title: The 3 A resolution structure of a D-galactose-binding protein for transport and chemotaxis in Escherichia coli.
abstract: X-ray diffraction studies of a D-galactose-binding protein essential for transport and chemotaxis in Escherichia coli have yielded a model of the polypeptide chain backbone. An initial polyalanine backbone trace was obtained at 3.2 A resolution by the molecular replacement technique, using a polyalanine search model derived from the refined structure of the L-arabinose-binding protein. Concurrently, a 3 A 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 two 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 four major hydrophobic clusters that stabilize the structure of the L-arabinose-binding protein are also present in the D-galactose-binding protein.
mesh_terms: D001426:Bacterial Proteins; D001692:Biological Transport; D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; D002633:Chemotaxis; D004926:Escherichia coli; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D001426:Bacterial Proteins; D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; C023550:galactose-binding protein