This chapter describes in vitro methods for measuring force and velocity of the actin-myosin interaction, using purified proteins. Myosin is a class of molecular motor that causes unidirectional movement of actin filaments, using the chemical energy obtained from the hydrolysis of ATP. In vitro motility assays provide an important approach to investigate myosin function, using only a small number of purified components. One intrinsic property of the myosin enzyme is its step size, which is defined as the average distance that a myosin moves an actin filament per ATP hydrolyzed. A tightly coupled model for myosin action depends on a one-to-one relationship between the release of ATP hydrolysis products and a force producing conformational change in myosin while bound to actin. The value of the step size could help to differentiate between these different models of myosin function. There is a minimum length of actin filament, dependent on the density of myosin on the surface, for continuous movement in the in vitro motility assay. Filaments longer than minimum length move continuously at the maximum speed, whereas shorter filaments dissociate from the surface in the presence of ATP. Another key to good movement in the in vitro motility assay is the quality of the myosin used. The in vitro motility assay can be extended to allow the measurement of force on single actin filaments by combining it with the technique of optical trapping.
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