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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.

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Comparison of the calculated intensities from the best-fit models and the observed ones of the three troponin-associated meridional reflections with the repeat of 384 Å. The observed data were measured from the highly orientated sols of native thin filaments (Oda, unpublished data). (A) The relaxed state/in the absence of Ca2+ ions and (B) the activated state/in the presence of Ca2+ ions. The intensity normalization is made so that the total intensities of the three reflections in each state are indentical.
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f9-6_13: Comparison of the calculated intensities from the best-fit models and the observed ones of the three troponin-associated meridional reflections with the repeat of 384 Å. The observed data were measured from the highly orientated sols of native thin filaments (Oda, unpublished data). (A) The relaxed state/in the absence of Ca2+ ions and (B) the activated state/in the presence of Ca2+ ions. The intensity normalization is made so that the total intensities of the three reflections in each state are indentical.

Mentions: We calculated the intensity profiles of the meridional reflections with a repeat of 384 Å from the troponin structures in the best-fit models in both states and compared with those observed in the X-ray diffraction patterns from the oriented sols of the native thin filaments in the absence and presence of Ca2+ ions (Oda, unpublishied data) using a method developed by Makino, Oda and Maeda (manuscript in preparation). Figure 9 shows the comparison of the intensity profiles of the first to the third order troponin-associated meridional reflections. Reasonable fits to the radial profiles were obtained in both states but the determination of the whole troponin structure is needed for further refinement.


X-ray fiber diffraction modeling of structural changes of the thin filament upon activation of live vertebrate skeletal muscles
Comparison of the calculated intensities from the best-fit models and the observed ones of the three troponin-associated meridional reflections with the repeat of 384 Å. The observed data were measured from the highly orientated sols of native thin filaments (Oda, unpublished data). (A) The relaxed state/in the absence of Ca2+ ions and (B) the activated state/in the presence of Ca2+ ions. The intensity normalization is made so that the total intensities of the three reflections in each state are indentical.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036664&req=5

f9-6_13: Comparison of the calculated intensities from the best-fit models and the observed ones of the three troponin-associated meridional reflections with the repeat of 384 Å. The observed data were measured from the highly orientated sols of native thin filaments (Oda, unpublished data). (A) The relaxed state/in the absence of Ca2+ ions and (B) the activated state/in the presence of Ca2+ ions. The intensity normalization is made so that the total intensities of the three reflections in each state are indentical.
Mentions: We calculated the intensity profiles of the meridional reflections with a repeat of 384 Å from the troponin structures in the best-fit models in both states and compared with those observed in the X-ray diffraction patterns from the oriented sols of the native thin filaments in the absence and presence of Ca2+ ions (Oda, unpublishied data) using a method developed by Makino, Oda and Maeda (manuscript in preparation). Figure 9 shows the comparison of the intensity profiles of the first to the third order troponin-associated meridional reflections. Reasonable fits to the radial profiles were obtained in both states but the determination of the whole troponin structure is needed for further refinement.

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