title: Structure of a sarcoplasmic calcium-binding protein from Nereis diversicolor refined at 2.0 A resolution.
abstract: The crystal structure of a sarcoplasmic Ca(2+)-binding protein (SCP) from the sandworm Nereis diversicolor has been determined and refined at 2.0 A resolution using restrained least-squares techniques. The two molecules in the crystallographic asymmetric unit, which are related by a non-crystallographic 2-fold axis, were refined independently. The refined model includes all 174 residues and three calcium ions for each molecule, as well as 213 water molecules. The root-mean-square difference in co-ordinates for backbone atoms and calcium ions of the two molecules is 0.51 A. The final crystallographic R-factor, based on 18,959 reflections in the range 2.0 A less than or equal to d less than or equal to 7.0 A, with intensities exceeding 2.0 sigma, is 0.182. Bond lengths and bond angles in the molecules have root-mean-square deviations from ideal values of 0.013 A and 2.2 degrees, respectively. SCP has four distinct domains with the typical helix-loop-helix (EF-hand) Ca(2+)-binding motif, although the second Ca(2+)-binding domain is not functional due to amino acid changes in the loop. The structure shows several unique features compared to other Ca(2+)-binding proteins with four EF-hand domains. The overall structure is highly compact and globular with a predominant hydrophobic core, unlike the extended dumbbell-shaped structure of calmodulin or troponin C. A hydrophobic tail at the COOH terminus adds to the structural stability by packing against a hydrophobic pocket created by the folding of the NH2 and COOH-terminal Ca(2+)-binding domain pairs. The first and second domains show different helix-packing arrangements from any previously described for Ca(2+)-binding proteins.
authors.affiliation: Department of Biochemistry, University of Alabama, Birmingham.
authors.forename: S
authors.initials: S
authors.lastname: Vijay-Kumar
authors.forename: W J
authors.initials: WJ
authors.lastname: Cook
mesh_terms: D000818:Animals; D000845:Annelida; D002118:Calcium / Q000737:chemistry; D002135:Calcium-Binding Proteins / Q000737:chemistry*; D006860:Hydrogen Bonding; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry; D011487:Protein Conformation; D012519:Sarcoplasmic Reticulum / Q000737:chemistry*; D012997:Solvents; D013696:Temperature; D014961:X-Ray Diffraction
chemical_list: D002135:Calcium-Binding Proteins; D012997:Solvents; D010100:Oxygen; D002118:Calcium