Collective Molecular Motor Transport

Cells constantly need to move molecules and organelles throughout their interior. RNA produced in the nucleus must be exported to then be translated into proteins. Some of these proteins must then be imported back into the nucleus. Others are packaged into membrane vesicles, which in turn must be trafficked to the plasma membrane and exocytosed from the cell. The list of such examples has hundreds of entries [1]. An energetically ‘cheap’ way to transport microscopic particles is through diffusion [2]. The trick is done by Brownian movements: all particles in cytoplasm are bombarded by countless water molecules, and if these particles are smaller than a micron in size, momentous imbalances in hits by these smaller molecules results in random movement. This paradigm is surprisingly effective when particles only must spread over short distances. However, there are three major caveats that in many cases make diffusion ineffective. (1) Diffusion is direction-less, so if the cell need transport from a source to a well-defined target (e.g. from ER to Golgi), diffusion alone does not work. (2) Distance travelled by diffusion increases as a square root of time, while directional movement with constant speed generates displacement that grows linearly with time. Therefore, long-distance diffusive movements are slow. (3) Objects that are larger typically diffuse slower, so membrane vesicles, for example, diffuse far too slowly for cellular functions.