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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.

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A histogram of needle displacements caused by single S1 molecules at low needle stiffness (n=135). The solid line indicates a double Gaussian with peaks at 14 nm and −13 nm fit to the data. The standard deviation of each Gaussian was similar to that of the thermal vibration of the needle used in solution.
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f5-1_1: A histogram of needle displacements caused by single S1 molecules at low needle stiffness (n=135). The solid line indicates a double Gaussian with peaks at 14 nm and −13 nm fit to the data. The standard deviation of each Gaussian was similar to that of the thermal vibration of the needle used in solution.

Mentions: We used actin bundles formed by α-actinin, in which actin filaments might be randomly incorporated. To investigate the polarity of actin filaments in a bundle, we observed movement of myosin filaments along actin bundles. Myosin filaments moved in both directions, indicating that some actin filaments in a bundle were oriented anti-parallel to the majority of the filaments (data not shown). Furthermore, in most cases, the histogram of all displacements of an S1 molecule interacting with the same actin bundle showed a bimodal distribution (Fig. 5). The distribution was well fit to a double Gaussian distribution, with centers in the positive and negative displacement regions which were almost equidistant relative to zero displacement. Furthermore, the spread of each Gaussian distribution was similar to that of the thermal fluctuation of the free needle. Therefore, the double Gaussian distribution should be due to an S1 molecule interacting with both parallel and anti-parallel actin filaments in the bundle. Judging from the number of displacements in the positive and negative regions (Fig. 5), about 20% or less of the actin filaments were anti-parallel.


Mechanism of muscle contraction based on stochastic properties of single actomyosin motors observed in vitro
A histogram of needle displacements caused by single S1 molecules at low needle stiffness (n=135). The solid line indicates a double Gaussian with peaks at 14 nm and −13 nm fit to the data. The standard deviation of each Gaussian was similar to that of the thermal vibration of the needle used in solution.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036627&req=5

f5-1_1: A histogram of needle displacements caused by single S1 molecules at low needle stiffness (n=135). The solid line indicates a double Gaussian with peaks at 14 nm and −13 nm fit to the data. The standard deviation of each Gaussian was similar to that of the thermal vibration of the needle used in solution.
Mentions: We used actin bundles formed by α-actinin, in which actin filaments might be randomly incorporated. To investigate the polarity of actin filaments in a bundle, we observed movement of myosin filaments along actin bundles. Myosin filaments moved in both directions, indicating that some actin filaments in a bundle were oriented anti-parallel to the majority of the filaments (data not shown). Furthermore, in most cases, the histogram of all displacements of an S1 molecule interacting with the same actin bundle showed a bimodal distribution (Fig. 5). The distribution was well fit to a double Gaussian distribution, with centers in the positive and negative displacement regions which were almost equidistant relative to zero displacement. Furthermore, the spread of each Gaussian distribution was similar to that of the thermal fluctuation of the free needle. Therefore, the double Gaussian distribution should be due to an S1 molecule interacting with both parallel and anti-parallel actin filaments in the bundle. Judging from the number of displacements in the positive and negative regions (Fig. 5), about 20% or less of the actin filaments were anti-parallel.

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