Leucine, isoleucine, valine-binding protein from Escherichia coli. Structure at 3.0-A resolution and location of the binding site.

The structure of the leucine, isoleucine, valine-binding protein, an integral part of the high-affinity, branched-chain aliphatic amino acid transport system in Escherichia coli, has been solved at 3.0-A resolution by x-ray crystallography. Five isomorphous heavy atom derivatives, including anomalous differences from a samarium derivative, were used. A model of the polypeptide chain backbone reveals two distinct, globular domains connected by three strands. Each domain consists of a beta-sheet core flanked by at least two helices on either side. Difference Fourier analyses of crystals soaked in L-leucine, L-isoleucine, or L-valine have located a single amino acid-binding site in the cleft formed by the two domains. Despite the lack of significant sequence homology, the bilobate and secondary structure observed were similar to that found in the structures of L-arabinose- and D-galactose-binding proteins previously determined in our laboratory.