Molecular shuttle gains light throttle

By Chhavi Sachdev, Technology Research News

A transport system small enough to move nanoscale objects like single molecules could eventually have far-reaching uses in nanoscale machinery and within the human body.

Researchers at the University of Washington have figured out how to control a nanoscale system that works like a railroad, complete with tracks, trains and stations. The setup could eventually allow novel materials to be built from the bottom up, said Viola Vogel, director of the center for nanotechnology and associate professor in bioengineering at the University of Washington. “This brings us a step closer to machines working on a molecular scale, the grand vision outlined by [physicist Richard] Feynman 40 years ago,” she said.

To make the tiny freight trains, the researchers used a nanoscale motor to move microtubule shuttles. They then loaded cargo onto the trains. However, the researchers needed to figure out how to control the trains, said Vogel. The fourth, most crucial step, therefore, revolved around " the question of guiding -- how can we direct the motion along nanoscale tracks?" she said.

The researchers isolated the motor protein kinesin to drive the shuttle. Motor proteins are the tiny molecules responsible for transporting small cargoes in nature and are fueled by the nucleotide adenosine triphosphate (ATP). Kinesin characteristically moves in a single direction along the lengths of microtubules, which are very thin protein tubes, such as the axon in a neuron.

The track is composed of kinesin fixed to a surface. The path is set for the microtubules because kinesin adheres to very tiny guiding channels on their surfaces, directing the motion of the free-floating microtubules. The process can be inverted, with the microtubules fixed to the surface, according to the researchers.

To load and unload specific cargo, the researchers showed that linking the enzyme biotin to the microtubules caused anything coated with the protein streptavidin to bind with it. The biotin/streptavidin interaction is well known and frequently used in bioengineering. They also proved that hooking cargo to the microtubules did not alter their speed or motion. While sometimes the tubules ‘derailed,’ the percentage of such accidents was small, the researchers said.

Their final challenge was to control the starting and the stopping of the microtubule freight trains. This was difficult because the kinesin motors are not very sensitive to changing ion concentration or pH levels, the two traditional methods of controlling molecules. Once started, the motors can also run for many hours without pausing because they consume very little energy from the ATP, the researchers said.

The researchers used ultraviolet light to turn the kinesin motor off and on. The light frees stored, or caged, ATP. “Think of the free ATP as the fuel in the motor, the caged ATP as the fuel in the gas tank, the kinesin as the motor and the hexokinase as the brake,” said Henry Hess, a postdoctoral researcher on the team. When an ultraviolet light is flashed, it acts on the inactive, caged ATP like a fuel pump and “turns caged ATP into free ATP," said Hess. This allows kinesin to push the microtubules around, he said.

“Without hexokinase, kinesin would run for hours on a single load of ATP. So we use hexokinase as [a] brake: It consumes the free ATP fairly fast, which in turn slows the kinesin,” Hess said. When all the free ATP is consumed by hexokinase and kinesin, the motors rest until the next flash of ultraviolet light, he said.

"Moving the shuttles in multiple, discrete steps under control from an external source of UV light is a key accomplishment of our research,” Vogel said.

The tiny railroad system “could have applications in materials, sensors, drugs, [and] information technology,” said Hess. “You could rip things apart and test the strength of the bonds holding them together; You could transport a ‘glue’ to a nanoscale crack on a surface and prevent the crack from spreading,” Hess said. The shuttles could help assemble those things that do not self-assemble and impose a specific order on the process. This would help further miniaturize microelectromechanical systems (MEMS) devices, he said.

The train could also be used as an assembly line, according to the researchers. A molecule loaded onto a shuttle could be transported along a series of reactor sites. The reactor sites could be nanometer-sized outlets where a molecule could be chemically modified by the voltage of a nearby electrode, Hess said.

The researchers’ next step is to “get a detailed understanding of the system parameters so that we can rationally engineer complex tracks,” said Vogel. They also plan to demonstrate the performance of the motor proteins in different applications. The motor protein shuttles will not be ready for practical use for at least another five years, Vogel said.

“The paper by Hess et al. is a nice piece of work,” said Fred Brouwer of the Institute of Molecular Chemistry at the University of Amsterdam. “Researchers coming from the field of biochemistry or molecular biology are well ahead of those who approach molecular shuttles in the synthetic way, because by making use of what nature has already developed they are already able to have more or less controlled binding/unloading of a cargo, together with movement along tracks, which could potentially also be unidirectional,” he said.

“The sizes of the systems, on the other hand, are very much bigger than those of synthetic molecules,” Brouwer said. “The microtubules moved by the biomolecular motors… are several micrometers long,” he said. This is about ten times larger than structures that can be made with current lithographic processes, he added.

The system size is not a drawback, according to the researchers. “The ratio in size between cargo and vehicle would be very similar to a bus transporting people,” said Hess.

Vogel and Hess’s colleagues were John Clemmens, Dong Qin at the University of Washington and Jonathon Howard who is also affiliated with the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Germany. They published their research in the April 24, 2001 issue of the journal Nano Letters. The research was funded by NASA.

Timeline:   > 5 years
Funding:  Government
TRN Categories:  Nanotechnology
Story Type:   News
Related Elements:  Technical paper, "Light-controlled molecular shuttles made from motor proteins carrying cargo on engineered surfaces," Nano Letters, April 24, 2001, vol. 1, No. 5, 235-239.


July 4/11, 2001

Page One

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Gender gap shows cyberspace bias

Software lets appliances speak softly

Molecular shuttle gains light throttle

Light-sensitive memory does not fade


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