The 3 A resolution structure of a D-galactose-binding protein for transport and chemotaxis in Escherichia coli.

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Abstract
X-ray diffraction studies of a D-galactose-binding protein essential for transport and chemotaxis in E. coli yielded a model of the polypeptide chain backbone. An initial polyalanine backbone trace was obtained at 3.2 .ANG. resolution by the molecular replacement technique, using a polyalanine search model derived from the refined structure of the L-arabinose-binding protein. A 3 .ANG. resolution electron-density map of the D-galactose receptor was determined from multiple isomorphous replacement (MIR) phases. The properly transformed initial polyalanine model superimposed on the MIR electron-density map proved to be an excellent guide in obtaining a final trace. The few changes made in the polyalanine model to improve the fit to the density were confined primarily to the COOH-terminal peptide and some loops connecting the elements of the secondary structure. Despite the lack of significant sequence homology, the overall course of the polypeptide backbone of the D-galactose-binding protein is remarkably similar to that of the L-arabinose-binding protein, the first structure in a series to be solved from this family of binding proteins. Both structures are elongated (axial ratios of 2:1) and composed of 2 globular domains. For both proteins, the arrangements of the elements of the secondary structure in both domains are identical; both lobes contain a core of .beta.-pleated sheet with a pair of helices on either side of the plane of the sheet. The 4 major hydrophobic clusters that stabilize the structure of the L-arabinose-binding protein are also present in the D-galactose-binding protein.