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Mechanism of muscle contraction based on stochastic properties of single actomyosin motors observed in vitro

View Article: PubMed Central - PubMed

ABSTRACT

We have previously measured the process of displacement generation by a single head of muscle myosin (S1) using scanning probe nanometry. Given that the myosin head was rigidly attached to a fairly large scanning probe, it was assumed to stably interact with an underlying actin filament without diffusing away as would be the case in muscle. The myosin head has been shown to step back and forth stochastically along an actin filament with actin monomer repeats of 5.5 nm and to produce a net movement in the forward direction. The myosin head underwent 5 forward steps to produce a maximum displacement of 30 nm per ATP at low load (<1 pN). Here, we measured the steps over a wide range of forces up to 4 pN. The size of the steps (∼5.5 nm) did not change as the load increased whereas the number of steps per displacement and the stepping rate both decreased. The rate of the 5.5-nm steps at various force levels produced a force-velocity curve of individual actomyosin motors. The force-velocity curve from the individual myosin heads was comparable to that reported in muscle, suggesting that the fundamental mechanical properties in muscle are basically due to the intrinsic stochastic nature of individual actomyosin motors. In order to explain multiple stochastic steps, we propose a model arguing that the thermally-driven step of a myosin head is biased in the forward direction by a potential slope along the actin helical pitch resulting from steric compatibility between the binding sites of actin and a myosin head. Furthermore, computer simulations show that multiple cooperating heads undergoing stochastic steps generate a long (>60 nm) sliding distance per ATP between actin and myosin filaments, i.e., the movement is loosely coupled to the ATPase cycle as observed in muscle.

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Stepping model based on preferential landing of the myosin head. (A) Potential slope along the actin half helical pitch due to the steric compatibility between the orientations of the binding sites of actin and the myosin head (see text for detail). (B) Mechanochemical coupling for the conventional model. The myosin head undergoes rapid attachment-detachment cycles with actin after ATP hydrolysis and then swings its neck domain (lever arm) to perform a powerstroke, coupled to Pi release77. (C) Mechanochemical coupling for the present model. The myosin head undergoes steps in the forward direction during attachment-detachment cycles. Coupled to Pi release, the head rotates the actin filamen60–64 (see Fig. 11) and stops the step, and isometric force may be generated7,54.
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f10-1_1: Stepping model based on preferential landing of the myosin head. (A) Potential slope along the actin half helical pitch due to the steric compatibility between the orientations of the binding sites of actin and the myosin head (see text for detail). (B) Mechanochemical coupling for the conventional model. The myosin head undergoes rapid attachment-detachment cycles with actin after ATP hydrolysis and then swings its neck domain (lever arm) to perform a powerstroke, coupled to Pi release77. (C) Mechanochemical coupling for the present model. The myosin head undergoes steps in the forward direction during attachment-detachment cycles. Coupled to Pi release, the head rotates the actin filamen60–64 (see Fig. 11) and stops the step, and isometric force may be generated7,54.

Mentions: How do the myosin steps define the potential? So far, several models have been proposed33,55,56. Here, we propose a simple model assuming more realistic situations in which the potential slope is produced by a steric compatibility between the orientations of the binding sites of actin and the myosin head. The actin filament has a double helical structure and the protofilament contains 7 monomers and rotates by 180° per half helical pitch. The tail (neck domain) of the myosin head is not rigid, so the myosin head attached to the probe can move along the actin helical pitch. However, the binding sites of actin monomers rotate along the helix relative to the myosin head attached to the probe and hence the steric compatibility between the orientations of the binding sites of the myosin head and the actin should change depending on their relative positions. Thus, this steric compatibility should result in a potential slope along the actin helical pitch. If the binding site of the head faces the right side of the actin filament fixed on a glass surface, the head could favorably bind to the actin on the right side of the filament but it would be unfavorable for the head to bind to the other sides (up and left sides) of the actin filament because the head would be required to bend and rotate (Fig. 10A, Upper). Thus, the potential slope that declines along the forward direction is produced along the half helical pitch (Fig. 10A, Lower). The idea of preferential landing was originally proposed by A. F. Huxley57 while the idea of preferential landing due to such a steric effect has been also argued for myosin55,58 (Iwaki et al., personal communication) and kinesin (Taniguchi et al., personal communication).


Mechanism of muscle contraction based on stochastic properties of single actomyosin motors observed in vitro
Stepping model based on preferential landing of the myosin head. (A) Potential slope along the actin half helical pitch due to the steric compatibility between the orientations of the binding sites of actin and the myosin head (see text for detail). (B) Mechanochemical coupling for the conventional model. The myosin head undergoes rapid attachment-detachment cycles with actin after ATP hydrolysis and then swings its neck domain (lever arm) to perform a powerstroke, coupled to Pi release77. (C) Mechanochemical coupling for the present model. The myosin head undergoes steps in the forward direction during attachment-detachment cycles. Coupled to Pi release, the head rotates the actin filamen60–64 (see Fig. 11) and stops the step, and isometric force may be generated7,54.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036627&req=5

f10-1_1: Stepping model based on preferential landing of the myosin head. (A) Potential slope along the actin half helical pitch due to the steric compatibility between the orientations of the binding sites of actin and the myosin head (see text for detail). (B) Mechanochemical coupling for the conventional model. The myosin head undergoes rapid attachment-detachment cycles with actin after ATP hydrolysis and then swings its neck domain (lever arm) to perform a powerstroke, coupled to Pi release77. (C) Mechanochemical coupling for the present model. The myosin head undergoes steps in the forward direction during attachment-detachment cycles. Coupled to Pi release, the head rotates the actin filamen60–64 (see Fig. 11) and stops the step, and isometric force may be generated7,54.
Mentions: How do the myosin steps define the potential? So far, several models have been proposed33,55,56. Here, we propose a simple model assuming more realistic situations in which the potential slope is produced by a steric compatibility between the orientations of the binding sites of actin and the myosin head. The actin filament has a double helical structure and the protofilament contains 7 monomers and rotates by 180° per half helical pitch. The tail (neck domain) of the myosin head is not rigid, so the myosin head attached to the probe can move along the actin helical pitch. However, the binding sites of actin monomers rotate along the helix relative to the myosin head attached to the probe and hence the steric compatibility between the orientations of the binding sites of the myosin head and the actin should change depending on their relative positions. Thus, this steric compatibility should result in a potential slope along the actin helical pitch. If the binding site of the head faces the right side of the actin filament fixed on a glass surface, the head could favorably bind to the actin on the right side of the filament but it would be unfavorable for the head to bind to the other sides (up and left sides) of the actin filament because the head would be required to bend and rotate (Fig. 10A, Upper). Thus, the potential slope that declines along the forward direction is produced along the half helical pitch (Fig. 10A, Lower). The idea of preferential landing was originally proposed by A. F. Huxley57 while the idea of preferential landing due to such a steric effect has been also argued for myosin55,58 (Iwaki et al., personal communication) and kinesin (Taniguchi et al., personal communication).

View Article: PubMed Central - PubMed

ABSTRACT

We have previously measured the process of displacement generation by a single head of muscle myosin (S1) using scanning probe nanometry. Given that the myosin head was rigidly attached to a fairly large scanning probe, it was assumed to stably interact with an underlying actin filament without diffusing away as would be the case in muscle. The myosin head has been shown to step back and forth stochastically along an actin filament with actin monomer repeats of 5.5 nm and to produce a net movement in the forward direction. The myosin head underwent 5 forward steps to produce a maximum displacement of 30 nm per ATP at low load (<1 pN). Here, we measured the steps over a wide range of forces up to 4 pN. The size of the steps (∼5.5 nm) did not change as the load increased whereas the number of steps per displacement and the stepping rate both decreased. The rate of the 5.5-nm steps at various force levels produced a force-velocity curve of individual actomyosin motors. The force-velocity curve from the individual myosin heads was comparable to that reported in muscle, suggesting that the fundamental mechanical properties in muscle are basically due to the intrinsic stochastic nature of individual actomyosin motors. In order to explain multiple stochastic steps, we propose a model arguing that the thermally-driven step of a myosin head is biased in the forward direction by a potential slope along the actin helical pitch resulting from steric compatibility between the binding sites of actin and a myosin head. Furthermore, computer simulations show that multiple cooperating heads undergoing stochastic steps generate a long (>60 nm) sliding distance per ATP between actin and myosin filaments, i.e., the movement is loosely coupled to the ATPase cycle as observed in muscle.

No MeSH data available.


Related in: MedlinePlus