ScienceWeek

BIOENGINEERING: BIOMOLECULAR MOTORS IN DEVICE DEVELOPMENT

The following points are made by Henry Hess (Science 2006 312:860):

1) Biomolecular motors, such as the motor protein kinesin, convert chemical energy derived from the hydrolysis of individual adenosine triphosphate (ATP) molecules into directed, stepwise motion [1]. This process enables them to actively transport designated cargo -- such as vesicles, RNA, or viruses -- to predetermined locations within cells. For engineers, active transport in biology inspires visions of nanofluidic systems for biosensing, of active materials that can rearrange their components, and of molecular conveyor belts and forklifts for nanometer-scale manufacturing.

2) Nanofluidic devices, which extend the lab-on-a-chip paradigm to systems with picoliter volumes and submicrometer channel diameters, present an immediate opportunity for the application of biomolecular motors. New work [2] shows that kinesin motor proteins can drive the directed transport of microtubules (filamentous assemblies of thousands of tubulin proteins) in closed channels with submicrometer dimensions. Controlled application of an external electric field steers the microtubules into either one of two arms of a Y junction.

3) The setup is an adaptation of the classic gliding motility assay [3], in which the kinesin motor proteins adhere to a surface via their rotationally flexible tails, bind to the leading ends of approaching microtubules with their two heads, and move the microtubules by stepping forward with alternating heads until they reach the trailing end and detach. In biological systems, the motors move and the microtubules are stationary. The key advantages of the inverted geometry used in the assay are that the microtubules are continuously bound to the surface over transport distances of more than a millimeter [4] and that the large microtubule allows the attachment of fluorescence tags for observation and of specific linkers for cargo binding [5].

4) Open or micrometer-scale closed channels have previously been fabricated to confine microtubule movements. Van den Heuvel et al [2] have now created closed channels with submicrometer dimensions. The channels not only provide better confinement, but they also mimic the dimensions of axons, in which motor-driven transport plays a central role. They may thus enable more realistic model studies at the system level of active transport in biology. Electric fields for active steering provide direct control over the paths of individual microtubules. By coupling fluorescence detection of microtubules with this control mechanism, van den Heuvel et al [2] have integrated optics, electronics, and molecular transport, thus introducing an element of real-time programmability.

References (abridged):

1. J. Howard, Mechanics of Motor Proteins and the Cytoskeleton (Sinauer, Sunderland, MA, 2001)

2. M. G. L. van den Heuvel, M. P. de Graaff, C. Dekker, Science 312, 910 (2006)

3. S. J. Kron, J. A. Spudich, Proc. Natl. Acad. Sci. U.S.A. 83, 6272 (1986)

4. P. Stracke, K. J. Bohm, J. Burgold, H. J. Schacht, E. Unger, Nanotechnology 11, 52 (2000)

5. M. Bachand, A. M. Trent, B. C. Bunker, G. D. Bachand, J. Nanosci. Nanotechnol. 5, 718 (2005)

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