New angle on myosin
Abstract
Motion and force in biological systems are produced by motor proteins, which reach out from one set of filaments or organelles and translate these relative to a second set of filaments. These motor proteins take the chemical energy of ATP and convert it into mechanical energy. The laws of physics dictate that the production of mechanical energy must involve the exertion of a force through a displacement. Recent work has shown that forces are of the order of 5–10 pN and displacements are between 5–10 nm (1, 2). Basically, there are two simple ways in which a motor protein could produce a force through a distance. In one model the motor protein could produce relative translation by attaching to one of the filaments followed by a shortening of some element, most likely the element tethering it to the first set of filaments, thereby effecting a translation. In the second model the motor protein could attach to the second set of filaments and then undergo a rotation, thus acting as a “rowing oar” propelling one set of filaments past the other. Studies of both the structure and function of myosin have suggested that it functions by the second of the two above hypotheses. Electron microscopy showed that myosin could assume different orientations in the filament lattice of the muscle, a conclusion that was supported by x-ray diffraction patterns (3, 4). In addition, it was shown that the head region of myosin interacting with actin could generate force and displacement, eliminating any structural element that could reasonably shorten (5). Thus the current paradigm for explaining the action of myosin is that at least a portion of the myosin head must rotate during the power stroke. The goal of the field for many years has been to identify which portions rotate …