The view that proteins are, in essence, nanoscale macromolecular machines is helping to foster a new paradigm in biology, with an interdisciplinary emphasis that incorporates strong elements of biophysics (1). Remarkable examples of cellular machinery abound, including ribosomes, polymerases, proteosomes, chaperonins, and spliceosomes. But perhaps no more spectacular example of proteins-as-machines exists than the motor proteins, also called mechanoenzymes. These molecular assemblies drive both rotary and linear motions in virtually all organisms. Rotary engines include the F1-F0 ATPase (ATP synthase) and the bacterial flagellar motor, both powered by currents of protons. Linear motors, which include members of the myosin, kinesin, and dynein superfamilies, are fueled by the hydrolysis of ATP.
Among the linear motors, kinesin is the only mechanoenzyme that is known to be processive—that is, it remains bound to its polymer substrate while undergoing multiple rounds of activity (23). Kinesin...
