Eukaryotic organisms synthesize a set of so-called motor proteins converting the chemical energy of nucleoside triphosphates into mechanical force, required for motility generation. A prominent motor protein is conventional kinesin, which is essentially involved in neuronal transport events. Walking in discrete nanometre steps along microtubule rails it transports specialized cargoes to cellular destinations. The transport direction is determined by both microtubule polarity and the intrinsic molecular structure of kinesin.
Purified kinesin and microtubules can be re-assembled in vitro into a force-generating unit able to transport artificial cargoes . Functioning in cell-free systems is one main reason why this kinesin-microtubule machinery, which can be regarded as a linear motor, is believed to become applicable for the development of devices that realize transport in nanometre steps . However, future applications require the solution of numerous methodological and technical problems, including velocity regulation, temporal stability, fuel supply, force generation into a desired direction, on and off switching, the determination of suitable materials, of the tolerable surfaces roughness, and of the minimal free vertical space still enabling kinesin functioning.
Our presentation summarizes recent results contributing to find out a basic configuration for constructing a kinesin-driven motor device: Using gliding microtubules as a model, we demonstrated that the transport velocity increased from about 0.6 µm/s to 2.0 µm/s by raising molar Mg2+/ATP ratio, elevating temperature, or lowering kinesin surface density. In contrast, polyhydroxy compounds slow down the transport. Kinesin needs a minimal free working space of about 100 nm height and works with constant velocity for hours under conditions of sufficient energy (ATP) supply. Individual gliding microtubules overcame surface height differences up to about 280 nm. Kinesin was observed to generate motility on a variety of materials, including glass, quartz, silicon, carbon, gold, and polystyrene.
Gliding microtubules can be aligned in isopolar fashion applying flow fields . After alignment the microtubules were chemically fixed , resulting in stable arrays of microtubule rails suitable for the unidirectional transport of kinesin-coated cargoes with dimension up to 20 µm. The cargo (even a small one, e.g., 100-nm beads) can change from one microtubule to an adjacent one, enabling transport distances in millimetre ranges, significantly exceeding the length of the individual microtubules (15 to 30 µm). On this basis, active transport rails of theoretically non-limited length might be assembled.
Velocity regulation, controlling transport direction, and the production of large areas with long and stable rails are considered to be crucial steps towards the development of motor protein-based transport devices, suitable for e.g. a controlled displacement of objects or specific substances over millimetre distances with nanometre precision.
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