title: A novel calcium binding site in the galactose-binding protein of bacterial transport and chemotaxis.
abstract: The refined 1.9-A resolution structure of the periplasmic D-galactose-binding protein (GBP) reveals a calcium ion surrounded by seven ligands, all protein oxygen atoms. A nine-residue loop (amino-acid positions 134-142), which is preceded by a beta-turn and followed by a beta-strand, provides five ligands from every second residue. The last two ligands are supplied by the carboxylate group of Glu 205. The entire GBP Ca2+-binding site adopts a conformation very similar to the site in the 'helix-loop-helix' or 'EF-hand' unit commonly found in intracellular calcium-binding proteins, but without the two helices. Structural analyses have also uncovered the sugar-binding site some 30 A from the calcium and a site for interacting with the membrane-bound trg chemotactic signal transducer approximately 45 A from the calcium. Our results show that a common tight calcium binding site of ancient origin can be tethered to different secondary structures. They also provide the first demonstration of a metal-binding site in a protein which is involved in bacterial active transport and chemotaxis.
mesh_terms: D000595:Amino Acid Sequence; D018407:Bacterial Physiological Phenomena; D001665:Binding Sites; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002135:Calcium-Binding Proteins; D002352:Carrier Proteins / Q000378:metabolism*; D002633:Chemotaxis; D005690:Galactose / Q000378:metabolism*; D008958:Models, Molecular; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; D011485:Protein Binding; D011487:Protein Conformation; D014961:X-Ray Diffraction
chemical_list: D002135:Calcium-Binding Proteins; D002352:Carrier Proteins; D009004:Monosaccharide Transport Proteins; D033902:Periplasmic Binding Proteins; C023550:galactose-binding protein; D002118:Calcium; D005690:Galactose