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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.

No MeSH data available.


Histograms of the step size just before the final plateau (A) and the remaining steps at the rising phase (B). Plateaus were determined as the greatest displacement achieved before myosin detaches from actin causing the displacement to return to zero.
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f8-1_1: Histograms of the step size just before the final plateau (A) and the remaining steps at the rising phase (B). Plateaus were determined as the greatest displacement achieved before myosin detaches from actin causing the displacement to return to zero.

Mentions: It may be possible that the multiple steps observed in the rising phase of the displacement resulted from two distinct events, i.e., thermal (passive) forward and backward jumps on the discrete binding site on an actin filament and active powerstrokes. According to the commonly accepted lever-arm theory, the powerstroke would be produced when the transition from a weakly-bound to a strongly-bound state occurred. Resting on this assumption, the step just before the final plateau should be an “active” powerstroke and its size should be equal to the amplitude of the average displacement (9.2 and 13 nm at high and low needle stiffness, respectively). We measured the size of the steps which occurred just before the final plateau (Fig. 8A) and the remainder of the steps of the rising phase (Fig. 8B). The histograms were indistinguishable and the peaks in both of them were around 6 nm. Therefore, all steps in the rising phase of displacements should be equivalent. Thus, this result excludes the possibility that only the last step in a displacement is caused by an active powerstroke and others are caused by passive fluctuations due to Brownian motion.


Mechanism of muscle contraction based on stochastic properties of single actomyosin motors observed in vitro
Histograms of the step size just before the final plateau (A) and the remaining steps at the rising phase (B). Plateaus were determined as the greatest displacement achieved before myosin detaches from actin causing the displacement to return to zero.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC5036627&req=5

f8-1_1: Histograms of the step size just before the final plateau (A) and the remaining steps at the rising phase (B). Plateaus were determined as the greatest displacement achieved before myosin detaches from actin causing the displacement to return to zero.
Mentions: It may be possible that the multiple steps observed in the rising phase of the displacement resulted from two distinct events, i.e., thermal (passive) forward and backward jumps on the discrete binding site on an actin filament and active powerstrokes. According to the commonly accepted lever-arm theory, the powerstroke would be produced when the transition from a weakly-bound to a strongly-bound state occurred. Resting on this assumption, the step just before the final plateau should be an “active” powerstroke and its size should be equal to the amplitude of the average displacement (9.2 and 13 nm at high and low needle stiffness, respectively). We measured the size of the steps which occurred just before the final plateau (Fig. 8A) and the remainder of the steps of the rising phase (Fig. 8B). The histograms were indistinguishable and the peaks in both of them were around 6 nm. Therefore, all steps in the rising phase of displacements should be equivalent. Thus, this result excludes the possibility that only the last step in a displacement is caused by an active powerstroke and others are caused by passive fluctuations due to Brownian motion.

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.