Although the biochemical properties of the actin/myosin interaction have been studied extensively using actin activation of myosin ATPase as an assay1,2, until recently no well-defined assay has been available to measure the mechanical properties of ATP-dependent movement of myosin along actin filaments. The first direct measurements of the rate of myosin movement in vitro3,4 used a naturally occurring, biochemically ill-defined array of actin filaments from the alga Nitella. We report here the construction of an oriented array of filaments reconstituted from purified muscle actin and the use of this array in a biochemically defined quantitative assay for the directed movement of myosin-coated polystyrene beads. We demonstrate for the first time that actin alone, linked to a substratum by a protein anchor, is sufficient to support movement of myosin at rates consistent with the speeds of muscle contraction and other forms of cell motility.
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