Dictyostelium has several isoforms of “unconventional” single-headed myosins that do not assemble into filaments (myosin I). In contrast, there is only one form of conventional myosin (myosin II) that self-assembles into bipolar thick filaments, and molecular genetic studies have shown that this myosin is an essential motor protein for several cell functions, including cytokinesis. Myosin II is phosphorylated on both the heavy chain and light chain, and these modifications regulate assembly and ATPase activity. A large fraction of the myosin II remains associated with structures that are resistant to extraction with nonionic detergents (the Triton cytoskeleton). These cytoskeletons retain the ability to contract in an ATP-dependent manner and can easily be isolated by low-speed centrifugation. A method for the rapid purification of myosin II from cytoskeletons is described. This method is simple and avoids the need for sonication, ultracentrifugation of large volumes, and overnight dialysis. The subunit composition, assembly properties, and enzyme activity are similar to those reported for myosin prepared from soluble extracts.
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