title: Calcium-dependent distance changes in binary and ternary complexes of troponin.
abstract: Calcium-dependent distance changes have been determined by resonance energy transfer in binary and ternary troponin complexes in order to collect evidence for the structural rearrangements which are part of the hypothetical trigger mechanism of skeletal muscle contraction. Donor and acceptor fluorophores were either intrinsic tryptophans in subunits with a favourable sequence from different species, quasi-intrinsic Tb3+ ions bound to troponin C or extrinsic labels attached to specific cysteine or methionine residues. All chemically modified subunits proved fully active in conferring calcium sensitivity onto myosin ATPase. Nine distances were determined between five sites which allowed construction of a three-dimensional lattice representing the spatial distribution of four sites in the ternary complex of troponin C, I and T. Distances in binary complexes were nearly unaltered upon addition of the third subunit. Regulatory calcium binding caused distance changes of the order of 0.7-1.1 nm. In view of the large displacements of the hypothetical mechanism, they turned out to be smaller than anticipated. The fluorophoric sites selected may be localized in a zone of the troponin complex which happens to be relatively little affected by the mechanism. Alternatively, amplification of the moderate changes seen here would require the complete set of thin filament proteins.
authors.affiliation: Lehr- und Forschungsgebiet Struktur und Funktion der Proteine, Institut für Biochemie, Rheinisch-Westfälische Technische Hochschule Aachen, Federal Republic of Germany.
authors.forename: H D
authors.initials: HD
authors.lastname: Schulzki
authors.forename: B
authors.initials: B
authors.lastname: Kramer
authors.forename: J
authors.initials: J
authors.lastname: Fleischhauer
authors.forename: D A
authors.initials: DA
authors.lastname: Mercola
authors.forename: A
authors.initials: A
authors.lastname: Wollmer
mesh_terms: D000818:Animals; D017317:Annexin A6; D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology*; D002135:Calcium-Binding Proteins / Q000378:metabolism; D003198:Computer Simulation; D004735:Energy Transfer; D008433:Mathematics; D008958:Models, Molecular; D009119:Muscle Contraction / Q000187:drug effects; D009132:Muscles / Q000378:metabolism; D011487:Protein Conformation; D011817:Rabbits; D013050:Spectrometry, Fluorescence; D014336:Troponin / Q000032:analysis / Q000378:metabolism*; D014422:Turkeys
chemical_list: D017317:Annexin A6; D002135:Calcium-Binding Proteins; D014336:Troponin; D002118:Calcium