Fluctuation of actin sliding over myosin thick filaments in vitro

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.

Direction and speed of microtubule movements driven by kinesin motors arranged on catchin thick filaments

“…Their orientation is, however, important for proper interpretation of such experiments [Sekimoto and Tawada, 1995;Imafuku et al, 1996]. Long thick filaments of bivalve muscles were used to overcome this problem, and it was concluded that some cooperative coordination exists between many motors on thick filaments in the actomyosin system [Noda et al, 2005]. Similarly, the present experimental system can be used for the kinesinmicrotubule motility system.…”

“…The images were recorded with a digital videocassette recorder (DSR-45, Sony, Tokyo, Japan). The images were then analyzed essentially as described previously [Imafuku et al, 1996;Noda et al, 2005]. In brief, the recorded images were transferred to a computer frame by frame, and the x and y coordinates of the leading end of the microtubules on each frame was digitized by using a mouse-driven cursor on the screen.…”