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la processes, some of which involve active assembly/disassembly of cytoskeletal substructures. Myosin II motors have been shown to function ce molecular motors using ATP hydrolysis to generate work and motion.1 Myosin II protein motors play The direct role of myosin II in the turnover of actin bundles has however not been elucidated, although doi:10.1016/j.jmb.2007.09.066 Available online at wwwkey roles in a number of important processes,2–8 some of which involve active assembly/disassem-bly of cytoskeletal substructures.2,3,9–11 Close exam-ination of the dynamics of certain cellular processes had revealed an intimate relationship between myosin II adenosine triphosphatase (ATPase) activ-ity and actin turnover rate.2,8,12–14 Myosin II motors function in actin bundle turnover in neuronal growth cones by promoting severing of the bundles into fragments that shrink over time8 and in the it is believed to involve filament unbundling15 and/ or depolymerization.8 Herein, we show that myosin II motors control actin bundle turnover in a concentration-dependent manner in vitro. We demonstrate that disassembly of actin bundles takes place in two stages—unbundling of the actin bundles into individual filaments and depolymer-ization of the filaments. We believe that myosin II motors may function similarly in vivo (e.g., in the disassembly of the contractile ring by tuning theEdited by J. Kar