Location of the ATPase site of myosin determined by three-dimensional electron microscopy

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Abstract
Both ATP hydrolysis by myosin and the accompanying cyclic association–dissociation of actin and myosin are essential for muscle contraction. It is important for understanding the molecular mechanism of contraction to know the three-dimensional locations of the two major functional sites of myosin: the ATPase site1 and the actin-binding site2,3. We have determined the position of the ATPase site of myosin using three-dimensional image reconstruction from electron micrographs and site-specific labelling with the avidin–biotin system1. The ATPase site is about 5 nm from the tip of the myosin head and is about 4 nm away from the actin-binding site of myosin. This is the first report of the three-dimensional location of an enzyme active site by electron microscopy.