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Many types of cellular motility, including muscle contraction, are driven by the cyclical interaction of myosin with actin, coupled to the breakdown of ATP. The current view of the mechanism is that myosin binds to actin with the products of ATP hydrolysis (ADP and phosphate) bound in the catalytic site (cross-bridge attachment). Then, as the products are released, myosin changes conformation to produce a movement or “working stroke”. In the absence of nucleotide, actin and myosin form a tightly bound “rigor” complex. Binding of a new ATP molecule to myosin causes the rigor complex to dissociate (cross-bridge detachment), and subsequent ATP hydrolysis resets the original myosin conformation so that the cycle can be repeated.