Cooperative transport by small teams of molecular motors

Life is intimately related to movement on many different time and length scales, from molecular movements to the motility of cells and organisms. One type of movement which is ubiquitous on the molecular and cellular scale, although not specific to the organic world, is Brownian motion or passive diffusion: Biomolecules, vesicles, organelles, and other subcellular particles constantly undergo random movements due to thermal fluctuations.1 Within cells, these random movements depend strongly on the size of the diffusing particles, because the effective viscosity of the cytoplasm increases with increasing particle size.2 While proteins typically diffuse through cytoplasm with diffusion coefficients in the range of μm/s to tens of μm/s and therefore explore the volume of a cell within a few minutes to several tens of minutes (for a typical cell size of a few tens of microns), a 100 nm sized organelle typically has a diffusion coefficient of ∼ 10μm/s within the cell,2 and would need ∼ 10 days to diffuse over the length of the cell. For fast and efficient transport of large cargoes, cells therefore use active transport based on the movements of molecular motors along cytoskeletal filaments.3,4,5 These molecular motors convert the chemical free energy released from the hydrolysis of ATP (adenosinetriphosphate) into directed motion and into mechanical work.