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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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Stiffness during the attachment of an acto-S1. The system stiffness was measured for individual displacements and the mean vaules were plotted against the plateau levels of displacements (see Fig. 1A). The data represents mean±s.e.m. Note that the system stiffness is constant over the range of displacement observed.
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f4-1_1: Stiffness during the attachment of an acto-S1. The system stiffness was measured for individual displacements and the mean vaules were plotted against the plateau levels of displacements (see Fig. 1A). The data represents mean±s.e.m. Note that the system stiffness is constant over the range of displacement observed.

Mentions: Since the dispacement of S1 was attenuated by the system compliance, the displacement of S1 was obtained by correcting the displacement of the needles for system compliance. The displacement of S1 was calculated as DS1=Dp× Ka/(Ka−Kp), where DS1 and Dp are the displacements of an S1 and the probe, respectively, Kp is the stiffness of the probe and Ka is the stiffness during the attachment of S1 to actin30,45. Each displacement was corrected for stiffness before and during the attachment. The above equation for correction, however, is only applicable if the system stiffness is approximately linear. Previous single molecule studies, which measured the displacement through the actin filament using optical tweezers, have shown that the system stiffness is likely highly nonlinear mainly due to the compliance resulting from the linkage between the actin filament and the bead31,46. Therefore, we first determined whether the system stiffness was linear or nonlinear. Figure 4 shows one example of the stiffness during acto-S1 attachment at the dwell time between steps, plotted against the amplitude of displacement (n=43). The system stiffness changed little over the range of individual displacements from −4 to 15 nm in our measurements. This result strongly suggests a linear force-extension relationship for the observed displacement range (<20 nm). The correction factor, Ka/(Ka−Kp), was calculated to be 1.05–2.5.


Mechanism of muscle contraction based on stochastic properties of single actomyosin motors observed in vitro
Stiffness during the attachment of an acto-S1. The system stiffness was measured for individual displacements and the mean vaules were plotted against the plateau levels of displacements (see Fig. 1A). The data represents mean±s.e.m. Note that the system stiffness is constant over the range of displacement observed.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036627&req=5

f4-1_1: Stiffness during the attachment of an acto-S1. The system stiffness was measured for individual displacements and the mean vaules were plotted against the plateau levels of displacements (see Fig. 1A). The data represents mean±s.e.m. Note that the system stiffness is constant over the range of displacement observed.
Mentions: Since the dispacement of S1 was attenuated by the system compliance, the displacement of S1 was obtained by correcting the displacement of the needles for system compliance. The displacement of S1 was calculated as DS1=Dp× Ka/(Ka−Kp), where DS1 and Dp are the displacements of an S1 and the probe, respectively, Kp is the stiffness of the probe and Ka is the stiffness during the attachment of S1 to actin30,45. Each displacement was corrected for stiffness before and during the attachment. The above equation for correction, however, is only applicable if the system stiffness is approximately linear. Previous single molecule studies, which measured the displacement through the actin filament using optical tweezers, have shown that the system stiffness is likely highly nonlinear mainly due to the compliance resulting from the linkage between the actin filament and the bead31,46. Therefore, we first determined whether the system stiffness was linear or nonlinear. Figure 4 shows one example of the stiffness during acto-S1 attachment at the dwell time between steps, plotted against the amplitude of displacement (n=43). The system stiffness changed little over the range of individual displacements from −4 to 15 nm in our measurements. This result strongly suggests a linear force-extension relationship for the observed displacement range (<20 nm). The correction factor, Ka/(Ka−Kp), was calculated to be 1.05–2.5.

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 (&lt;1 pN). Here, we measured the steps over a wide range of forces up to 4 pN. The size of the steps (&sim;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 (&gt;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