A comparison of muscle thin filament models obtained from electron microscopy reconstructions and low-angle X-ray fibre diagrams from non-overlap muscle

J Struct Biol. 2006 Aug;155(2):273-84. doi: 10.1016/j.jsb.2006.02.020. Epub 2006 May 7.


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.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Extramural

MeSH terms

  • Actin Cytoskeleton / chemistry*
  • Actin Cytoskeleton / metabolism
  • Actin Cytoskeleton / ultrastructure
  • Animals
  • Calcium / chemistry
  • Calcium / metabolism
  • Computer Simulation
  • Humans
  • Microfilament Proteins / chemistry*
  • Microfilament Proteins / metabolism
  • Microfilament Proteins / ultrastructure
  • Microscopy, Electron / methods
  • Models, Biological
  • Models, Molecular
  • Muscle Contraction
  • Muscles / metabolism
  • Muscles / physiology
  • Protein Structure, Secondary
  • Protein Structure, Tertiary
  • Tropomyosin / chemistry
  • Tropomyosin / metabolism
  • Tropomyosin / ultrastructure
  • Troponin / chemistry
  • Troponin / metabolism
  • Troponin / ultrastructure
  • X-Ray Diffraction / methods


  • Microfilament Proteins
  • Tropomyosin
  • Troponin
  • Calcium