy chain is not available in the assembly medium, a 2nd L chain plays the structural role of the H chain in the formation of a dimer. In the present work, the .lambda.-type L chains were dissociated from the H chains of a serum IgGl from the patient Mcg and reassembled noncovalently into a dimer. The reassembly process was completed by allowing the penultimate half-cystine residues to form an interchain disulfide bond. The covalently linked dimer was compared with the Mcg urinary Bence-Jones dimer, for which an atomic model has been fitted to a 2.3-.ANG. electron density map. The assembled dimer and the native Bence-Jones protein were indistinguishable in their chromatographic and electrophoretic properties, as well as in their activity in the binding of bis(dinitrophenyl)lysine. The L chains can apparently be converted into the two types of Bence-Jones conformational isomers. The procedure was also reversed: the two Bence-Jones isomers were dissociated and reassembled as the single type of isomer associating with each of two H chains in the IgGl protein. The change in activity occurring when a L chain associates with a H chain instead of a 2nd L chain is illustrated by the fact that the Mcg IgGl immunoglobulin does not bind bis(dinitrophenyl)lysine in measurable amounts.