Crystal and Molecular Structure of a Dimer Composed of the Variable Portions of the Bence-Jones Protein REI

Abstract

The structure of the variable part of a χ-type Bence-Jones protein RE1 has been determined at a resolution of 0.28 nm. It forms a dimer in the crystal related by a local diad, held together by hydrogen bonding interactions of residues Tyr-36, Gln-38, Ala-43, Pro-44, Tyr-87, Gln-89 and Phe-98, which are largely conserved in light chains. The structure consists of two hydrogen-bonded sheets covering a hydrophobic interior made up of mostly conserved amino-acid side chains. Approximately 50% of the residues form β-pleated sheets. Several of the pleated sheet strands are connected by hair-pin bends, which contain glycine residues conserved in most light chains. The arrangement of the hypervariable segments especially in comparison with the Fab structure suggests that the V dimers form a primitive antibody. The folding of the poly-peptide chain and the spatial relationship of the two monomers appear to be the same as in the λ-type Bence-Jones protein dimer and the Fab fragment.