‘Life is motion’: multiscale motility of molecular motors

Life is intimately related to complex patterns of directed movement. It is quite remarkable that all of this movement is based on filaments and motor molecules which perform mechanical work on the nanometer scale. This article reviews recent theoretical work on the motility of molecular motors and motor particles that bind to cytoskeletal filaments and walk along these filaments in a directed fashion. It is emphasized that these systems exhibit several motility regimes which are well seperated in time. In their bound state, the motor particles move with a typical velocity of about 1μm/s. The motor cycles underlying this bound motor movement can be understood in terms of driven Brownian ratchets and networks. On larger length and time scales, the motor particles unbind from the filaments and undergo peculiar motor walks consisting of many diffusional encounters with the filaments. If the mutual exclusion (or hardcore repulsion) of these motor particles is taken into account, one finds a variety of cooperative phenomena and self-organized processes: build-up of traffic jams; active structure formation leading to steady states with spatially nonuniform density and current patterns; and active phase transitions between different steady states far from equilibrium. A particularly simple active phase transition with spontaneous symmetry breaking is predicted to occur in systems with two species of motor particles which walk on the filaments in opposite directions.