title: The structure of cardiac troponin C regulatory domain with bound Cd2+ reveals a closed conformation and unique ion coordination.
abstract: The amino-terminal domain of cardiac troponin C (cNTnC) is an essential Ca(2+) sensor found in cardiomyocytes. It undergoes a conformational change upon Ca(2+) binding and transduces the signal to the rest of the troponin complex to initiate cardiac muscle contraction. Two classical EF-hand motifs (EF1 and EF2) are present in cNTnC. Under physiological conditions, only EF2 binds Ca(2+); EF1 is a vestigial site that has lost its function in binding Ca(2+) owing to amino-acid sequence changes during evolution. Proteins with EF-hand motifs are capable of binding divalent cations other than calcium. Here, the crystal structure of wild-type (WT) human cNTnC in complex with Cd(2+) is presented. The structure of Cd(2+)-bound cNTnC with the disease-related mutation L29Q, as well as a structure with the residue differences D2N, V28I, L29Q and G30D (NIQD), which have been shown to have functional importance in Ca(2+) sensing at lower temperatures in ectothermic species, have also been determined. The structures resemble the overall conformation of NMR structures of Ca(2+)-bound cNTnC, but differ significantly from a previous crystal structure of Cd(2+)-bound cNTnC in complex with deoxycholic acid. The subtle structural changes observed in the region near the mutations may play a role in the increased Ca(2+) affinity. The 1.4 Å resolution WT cNTnC structure, which is the highest resolution structure yet obtained for cardiac troponin C, reveals a Cd(2+) ion coordinated in the canonical pentagonal bipyramidal geometry in EF2 despite three residues in the loop being disordered. A Cd(2+) ion found in the vestigial ion-binding site of EF1 is coordinated in a noncanonical `distorted' octahedral geometry. A comparison of the ion coordination observed within EF-hand-containing proteins for which structures have been solved in the presence of Cd(2+) is presented. A refolded WT cNTnC structure is also presented.
authors: Department of Molecular Biology and Biochemistry, Simon Fraser University, South Science Building, 8888 University Drive, Burnaby, British Columbia V5A 1S6, Canada.
authors: Xiaolu Linda
authors: XL
authors: Zhang
authors: Glen F
authors: GF
authors: Tibbits
authors: Mark
authors: M
authors: Paetzel
mesh_terms: D020816:Amino Acid Motifs; D001665:Binding Sites; D002104:Cadmium / Q000378:metabolism* / Q000633:toxicity; D002118:Calcium / Q000378:metabolism; D018360:Crystallography, X-Ray; D003545:Cysteine / Q000737:chemistry; D006331:Heart Diseases / Q000139:chemically induced / Q000235:genetics; D008958:Models, Molecular; D009154:Mutation; D011487:Protein Conformation; D017510:Protein Folding; D017434:Protein Structure, Tertiary; D019209:Troponin C / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*
chemical_list: D019209:Troponin C; D002104:Cadmium; D003545:Cysteine; D002118:Calcium
keywords: EF-hand motifs; cadmium; calcium-binding proteins; cardiac troponin C; ion coordination