Visualization of single actin filaments by fluorescence microscopy led to the development of new in vitro assyas for analysing actomyosin-based motility at the molecular level. The ability to manipulate actin filaments with a microneedle or an optical trap combined with position-sensitive detectors has enabled direct measurements of nanometre displacements and piconewton forces exerted by individual myosin molecules. To elucidate how myosin generates movement, it is necessary to understand how ATP hydrolysis is coupled to mechanical work at the level of the single molecule. But the most sensitive microscopic ATPase assay available still requires over 1,000 myosins. To enhance the sensitivity of such assays, we have refined epifluorescence and total internal reflection microscopies to visualize single fluorescent dye molecules. We report here that this apporach can be used directly to image single fluorescently labelled myosin molecules and detect individual ATP turnover reactions. In contrast to previously reported single fluorescent molecule imaging methods, which used speciments immobilized on an air-dried surface, our method allows video-rate imaging of single molecules in aqueous solution, and hence can be applied to the study of many types of enzymes and biomolecules.
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