Motor protein-driven unidirectional transport of micrometer-sized cargoes across isopolar microtubule arrays

Conventional kinesin is a motor protein which translocates organelles from cell centre to cell periphery along specialized filamentous tracks, called microtubules. The direction of translocation is determined by microtubule polarity. This process of biological force generation can be simulated outside cells with kinesin-coated particles actively moving along immobilized microtubules. The in vitro approaches of kinesin-mediated transport described so far had the disadvantage that concerning their polarity the microtubules were randomly distributed resulting in random transport direction. The present paper demonstrates the unidirectional translocation of kinesin-coated cargoes across arrays of microtubules aligned not only in a geometrically parallel but also in an isopolar fashion. As cargo, glass, gold, and polystyrene beads with diameters between 1 and 10 µm were used. Independent of material and size, these beads were observed to move unidirectionally with average velocities of 0.3-1.0 µm s-1 over distances up to 2.2 mm. Moreover, the isopolar microtubule arrays even enabled the transport of large flat glass particles with an area of up to 24 µm×12 µm and 2-5 µm thickness which obviously contacted more than one microtubule. The controlling transport direction is considered to be an essential step for future developments of motor protein-based microdevices working in nanometre steps.