Limits...
X-ray fiber diffraction modeling of structural changes of the thin filament upon activation of live vertebrate skeletal muscles

View Article: PubMed Central - PubMed

ABSTRACT

In order to clarify the structural changes of the thin filaments related to the regulation mechanism in skeletal muscle contraction, the intensities of thin filament-based reflections in the X-ray fiber diffraction patterns from live frog skeletal muscles at non-filament overlap length were investigated in the relaxed state and upon activation. Modeling the structural changes of the whole thin filament due to Ca2+-activation was systematically performed using the crystallographic data of constituent molecules (actin, tropomyosin and troponin core domain) as starting points in order to determine the structural changes of the regulatory proteins and actin. The results showed that the globular core domain of troponin moved toward the filament axis by ∼6 Å and rotated by ∼16° anticlockwise (viewed from the pointed end) around the filament axis by Ca2+-binding to troponin C, and that tropomyosin together with the tail of troponin T moved azimuthally toward the inner domains of actin by ∼12° and radially by ∼7 Å from the relaxed position possibly to partially open the myosin binding region of actin. The domain structure of the actin molecule in F-actin we obtained for frog muscle thin filament was slightly different from that of the Holmes F-actin model in the relaxed state, and upon activation, all subdomains of actin moved in the direction to closing the nucleotide-binding pocket, making the actin molecule more compact. We suggest that the troponin movements and the structural changes within actin molecule upon activation are also crucial components of the regulation mechanism in addition to the steric blocking movement of tropomyosin.

No MeSH data available.


Related in: MedlinePlus

Dispositional changes of actin, tropomyosin and the troponin core domain in the best-fit models. (A) Structural change in actin. The central figure is viewed along the fiber axis (z axis), and the left and right ones are rotated by ±90° around the fiber axis. Subdomains 1, 2, 3 and 4 are colored as in Fig. 5. The top panel shows the changes from the Holmes actin in the best-fit model in the relaxed state. The bottom panel shows the changes in the transition of the relaxed state to the activated state. The direction of change of subdomains in actin is depicted by an arrow. (B) The positional change of tropomyosin (viewed from the pointed/M-line end). The left is in the relaxed state and the right is in the activated state. In each state, the radial distance of tropomyosin from the filament axis is indicated by yellow. In the activated state, the azimuthal angle change of tropomyosin around the filament axis is denoted by an arrow. (C) The dispositional and orientation changes of the troponin core domain. The left is in the relaxed state and the right is in the activated state. The top panel (“Top View”) is viewed from the pointed/M-line end, and the bottom panel (“Side View”) is viewed perpendicular to the filament axis (y axis). In the top panel, the radial distance of the troponin core domain from the filament axis is indicated by a red line. In the bottom panel, the change in the rotation angle around the y axis is denoted by a curved arrow. The color assignment for the constituent molecules is the same as in Fig. 3.
© Copyright Policy
Related In: Results  -  Collection


getmorefigures.php?uid=PMC5036664&req=5

f8-6_13: Dispositional changes of actin, tropomyosin and the troponin core domain in the best-fit models. (A) Structural change in actin. The central figure is viewed along the fiber axis (z axis), and the left and right ones are rotated by ±90° around the fiber axis. Subdomains 1, 2, 3 and 4 are colored as in Fig. 5. The top panel shows the changes from the Holmes actin in the best-fit model in the relaxed state. The bottom panel shows the changes in the transition of the relaxed state to the activated state. The direction of change of subdomains in actin is depicted by an arrow. (B) The positional change of tropomyosin (viewed from the pointed/M-line end). The left is in the relaxed state and the right is in the activated state. In each state, the radial distance of tropomyosin from the filament axis is indicated by yellow. In the activated state, the azimuthal angle change of tropomyosin around the filament axis is denoted by an arrow. (C) The dispositional and orientation changes of the troponin core domain. The left is in the relaxed state and the right is in the activated state. The top panel (“Top View”) is viewed from the pointed/M-line end, and the bottom panel (“Side View”) is viewed perpendicular to the filament axis (y axis). In the top panel, the radial distance of the troponin core domain from the filament axis is indicated by a red line. In the bottom panel, the change in the rotation angle around the y axis is denoted by a curved arrow. The color assignment for the constituent molecules is the same as in Fig. 3.

Mentions: The changes in the center of gravity of each of four subdomains were calculated from the small movements of the segments within each subdomain of actin (Fig. 8A). The positional differences of the actin subdomains in the relaxed model from their positions in the Holmes model are depicted in the upper panel in Fig. 8A. All subdomains moved toward the filament axis from the positions in the Holmes model, and subdomains 1 and 2 moved close to each other. The largest may be the motion of subdomain 3 along the x and y axes. This is the actin conformation in the thin filament of frog skeletal muscle in the relaxed state.


X-ray fiber diffraction modeling of structural changes of the thin filament upon activation of live vertebrate skeletal muscles
Dispositional changes of actin, tropomyosin and the troponin core domain in the best-fit models. (A) Structural change in actin. The central figure is viewed along the fiber axis (z axis), and the left and right ones are rotated by ±90° around the fiber axis. Subdomains 1, 2, 3 and 4 are colored as in Fig. 5. The top panel shows the changes from the Holmes actin in the best-fit model in the relaxed state. The bottom panel shows the changes in the transition of the relaxed state to the activated state. The direction of change of subdomains in actin is depicted by an arrow. (B) The positional change of tropomyosin (viewed from the pointed/M-line end). The left is in the relaxed state and the right is in the activated state. In each state, the radial distance of tropomyosin from the filament axis is indicated by yellow. In the activated state, the azimuthal angle change of tropomyosin around the filament axis is denoted by an arrow. (C) The dispositional and orientation changes of the troponin core domain. The left is in the relaxed state and the right is in the activated state. The top panel (“Top View”) is viewed from the pointed/M-line end, and the bottom panel (“Side View”) is viewed perpendicular to the filament axis (y axis). In the top panel, the radial distance of the troponin core domain from the filament axis is indicated by a red line. In the bottom panel, the change in the rotation angle around the y axis is denoted by a curved arrow. The color assignment for the constituent molecules is the same as in Fig. 3.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC5036664&req=5

f8-6_13: Dispositional changes of actin, tropomyosin and the troponin core domain in the best-fit models. (A) Structural change in actin. The central figure is viewed along the fiber axis (z axis), and the left and right ones are rotated by ±90° around the fiber axis. Subdomains 1, 2, 3 and 4 are colored as in Fig. 5. The top panel shows the changes from the Holmes actin in the best-fit model in the relaxed state. The bottom panel shows the changes in the transition of the relaxed state to the activated state. The direction of change of subdomains in actin is depicted by an arrow. (B) The positional change of tropomyosin (viewed from the pointed/M-line end). The left is in the relaxed state and the right is in the activated state. In each state, the radial distance of tropomyosin from the filament axis is indicated by yellow. In the activated state, the azimuthal angle change of tropomyosin around the filament axis is denoted by an arrow. (C) The dispositional and orientation changes of the troponin core domain. The left is in the relaxed state and the right is in the activated state. The top panel (“Top View”) is viewed from the pointed/M-line end, and the bottom panel (“Side View”) is viewed perpendicular to the filament axis (y axis). In the top panel, the radial distance of the troponin core domain from the filament axis is indicated by a red line. In the bottom panel, the change in the rotation angle around the y axis is denoted by a curved arrow. The color assignment for the constituent molecules is the same as in Fig. 3.
Mentions: The changes in the center of gravity of each of four subdomains were calculated from the small movements of the segments within each subdomain of actin (Fig. 8A). The positional differences of the actin subdomains in the relaxed model from their positions in the Holmes model are depicted in the upper panel in Fig. 8A. All subdomains moved toward the filament axis from the positions in the Holmes model, and subdomains 1 and 2 moved close to each other. The largest may be the motion of subdomain 3 along the x and y axes. This is the actin conformation in the thin filament of frog skeletal muscle in the relaxed state.

View Article: PubMed Central - PubMed

ABSTRACT

In order to clarify the structural changes of the thin filaments related to the regulation mechanism in skeletal muscle contraction, the intensities of thin filament-based reflections in the X-ray fiber diffraction patterns from live frog skeletal muscles at non-filament overlap length were investigated in the relaxed state and upon activation. Modeling the structural changes of the whole thin filament due to Ca2+-activation was systematically performed using the crystallographic data of constituent molecules (actin, tropomyosin and troponin core domain) as starting points in order to determine the structural changes of the regulatory proteins and actin. The results showed that the globular core domain of troponin moved toward the filament axis by ∼6 Å and rotated by ∼16° anticlockwise (viewed from the pointed end) around the filament axis by Ca2+-binding to troponin C, and that tropomyosin together with the tail of troponin T moved azimuthally toward the inner domains of actin by ∼12° and radially by ∼7 Å from the relaxed position possibly to partially open the myosin binding region of actin. The domain structure of the actin molecule in F-actin we obtained for frog muscle thin filament was slightly different from that of the Holmes F-actin model in the relaxed state, and upon activation, all subdomains of actin moved in the direction to closing the nucleotide-binding pocket, making the actin molecule more compact. We suggest that the troponin movements and the structural changes within actin molecule upon activation are also crucial components of the regulation mechanism in addition to the steric blocking movement of tropomyosin.

No MeSH data available.


Related in: MedlinePlus