Biological motors sort molecules one-by-one

Paul Boughton

Researchers from Delft University of Technology's Kavli Institute of Nanoscience have discovered how to use the motors of biological cells in extremely small channels on a chip.

Based on this, they have built a transport system that uses electrical charges to direct the molecules individually. To demonstrate this, the Delft researchers sorted the individual molecules according to their colour. The research findings are being published in Science on 12 May 2006.

The biological cell is a complex of many different small protein factories. The necessary transportation of materials within the cell occurs across a network of microtubules: long, tubular-shaped proteins that extend in a star-shaped formation from the nucleus of the cell to the walls of the cell. Molecular bio-motors, such as the enzyme kinesin, can walk in small steps (of 8nm) with a load of material along these microtubule-networks and thus provide transport within the cell.

Fascinated by these biological motors, the researchers at Delft University of Technology's Kavli Institute of Nanoscience are currently exploring the possibility of inserting these kinesin-motors and microtubules in an electrically directed transport system that is made by the researchers using nano-fabrication techniques.

The researchers turned the system around: the kinesin-motors are fastened in large quantities on a surface with their 'feet' up; the microtubules (measuring approximately 1 to 15 micrometers in length) were then transported over the 'carpet' of motors. The microtubules are, as it were, 'crowd surfing' over the sea of small kinesin motors. A particular challenge of the research was to ensure beforehand that the microtubule tubes could be transported in a determined direction and were not dislodged by collisions of the motor carpet.

PhD student Martin van den Heuvel, master student Martijn de Graaff and group leader professor Cees Dekker have, for what is believed to be the first time, successfully controlled and addressed individual microtubules. An important step in this was to allow microtubule-transport to occur in small closed liquid channels. This made it possible to apply a strong electrical field locally at the Y-junction in the channels. Because of this, the electrical force could be exerted on the individual microtubules. The researchers discovered that by using this electrical force they could push the front of the microtubule into the determined direction.

To demonstrate this, the researchers allowed a mixture of green and red fluorescent microtubules to arrive at a Y-junction. By changing the direction of the electrical force, depending on the colour of the microtubule, the Delft researchers were able to collect the green and red microtubules in different reservoirs.

By applying their approach to the nano-channels, the researchers were able to steer individual microtubules and prevent the microtubules from derailing from their tracks. At the same time, the researchers discovered that their work contained a third interesting aspect: the closed channels offered the possibility to observe the electrical transport of freely suspended microtubules, thus proving that the speed of the microtubules under an external electrical field is largely dependent on the orientation of the cylinder-shaped molecules. This was the first time that this orientation-dependency of the electrophoretic mobility was observed.
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