Kinesins belong to the group of motor proteins. They walk along microtubule rails in 8-nm steps, converting the chemical energy of adenosine triphosphate into mechanical force required for intracellular translocation of organelles. A prominent member of the kinesin protein superfamily is the so-called conventional kinesin, which is mainly involved in neuronal transport events. It is possible to purify both the kinesin and the microtubules to re-assemble this biological force generating system under in vitro conditions. So, kinesin-coated polymeric beads were found to be able to translocate along microtubules immobilized onto glass surfaces.
A characteristic attribute of kinesin-dependent transport is its directionality, which depends on microtubule polarity and the intrinsic molecular structure of kinesin. The in vitro approaches of kinesin-mediated transport, described so far, used microtubules either distributed irregularly or aligned parallel in geometrically fashion only [1,2], resulting in random or opposite transport directions, respectively. To avoid counteractive forces, the directed transport of a large cargo, binding to more than one microtubule, needs microtubule rails aligned with same polarity. Recently, we have shown that microtubules align in isopolar fashion during gliding across kinesin-coated glass surfaces by application of strong flow fields . However, after abolishing the flow field, gliding became disordered again. In this study, we immobilized arrays of flow-field aligned isopolar microtubules by treatment with glutaraldehyde  to produce large dense stable rails and demonstrated their suitability for the unidirectional translocation of kinesin-coated micrometer-sized gold and glass beads as well as thin sheets. Single beads were shown to cover distances up to 1.3 mm, significantly exceeding the length of the individual microtubules (15 to 30 µm). Discontinuities within the microtubule rail system (gaps between the end of one microtubule and the following one) were observed to be overcome. On this basis, due to microtubule cooperativity active transport rails of theoretically non-limited length might be assembled.
Controlling transport direction and the production of long and stable rails can be regarded as an important step towards the development of motorprotein-based micro transport devices, suitable for e.g. a controlled displacement of objects in nanometre steps or a controlled and specific substance transport over micrometre to millimetre distances.
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This study was performed on the basis of research projects granted by the Deutsche Forschungsgemeinschaft (DFG INK 22/Bl-1/A5; DFG Un82/5-1).