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Class II myosin heavy chains have `heads' that hydrolyse ATP and `tails' required for filament formation. Dictyostelium is a useful system for investigating the importance of these regions in myosin function, since it has only one heavy chain gene, which is essential for growth in suspension, cytokinesis and full development. Edward Korn and co-workers have constructed myosin II chimeras in which the tail domain of Dictyostelium myosin II is replaced by that of Acanthamoeba myosin II or a chicken smooth muscle myosin II; these exhibit unregulated actin-activated ATPase activity and form unusual filaments. Korn and co-workers now show that mutants expressing these chimeras fail to develop fully but can grow in suspension and undergo cytokinesis (see p. 4237), concluding that regulation of ATPase activity and normal filament assembly are not essential for certain key motor functions. Significantly, the authors demonstrate correct localization of the chimeras to the cleavage furrow — a property known to require the tail — despite their significantly different tail sequences. Cleavage furrow localization thus probably depends not on a specific sequence but on a general property of the tail, such as its coiled-coil structure.
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