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Abstract

Myosin II, the myosin which has provided the most biochemical and structural data, is dimeric consisting of a long coiled-coil region with the motor domain flexibly attached to the N-terminal end of the coiled-coil. The motor domain (subfragment 1, S1, or cross-bridge) is obtained by proteolytic cleavage of myosin. S1 is a minimal model for the transport of actin filaments by myosin. S1 contains the ATPase, the actin-binding site which is regulated by ATP binding, and a long lever arm, which moves actin past myosin by a swinging motion. The lever arm is buttressed by two calmodulin-like light chains. X-ray crystallography of S1 and S1 prepared without the lever arm shows that the backbone of the molecule is a seven-stranded β-sheet. The active site is very similar to the G-proteins and contains a P-loop and switch 1 and switch 2 elements. Moreover,the motor domain can be found in three conformations that correspond with three of the four states predicted by the Lymn-Taylor cross-bridge cycle. The rotation of the lever arm (power stroke) is coupled to changes in the relative positions of switch 2 and the P-loop after ATP binding. This rotation is reversed on products release (the power stroke). The movement of switch 1, which accompanies ATP binding, is coupled with an opening or closing of a cleft in the actin-binding site, which leads to large changes in the actin affinity. Myosin V yields crystallographic data on the strong actin-binding form of the cross-bridge. Strong binding to actin is accompanied by a bending of the central β-sheet, which enables products release and hence induces the power stroke. All myosins probably use the same mechanism.