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The cooperative action of many molecular motors is essential for dynamicprocesses such as cell motility and mitosis. This action can be studied byusing motility assays in which the motion of cytoskeletal filaments over asurface coated with motor proteins is tracked. In previous studies ofactin-myosin II systems, fast directional motion was observed, reflecting thetendency of myosin II motors to propagate unidirectionally along actinfilaments. Here, we present a motility assay with actin bundles consisting ofshort filamentous segments with randomly alternating polarities. These actintracks exhibit bidirectional motion with macroscopically large time intervals(of the order of several seconds) between direction reversals. Analysis of thisbidirectional motion reveals that the characteristic reversal time,$\tau_{rev}$, does not depend on the size of the moving bundle or on the numberof motors, $N$. This observation contradicts previous theoretical calculationsbased on a two-state ratchet model [Badoual et al., Proc. Natl. Acad. Sci. USA,vol. 99, p. 6696 (2002)], predicting an exponential increase of $\tau_{rev}$with $N$. We present a modified version of this model that takes into accountthe elastic energy due to the stretching of the actin track by the myosin IImotors. The new model yields a very good quantitative agreement with theexperimental results.

Bidirectional cooperative motion of myosin-II motors on actin tracks with randomly alternating polarities