Muscle Contraction Mechanisms: Use of Synchrotron X‐ray Diffraction


Muscle contraction occurs when the constitutive proteins, actin and myosin, interact via crossbridge formation, powered by the hydrolysis of adenosine triphosphate (ATP). It is a physical process that transduces chemical energy into mechanical work, producing directional motion. The central question is how force generation and movement in muscle contraction are associated with major conformational changes in contractile and regulatory proteins. The best available technique for such an approach is X‐ray diffraction which can provide structural information with a submolecular resolution. X‐ray diffraction with intense synchrotron radiation has demonstrated a structural basis for the molecular mechanism underlying muscle contraction with high spatial‐ and time‐resolution. The structural alterations in vertebrate skeletal muscles undergoing contraction by synchrotron X‐ray diffraction are outlined by putting a lot of weight on the thin filament as the locus of actomyosin interaction.

Key Concepts:

  • Muscle contraction occurs when constitutive proteins actin and myosin in muscle interact with each other, powered by the hydrolysis of adenosine triphosphate (ATP), leading to a force generation or a shortening of the sarcomere.

  • Sliding filament theory is a proposal on the basis of the discovery that when a sarcomere shortens, two types of filaments slide each other with little change in their lengths, resulting in an overall shortening of muscle.

  • Elastic elements sustaining the tension that muscle exerts during contraction are thought to be resided somewhere in a sarcomere. In the current hypothesis, the only elastic elements are assumed to reside somewhere around or within the myosin heads.

  • Extensibility of the thin actin filament: The force is transmitted to the ends of the contractile unit through the thin actin filaments, which have been thought to contribute very little compliance to the mechanochemical machinery. Evidence that the thin actin filaments are purely elastic under active force generation has been obtained.

  • Twisting of the thin actin filament suggests that the force acting at a myosin crossbridge contains torque components around the axis of the thin actin filament.

  • X‐ray diffraction is the interference of the X‐rays scattered from the electron densities of the matter, and the Fourier transformation of the interference pattern yields the structure of the matter.

  • Synchrotron radiations are electromagnetic waves that are emitted in the tangential direction of the orbit when electrons or positrons circulating near the speed of light in an accelerating ring (synchrotron). Synchrotron X‐rays with high brilliance are an indispensable tool for muscle structural research.

Keywords: muscle contraction; muscle regulation; thin filament; thick filament; extensibility of thin filament; twisting of thin filament; X‐ray fibre diffraction; synchrotron radiation

Figure 1.

(a) Hirarchical structure of vertebrate skeletal muscle and structure of a sarcomere. A, thin actin filament and M, thick myosin filament in a cross‐section. (b) Shortening of the sarcomere in muscle contraction. (c) A schematic diagram of the interaction of two‐headed myosin crossbridges and a thin actin filament, coupled with the hydrolysis of ATP. Reproduced from Wakabayashi et al.. With permission from the Japanese Society for Synchrotron Radiation Research.

Figure 2.

X‐ray diffraction patterns from live frog skeletal muscles taken with an image plate detector (see Wakabayashi and Amemiya, ) at the small‐angle X‐ray scattering/diffraction beamline of the Photon Factory (Japan). (a) In the resting state; (b) during an isometric contraction and (c) in the rigor state. Vertical direction is parallel to the fibre axis of muscle. M, the meridional axis; E, the equatorial axis; A2.7, the 2.7‐nm actin meridional reflection; A5.1 and A5.9, the 5.1‐ and 5.9‐nm actin layer‐line reflections, respectively and M14.4–M2.9, the myosin‐based reflections with the 3rd (14.4‐nm), 6th (7.2‐nm), 9th (4.8‐nm), 11th (3.9‐nm) and 15th (2.9‐nm) orders of the 42.9‐ or 43.5‐nm basic repeat.

Figure 3.

High‐resolution X‐ray diffraction patterns from live frog skeletal muscles taken with a CCD detector at the BioCAT beamline of the Advanced Photon Sources (USA). Exposure time, 400 ms. The left side is the pattern in the resting state and the right side is the one during an isometric contraction with the meridional axis (M) coincident. E, the equatorial axis; A2nd‐A1.37; the second actin layer line, the 5.9‐, 5.1‐ 2.9‐, 2.7‐ 1.87‐ and 1.37‐nm actin reflections, respectively; M3–M15, the myosin‐based reflections with the 3rd to 15th orders of the basic repeat.

Figure 4.

Modelling of the structure of thin actin filaments from live frog skeletal muscle using the atomic position data of constitutive molecules. (a) A resting model and (b) a contracting model, in which the constitutive molecules are represented by the space‐filling of spheres. Above, a perspective view with a pointed end (towards the M‐line in the sarcomere) upward; below, a cross‐sectional view. F‐actin is shown by blue balls, in which the subdomains (1–4) of an actin monomer as defined in the crystal structure (Holmes et al., ) are shown in red, orange, magenta and pink. Tropomyosin (TM) is shown by the two strings of white balls and the troponin (Tn) subunits are shown by cyan balls (TnC), green balls (TnI) and yellow balls (TnT). The arrows indicate the direction of movement of each actin subdomain and tropomyosin in the transition from rest to contraction; the numerical values represent a magnitude of movement (in angstrom units). The movements of TM and Tn in the transition from rest are shown by arrow in the cross‐sectional view. Reproduced from Sugimoto et al.. With permission from Biochemical and Biophysical Research Communications.

Figure 5.

Configurations of two‐headed myosin crossbridges along the thick filament backbone in the optimal models. (a) The resting model in the singlet repeating region. (b) The resting model in the triplet repeating region. (c) The contracting model in the singlet repeating region. (d) The contracting model in the triplet repeating region. (See text for the singlet and triplet repeating regions.) The z‐axis orients in the direction of the Z‐line in the sarcomere and is parallel to the filament axis. The backbone (gray) is indicated as a structureless cylinder. Reproduced from Oshima et al.. With permission from Journal of Molecular Biology.

Figure 6.

Spacing changes from the resting values of the 2.7‐nm actin‐ and the myosin‐based meridional reflections against the force (P/P0) relative to the maximum isometric force (P0). Data from the rigorised muscles as a function of different amount of the stretch (open symbols) and that obtained from the overstretched muscle on activation (red triangle) and that from resting skinned rabbit muscle at the low temperature (green double triangle) are also shown. (b) Histograms of the axial spacing changes the 2.7‐, 5.1‐ and 5.9‐nm actin‐based reflections in the transition from rest to activation of the overstretched muscle, rest to the contraction of the muscle at a full‐filament overlap length, and the contracting muscle to stretch (∼1.4 P0). Values on the histograms denote the average value. (c) Spacing changes from resting values of the 14.3‐ or 14.5‐nm‐based myosin meridional reflections (M3, M6, M9 and M15) in the same transitions as in (b).

Figure 7.

Time courses of the intensity changes of the first troponin meridional reflection (Tn1). The red circles denote the time course of Tn1 from muscles at full‐filament overlap length, and the open circle, that of Tn1 from overstretched muscles on activation. The solid curve shows the tension change, and the red horizontal bar denotes the period of stimulation. Reproduced from Sugimoto et al.. With permission from Biochemical and Biophysical Research Commnucations.

Figure 8.

Conformational change of the myosin heads (S1) in the presence of ATP. (a) Comparison of the X‐ray solution scattering curves taken from S1 samples with no nucleotide (black) and in the presence of MgATP (red). In this plot (I(S) versus SRg), the deviation of the scattering curve of S1 in ATP solution from that with no nucleotide beyond SRg=0.4 (S is a scattering vector length defined as 2sinθ/λ where 2θ is the scattering angle, λ the wavelength of X‐rays and Rg the radius of gyration of S1) reflects the distinct shape change during the hydrolysis of ATP. (b) Modelling of the conformational change of S1 in the presence of ATP by using the atomic position data of S1. When the two models coincide with their proximal domain (the catalytic domain), the distal tip of the molecule tail (a light‐chain binding domain) moves as shown by arrows (vertically (tilt) and azimuthally (twist)). In this figure, the imaginary actin filament axis is aligned vertically at the left‐hand side of the models. Dark blue, S1 no nucleotide and cyan, S1 in MgATP. (c) The radius of gyration values (Rg) of various nucleotide‐bound S1 samples (in angstrom units). The change in Rg represents the global conformational alteration. S1 in ATP denotes S1 in solution containing ATP and S1.ADP‐pPDM, S1‐bound adenosine diphosphate (ADP) crosslinked with pPDM (p‐phenylenedimaleimide). S1.ATPγS and S1.ADP denote S1‐bound ATPγS and ADP which mimic an S1‐bound ATP state and S1‐bound ADP state, respectively, in the hydrolysis cycle of ATP. S1.ADP.AlF4, S1.ADP.BeFx and S1.ADP.Vi (Vi, vanadate) denote S1‐bound ADP and various phosphate analogues which mimic the key intermediate state (M.ADP.Pi where Pi is inorganic phosphate) of the ATP hydrolysis cycle.



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Further Reading

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Wakabayashi K and Yagi N (1999) Muscle contraction: challenges for synchrotron radiation. Journal of Synchrotron Radiation 6: 875–890.

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Wakabayashi, Katsuzo, Sugimoto, Yasunobu, Takezawa, Yasunori, Oshima, Kanji, Matsuo, Tatsuhito, Ueno, Yutaka, and Irving, Thomas C(May 2010) Muscle Contraction Mechanisms: Use of Synchrotron X‐ray Diffraction. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0000675.pub2]