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

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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.


Potential profile for biased Brownian steps. (A) Biased Brownian steps of a myosin head along an actin filament (Upper) and asymmetric potential of the activation energy (Lower). (B) Ratio of forward and backward steps at various loads (open circles) and difference between the maximum potential barriers for forward and backward steps, Δu (filled triangles) (Supplement Movie 1).
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f9-1_1: Potential profile for biased Brownian steps. (A) Biased Brownian steps of a myosin head along an actin filament (Upper) and asymmetric potential of the activation energy (Lower). (B) Ratio of forward and backward steps at various loads (open circles) and difference between the maximum potential barriers for forward and backward steps, Δu (filled triangles) (Supplement Movie 1).

Mentions: The stepping motion of an S1 molecule was not always smooth and sometimes moved towards the opposite direction along an actin filament. The size of the steps was ∼5.5 nm for steps in both the forward and backward directions. This step size coincides with the distance between adjacent actin monomers in one strand of an actin filament (Fig. 9A). Furthermore, the number of steps ranged randomly from one to five during no more than one ATP hydrolysis cycle, i.e., the 5.5-nm steps were not tightly coupled to the ATP hydrolysis cycle. The stochastic features of this stepping motion and the step size strongly suggest that the myosin head walks or slides along the actin monomer repeat driven by Brownian motion. Because the majority of steps occurred in one direction, they should not result from pure thermal diffusion but rather be biased in one direction (forward), i.e., this process is active, not passive.


Mechanism of muscle contraction based on stochastic properties of single actomyosin motors observed in vitro
Potential profile for biased Brownian steps. (A) Biased Brownian steps of a myosin head along an actin filament (Upper) and asymmetric potential of the activation energy (Lower). (B) Ratio of forward and backward steps at various loads (open circles) and difference between the maximum potential barriers for forward and backward steps, Δu (filled triangles) (Supplement Movie 1).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036627&req=5

f9-1_1: Potential profile for biased Brownian steps. (A) Biased Brownian steps of a myosin head along an actin filament (Upper) and asymmetric potential of the activation energy (Lower). (B) Ratio of forward and backward steps at various loads (open circles) and difference between the maximum potential barriers for forward and backward steps, Δu (filled triangles) (Supplement Movie 1).
Mentions: The stepping motion of an S1 molecule was not always smooth and sometimes moved towards the opposite direction along an actin filament. The size of the steps was ∼5.5 nm for steps in both the forward and backward directions. This step size coincides with the distance between adjacent actin monomers in one strand of an actin filament (Fig. 9A). Furthermore, the number of steps ranged randomly from one to five during no more than one ATP hydrolysis cycle, i.e., the 5.5-nm steps were not tightly coupled to the ATP hydrolysis cycle. The stochastic features of this stepping motion and the step size strongly suggest that the myosin head walks or slides along the actin monomer repeat driven by Brownian motion. Because the majority of steps occurred in one direction, they should not result from pure thermal diffusion but rather be biased in one direction (forward), i.e., this process is active, not passive.

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.