Motility cycle model of muscle myosin & kinesin.

(A) Muscle myosin. Frame 1: Muscle myosin is a dimer of two identical motor heads (catalytic cores are blue; lever arms in the prestroke ADP-Pi state are yellow & bound weakly to actin), which are anchored to the thick filament (top) by a coiled coil. Frame 2: One head docks properly onto an actin binding site (green). The two myosin heads act independently, and only one attaches to actin at a time. Frame 3: Actin docking causes Pi release. The lever arm then swings to the poststroke, ADP-bound state (red), which moves the actin filament by ~100 Å. Frame 4: After completing the stroke, ADP dissociates and ATP binds to the active site, which rapidly reverts the catalytic core to its weak-binding actin state. The lever arm will then recock back to its prestroke state (i.e., back to frame 1). (B) Kinesin. Unlike myosin, the two heads of the kinesin dimer work in a coordinated manner to move processively along the track. Frame 1: Each catalytic core (blue) is bound to a tubulin heterodimer (green subunit and white subunit) along a MT. Frame 2: Neck linker docking is completed by the leading head (yellow), which throws the partner head forward by 160 Å (arrow) toward the next tubulin binding site. Frame 3: the new leading head docks tightly onto the binding site, which completes the 80 Å motion of the attached cargo. The trailing head hydrolyzes ATP to ADP-Pi. Frame 4: After ADP dissociates, an ATP binds to the leading head and the neck linker begins to zipper onto the core. The trailing head, is in the process of being thrown forward.