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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.


Correlation between the effective diffusion coefficient and the sliding velocity. Each point represents a data set (mean±s.e.m.) obtained with a separate animal of either M. galloprovincialis (open squares) or S. virgatus (filled circles). There are two data points at (Velocity=1.6, Dm= 0.015), which is marked by (2) in the panel. Assuming a linear correlation between the x- and y-axes, we obtained 0.60 for the linear-correlation coefficient r. The probability that a random sample of Ns (=18) uncorrelated experimental data points would yield an experimental linear-correlation coefficient as large as or larger than the observed value of /r/ was calculated from the equation of Bevington & Robinson18, and the result was <0.85%. This small value indicates that Dm and the velocity are unlikely to be uncorrelated.
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f5-1_45: Correlation between the effective diffusion coefficient and the sliding velocity. Each point represents a data set (mean±s.e.m.) obtained with a separate animal of either M. galloprovincialis (open squares) or S. virgatus (filled circles). There are two data points at (Velocity=1.6, Dm= 0.015), which is marked by (2) in the panel. Assuming a linear correlation between the x- and y-axes, we obtained 0.60 for the linear-correlation coefficient r. The probability that a random sample of Ns (=18) uncorrelated experimental data points would yield an experimental linear-correlation coefficient as large as or larger than the observed value of /r/ was calculated from the equation of Bevington & Robinson18, and the result was <0.85%. This small value indicates that Dm and the velocity are unlikely to be uncorrelated.

Mentions: We confirmed the length-independence of both the sliding velocity and effective diffusion coefficient of actin filaments, using myosin filaments isolated from 16 additional different animals (9 animals of a bivalve species, S. virgatus, and 7 animals of another bivalve species, M. galloprovincialis) (data not shown). The sliding velocity of actin filaments over myosin filaments isolated from M. galloprovincialis is faster than that over myosin filaments isolated from S. virgatus, and the effective diffusion coefficient of actin filaments over the former myosin filaments is correspondingly larger than that over the latter myosin filaments on the average (compare the data points shown by the open square marks and those shown by the filled circle marks in Fig. 5). Although the sliding velocity of the actin filaments over different myosin filaments isolated from a single animal of a bivalve species appears to be almost the same (see the small error bars for the velocity of each point in Fig. 5), the actin sliding velocity over myosin filaments varied to some extent from animal to animal even within each species of the bivalve, and the effective diffusion coefficient also varied. There thus appears to be a positive correlation between the effective diffusion coefficient and the sliding velocity of actin filaments (Fig. 5).


Fluctuation of actin sliding over myosin thick filaments in vitro
Correlation between the effective diffusion coefficient and the sliding velocity. Each point represents a data set (mean±s.e.m.) obtained with a separate animal of either M. galloprovincialis (open squares) or S. virgatus (filled circles). There are two data points at (Velocity=1.6, Dm= 0.015), which is marked by (2) in the panel. Assuming a linear correlation between the x- and y-axes, we obtained 0.60 for the linear-correlation coefficient r. The probability that a random sample of Ns (=18) uncorrelated experimental data points would yield an experimental linear-correlation coefficient as large as or larger than the observed value of /r/ was calculated from the equation of Bevington & Robinson18, and the result was <0.85%. This small value indicates that Dm and the velocity are unlikely to be uncorrelated.
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Related In: Results  -  Collection

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f5-1_45: Correlation between the effective diffusion coefficient and the sliding velocity. Each point represents a data set (mean±s.e.m.) obtained with a separate animal of either M. galloprovincialis (open squares) or S. virgatus (filled circles). There are two data points at (Velocity=1.6, Dm= 0.015), which is marked by (2) in the panel. Assuming a linear correlation between the x- and y-axes, we obtained 0.60 for the linear-correlation coefficient r. The probability that a random sample of Ns (=18) uncorrelated experimental data points would yield an experimental linear-correlation coefficient as large as or larger than the observed value of /r/ was calculated from the equation of Bevington & Robinson18, and the result was <0.85%. This small value indicates that Dm and the velocity are unlikely to be uncorrelated.
Mentions: We confirmed the length-independence of both the sliding velocity and effective diffusion coefficient of actin filaments, using myosin filaments isolated from 16 additional different animals (9 animals of a bivalve species, S. virgatus, and 7 animals of another bivalve species, M. galloprovincialis) (data not shown). The sliding velocity of actin filaments over myosin filaments isolated from M. galloprovincialis is faster than that over myosin filaments isolated from S. virgatus, and the effective diffusion coefficient of actin filaments over the former myosin filaments is correspondingly larger than that over the latter myosin filaments on the average (compare the data points shown by the open square marks and those shown by the filled circle marks in Fig. 5). Although the sliding velocity of the actin filaments over different myosin filaments isolated from a single animal of a bivalve species appears to be almost the same (see the small error bars for the velocity of each point in Fig. 5), the actin sliding velocity over myosin filaments varied to some extent from animal to animal even within each species of the bivalve, and the effective diffusion coefficient also varied. There thus appears to be a positive correlation between the effective diffusion coefficient and the sliding velocity of actin filaments (Fig. 5).

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.