title: A comparison of muscle thin filament models obtained from electron microscopy reconstructions and low-angle X-ray fibre diagrams from non-overlap muscle.
abstract: The regulation of striated muscle contraction involves changes in the interactions of troponin and tropomyosin with actin thin filaments. In resting muscle, myosin-binding sites on actin are thought to be blocked by the coiled-coil protein tropomyosin. During muscle activation, Ca2+ binding to troponin alters the tropomyosin position on actin, resulting in cyclic actin-myosin interactions that accompany muscle contraction. Evidence for this steric regulation by troponin-tropomyosin comes from X-ray data [Haselgrove, J.C., 1972. X-ray evidence for a conformational change in the actin-containing filaments of verterbrate striated muscle. Cold Spring Habor Symp. Quant. Biol. 37, 341-352; Huxley, H.E., 1972. Structural changes in actin and myosin-containing filaments during contraction. Cold Spring Habor Symp. Quant. Biol. 37, 361-376; Parry, D.A., Squire, J.M., 1973. Structural role of tropomyosin in muscle regulation: analysis of the X-ray diffraction patterns from relaxed and contracting muscles. J. Mol. Biol. 75, 33-55] and electron microscope (EM) data [Spudich, J.A., Huxley, H.E., Finch, J., 1972. Regulation of skeletal muscle contraction. II. Structural studies of the interaction of the tropomyosin-troponin complex with actin. J. Mol. Biol. 72, 619-632; O'Brien, E.J., Gillis, J.M., Couch, J., 1975. Symmetry and molecular arrangement in paracrystals of reconstituted muscle thin filaments. J. Mol. Biol. 99, 461-475; Lehman, W., Craig, R., Vibert, P., 1994. Ca2+-induced tropomyosin movement in Limulus thin filaments revealed by three-dimensional reconstruction. Nature 368, 65-67] each with its own particular strengths and limitations. Here we bring together some of the latest information from EM analysis of single thin filaments from Pirani et al. [Pirani, A., Xu, C., Hatch, V., Craig, R., Tobacman, L.S., Lehman, W. (2005). Single particle analysis of relaxed and activated muscle thin filaments. J. Mol. Biol. 346, 761-772], with synchrotron X-ray data from non-overlapped muscle fibres to refine the models of the striated muscle thin filament. This was done by incorporating current atomic-resolution structures of actin, tropomyosin, troponin and myosin subfragment-1. Fitting these atomic coordinates to EM reconstructions, we present atomic models of the thin filament that are entirely consistent with a steric regulatory mechanism. Furthermore, fitting the atomic models against diffraction data from skinned muscle fibres, stretched to non-overlap to preclude crossbridge binding, produced very similar results, including a large Ca2+-induced shift in tropomyosin azimuthal location but little change in the actin structure or apparent alteration in troponin position.
authors.affiliation: Arbeitsgruppe Biophysik, Max Planck Institut für medizinische Forschung, 69120 Heidelberg, Germany.
authors.forename: Katrina J V
authors.initials: KJ
authors.lastname: Poole
authors.forename: Michael
authors.initials: M
authors.lastname: Lorenz
authors.forename: Gwyndaf
authors.initials: G
authors.lastname: Evans
authors.forename: Gerd
authors.initials: G
authors.lastname: Rosenbaum
authors.forename: Alnoor
authors.initials: A
authors.lastname: Pirani
authors.forename: Roger
authors.initials: R
authors.lastname: Craig
authors.forename: Larry S
authors.initials: LS
authors.lastname: Tobacman
authors.forename: William
authors.initials: W
authors.lastname: Lehman
authors.forename: Kenneth C
authors.initials: KC
authors.lastname: Holmes
mesh_terms: D008841:Actin Cytoskeleton / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure; D000818:Animals; D002118:Calcium / Q000737:chemistry / Q000378:metabolism; D003198:Computer Simulation; D006801:Humans; D008840:Microfilament Proteins / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure; D008854:Microscopy, Electron / Q000379:methods; D008954:Models, Biological; D008958:Models, Molecular; D009119:Muscle Contraction; D009132:Muscles / Q000378:metabolism / Q000502:physiology; D017433:Protein Structure, Secondary; D017434:Protein Structure, Tertiary; D014335:Tropomyosin / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure; D014336:Troponin / Q000737:chemistry / Q000378:metabolism / Q000648:ultrastructure; D014961:X-Ray Diffraction / Q000379:methods
chemical_list: D008840:Microfilament Proteins; D014335:Tropomyosin; D014336:Troponin; D002118:Calcium