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Fluctuation of actin sliding over myosin thick filaments in vitro

View Article: PubMed Central - PubMed

ABSTRACT

It is customarily thought that myosin motors act as independent force-generators in both isotonic unloaded shortening as well as isometric contraction of muscle. We tested this assumption regarding unloaded shortening, by analyzing the fluctuation of the actin sliding movement over long native thick filaments from molluscan smooth muscle in vitro. This analysis is based on the prediction that the effective diffusion coefficient of actin, a measure of the fluctuation, is proportional to the inverse of the number of myosin motors generating the sliding movement of an actin filament, hence proportional to the inverse of the actin length, when the actions of the motors are stochastic and statistically independent. Contrary to this prediction, we found the effective diffusion coefficient to be virtually independent of, and thus not proportional to, the inverse of the actin length. This result shows that the myosin motors are not independent force-generators when generating the continuous sliding movement of actin in vitro and that the sliding motion is a macroscopic manifestation of the cooperative actions of the microscopic ensemble motors.

No MeSH data available.


Related in: MedlinePlus

An example of the movement of an actin filament over a native thick filament from the molluscan smooth muscle. The lengths of the actin filament and the myosin filament were 1.8 and 48.1 μm respectively. A, Trajectory (127 positional data points) of the front tip of the actin filament in the x–y plane. The filled circles show the tip position in each video frame. The center perpendicular line is a linear regression line applied to the original data points. B, Positions of the actin tip projected onto the linear regression line in A, the longitudinal components, as a function of time. The filled line is a linear regression line applied to the longitudinal components. The slope of the line yields a value of 1.33 μm/s for the average sliding velocity.
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f2-1_45: An example of the movement of an actin filament over a native thick filament from the molluscan smooth muscle. The lengths of the actin filament and the myosin filament were 1.8 and 48.1 μm respectively. A, Trajectory (127 positional data points) of the front tip of the actin filament in the x–y plane. The filled circles show the tip position in each video frame. The center perpendicular line is a linear regression line applied to the original data points. B, Positions of the actin tip projected onto the linear regression line in A, the longitudinal components, as a function of time. The filled line is a linear regression line applied to the longitudinal components. The slope of the line yields a value of 1.33 μm/s for the average sliding velocity.

Mentions: Fig. 2A shows an example of the original positional trajectory of an actin filament sliding over a native thick filament isolated from a single bivalve animal. To eliminate any minor lateral components of the actin displacements, the trajectory was fitted to a straight line (Fig. 2A), and each position was projected perpendicularly onto the fitted line. The scatter of the lateral components appears almost random (Fig. 2A), thus indicating a satisfactory fitting. The distribution of the lateral components fits a Gaussian distribution with a standard deviation (=σlc) of 71 nm and the mean=0 (see Supplementary Figure 1A). This indicates that 95% of the lateral components fall within ±2σlc (=142 nm, approximately equal to the pixel-pixel unit distances). Therefore, the lateral components represent mostly the uncertainties of the position measurements due to digitization. The lateral components did not depend on the actin length (see Supplementary Figure 1B). This result together with the fact that the actin trajectory was fitted to a straight line indicates that actin did not wobble during its sliding movement within the accuracy of positional measurements in the present experiments. Fig. 2B shows the longitudinal displacement after the adjustments as a function of time. The slope of the fitted regression line in Fig. 2B yields a value of 1.33 μm/s for the average sliding velocity.


Fluctuation of actin sliding over myosin thick filaments in vitro
An example of the movement of an actin filament over a native thick filament from the molluscan smooth muscle. The lengths of the actin filament and the myosin filament were 1.8 and 48.1 μm respectively. A, Trajectory (127 positional data points) of the front tip of the actin filament in the x–y plane. The filled circles show the tip position in each video frame. The center perpendicular line is a linear regression line applied to the original data points. B, Positions of the actin tip projected onto the linear regression line in A, the longitudinal components, as a function of time. The filled line is a linear regression line applied to the longitudinal components. The slope of the line yields a value of 1.33 μm/s for the average sliding velocity.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5036633&req=5

f2-1_45: An example of the movement of an actin filament over a native thick filament from the molluscan smooth muscle. The lengths of the actin filament and the myosin filament were 1.8 and 48.1 μm respectively. A, Trajectory (127 positional data points) of the front tip of the actin filament in the x–y plane. The filled circles show the tip position in each video frame. The center perpendicular line is a linear regression line applied to the original data points. B, Positions of the actin tip projected onto the linear regression line in A, the longitudinal components, as a function of time. The filled line is a linear regression line applied to the longitudinal components. The slope of the line yields a value of 1.33 μm/s for the average sliding velocity.
Mentions: Fig. 2A shows an example of the original positional trajectory of an actin filament sliding over a native thick filament isolated from a single bivalve animal. To eliminate any minor lateral components of the actin displacements, the trajectory was fitted to a straight line (Fig. 2A), and each position was projected perpendicularly onto the fitted line. The scatter of the lateral components appears almost random (Fig. 2A), thus indicating a satisfactory fitting. The distribution of the lateral components fits a Gaussian distribution with a standard deviation (=σlc) of 71 nm and the mean=0 (see Supplementary Figure 1A). This indicates that 95% of the lateral components fall within ±2σlc (=142 nm, approximately equal to the pixel-pixel unit distances). Therefore, the lateral components represent mostly the uncertainties of the position measurements due to digitization. The lateral components did not depend on the actin length (see Supplementary Figure 1B). This result together with the fact that the actin trajectory was fitted to a straight line indicates that actin did not wobble during its sliding movement within the accuracy of positional measurements in the present experiments. Fig. 2B shows the longitudinal displacement after the adjustments as a function of time. The slope of the fitted regression line in Fig. 2B yields a value of 1.33 μm/s for the average sliding velocity.

View Article: PubMed Central - PubMed

ABSTRACT

It is customarily thought that myosin motors act as independent force-generators in both isotonic unloaded shortening as well as isometric contraction of muscle. We tested this assumption regarding unloaded shortening, by analyzing the fluctuation of the actin sliding movement over long native thick filaments from molluscan smooth muscle in vitro. This analysis is based on the prediction that the effective diffusion coefficient of actin, a measure of the fluctuation, is proportional to the inverse of the number of myosin motors generating the sliding movement of an actin filament, hence proportional to the inverse of the actin length, when the actions of the motors are stochastic and statistically independent. Contrary to this prediction, we found the effective diffusion coefficient to be virtually independent of, and thus not proportional to, the inverse of the actin length. This result shows that the myosin motors are not independent force-generators when generating the continuous sliding movement of actin in vitro and that the sliding motion is a macroscopic manifestation of the cooperative actions of the microscopic ensemble motors.

No MeSH data available.


Related in: MedlinePlus