title: Novel arrangement of immunoglobulin variable domains: X-ray crystallographic analysis of the lambda-chain dimer Bence-Jones protein Loc.
abstract: We have characterized and crystallized a human lambda I light-chain dimer, Bence-Jones protein Loc, which has variable (V) region antigenic determinants characteristic for the lambda I subgroup and constant (C) region determinants of the C lambda I gene Mcg. The crystal structure was determined to 3-A resolution; the R factor is 0.27. The angle formed by the twofold axes of the V and C domains, the "elbow bend", is 97 degrees, the smallest found so far for an antibody fragment. The antigen-binding site formed by the two V domains of the Loc light chain differs significantly from those of other immunoglobulin molecules (light-chain dimers and Fab fragments) for which X-ray crystallographic data are available. Whereas, in other antibody fragments, the V domains are related by a local twofold axis, a local twofold screw axis with a translational component of 3.5 A relates the V domains in protein Loc. In contrast to the classic antigen binding "pocket" formed by V domain interactions in the previously characterized antibody structures, the V region associations in protein Loc result in a central protrusion in the binding site, with grooves on two sides of the protrusion. The structure of protein Loc indicates that immunoglobulins are physically capable of forming a more diverse spectrum of antigen-binding sites than has been heretofore apparent. Moreover, the unusual protruding nature of the binding site may be analogous to structures required for some anti-idiotypic antibodies. Further, the complementarity-determining residues form parts of two independent grooves.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D001536:Bence Jones Protein / Q000302:isolation & purification / Q000652:urine; D006801:Humans; D007147:Immunoglobulin Light Chains; D007135:Immunoglobulin Variable Region; D007146:Immunoglobulin lambda-Chains; D008958:Models, Molecular; D009101:Multiple Myeloma / Q000276:immunology; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D007147:Immunoglobulin Light Chains; D007135:Immunoglobulin Variable Region; D007146:Immunoglobulin lambda-Chains; D001536:Bence Jones Protein