Abstract
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.
Highlights
Despitelittle sequence homology [21, 22], the secondary structurepackingineach of theLIV-BPdomains is very similar to that found in the corresponding domains of the bilobate ABP (Fig. 3b) and D-galactose-binding protein
Since the LIV-BP crystals used in the present structural analysis are devoid of bound leucine, difference
Note the wide cleft between thetwo domains in LIV-BP and the narrow cleft in ABP. a, contoured difference Fourier from a crystal soaked in50 mM L-leucine to locate the binding site is superimposed on the LIV-BP trace; only one major site is apparent. b, located in the cleft of the ABP crystal structure is a bound L-arabinose molecule
Summary
3.0-A Structure of Leucine, Isoleucine, Valine-binding Protein Despitelittle sequence homology [21, 22], the secondary structurepackingineach of theLIV-BPdomains is very similar to that found in the corresponding domains of the bilobate ABP (Fig. 3b) and D-galactose-binding protein. Since the LIV-BP crystals used in the present structural analysis are devoid of bound leucine (see above), difference
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