Motor proteins are force-generating nanomachines that are highly adaptable to their everchanging biological environments and have a high energy conversion efficiency. Here weconstructed an imaging system that uses optical tweezers and a DnA handle to visualizeelementary mechanical processes of a nanomachine under load. We apply our system tomyosin-V, a well-known motor protein that takes 72nm ‘hand-over-hand’ steps composedof a ‘lever-arm swing’ and a ‘Brownian search-and-catch’. We find that the lever-arm swinggenerates a large proportion of the force at low load (<0.5pn), resulting in 3kBT of work. Athigh load (1.9pn), however, the contribution of the Brownian search-and-catch increases todominate, reaching 13kBT of work. We believe the ability to switch between these two forcegeneration modes facilitates myosin-V function at high efficiency while operating in a dynamicintracellular environment.
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