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Human cells inspire mini-robot makers
By Kate Dalke

Inspired by the motor proteins in our own cells, researchers have designed tiny machines that could someday move single molecules and assemble them into complex structures. In a new study, they used the machines to map microscopic structures more accurately than is possible with conventional microscopes.

The machines are robots built from structures in our cells called microtubules and motor proteins. Motor proteins are like self-propelled cars that carry cargo along tracks. These tracks are tubes of stacked proteins known as microtubules.

Images of the surface become clearer as time and the number of frames increases.

"The nice thing about these [robots] is that they move by themselves," says Henry Hess of the University of Washington in Seattle, who led the study. The robots are not attached to an arm or a power source.

"You can scan surfaces that are inaccessible," Hess adds. The machines are only few nanometers in length (one nanometer equals one-billionth of a meter).

"Motor proteins generate more force, have better fuel efficiency, and are smaller in size than any man-made nanomotor," the researchers write in Nano Letters.

The scientists were able to map a test surface made of thin polyurethane with one-micrometer high bumps. They coated the surface with motor proteins and then released a solution of microtubules.

Microtubule tubes hitch up with motor proteins (above) and are propelled along the surface (below).

The microtubules bound to the motor proteins, propelling the microtubules in random directions. The microtubules had fluorescent tags, a little like a headlamp, which allowed the researchers to track their movements.

After 500 fluorescence images of the moving microtubules were taken the information about all the microtubule paths was condensed into a single image which showed the surface topography.

"We can derive information about the surface simply by tracking the robot, provided that its position can be detected accurately and that its path or the detected signal is sensitive to surface properties," the scientists write in Nano Letters.

In a separate experiment, the researchers created a nano-sized instrument capable of measuring forces between molecules. These types of instruments could be used to study the binding of proteins within cells.

The instrument calculates the forces that intermolecular bonds can withstand before breaking and mimics the wear and tear such bonds experience within the body.

The tool "measures what actually goes on in biology," says Hess, citing the example of how a cell uses these bonds to crawl along a surface.

Ultimately, the researchers are working towards building a nanoscale conveyor belt that would allow the movement and assembly of structures from single molecules. In recent studies, they have already demonstrated the microtubules' ability to pick up tiny beads in solution and transport them across a surface.

"There are no 'good' artificial nano-motors," says Hess. "We try to integrate solutions found by nature like the motor proteins in nanotechnology."

See related GNN article
»A new generation of protein nanoarrays

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Hess, H. et al. A piconewton forcemeter assembled from microtubules and kinesins. Nano Lett 2, 1113-1115 (October 9, 2002).
Hess, H. et al. Surface imaging by self-propelled nanoscale probes. Nano Lett 2, 113-116 (February 2002).
Hess, H. et al. Light-controlled molecular shuttles made from motor proteins carrying cargo on engineered surfaces. Nano Lett 1, 235-239 (May 2001).

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