NANOTECHNOLOGY:
Borrowing From Biology to Power the Petite

Cells are packed with protein-based motors powered by the chemical fuel of life, adenosine triphosphate, or ATP. These motors ferry cargo, flex muscles, and even copy DNA. And at a recent meeting,^* two groups, one led by Carlo Montemagno of Cornell University in Ithaca, New York, and the other by Viola Vogel of the University of Washington in Seattle, reported taking the first baby steps toward harnessing these motors to power nanotechnology devices. Like molecular mechanics, the researchers have unbolted the motors from their cellular moorings, remounted them on engineered surfaces, and demonstrated that they can in fact perform work, such as twirling microscopic plastic beads. "What we're really trying to do is make engineered systems that tap into the energy system of life," says Montemagno.

The effort still has a long way to go. But the early work is already generating enthusiasm in the community. "I think it's a very productive path to follow," says Al Globus, a nanotechnology expert at the National Aeronautics and Space Administration's Ames Research Center, Moffat Field, California. If the effort does pan out, it could help researchers make everything from tiny pumps that release lifesaving drugs when needed to futuristic materials that heal themselves when damaged.

For their molecular motor, Montemagno and his colleagues turned to one of the cell's heavy lifters: ATPase, a complex of nine types of proteins that work together to generate ATP. While tiny--it measures just 12 nanometers across and 12 high--this cellular motor is remarkably sophisticated, containing a cylinder of six proteins surrounding a central shaft. ATPase converts the movement of protons within the cell's energy powerhouse, the mitochondrion, into a mechanical rotation of the shaft, a motion that helps catalyze the formation of ATP. But the motor can also run in reverse, burning ATPs to rotate the shaft and move protons.

Last year, Hiroyuki Noji and his colleagues at the Tokyo Institute of Technology and Keio University in Yokohama, Japan, captured this rotational motion on camera for the first time (see Science, 4 December, p. 1844). They dangled a fluorescent-tagged molecule off the end of the shaft, fed the motor ATP, then put it through a microscope and took sequential pictures of the shaft as it rotated in circles around the cylinder.

Based on the number of rotations produced by a given amount of ATP, the researchers calculated that the motor operates at near 100% efficiency-- "well above the efficiency of motors we're capable of building," says Montemagno. "If the motor was as big as a person, it would be able to spin a telephone pole about 2 kilometers long about one revolution per second."

That result inspired Montemagno and his Cornell colleagues--George Bachand, Scott Stelick, and Marlene Bachand--to see if they could use the ATPase rotary motor to move man-made objects. They started by genetically engineering two changes into ATPase proteins, one to stick the motors to metal surfaces and the other to provide an attachment site for the beads that they wanted the motor to move.

To make the first change, the team added an amino acid sequence loaded with histidine, which binds tightly to metals, to the base of the proteins that form the motor's cylinder. Next they used electron beam lithography to pattern an array of nickel islands--each roughly 40 nanometers across--atop a glass microscope cover slip. When they then spritzed water on top to keep the proteins happy and added the motors, the base of the cylinders bound to the nickel islands, causing the motors to stand upright.

To attach the beads, which were made of plastic or a plastic/iron composite and coated with a small organic molecule called biotin, Montemagno and his colleagues added cystine, a sulfur-containing amino acid, to the top of the central shaft. That allowed the shaft to grab a small sulfur-binding protein called streptavidin, which could in turn bind the biotin-coated beads. When the researchers then added ATP fuel to the solution atop the slide and used a laser-based interferometer to track the beads' movement, they could see their array of motors twirling in endless loops, like a dance floor of nano-sized dervishes. "I had the thing running for well over 2 hours at a time," says Montemagno. "It was seriously cool."

But whirling beads--impressive as they may be--are still a long way from nanorobots rooting through the body. So Montemagno's team is pressing ahead. They're currently working on replacing the beads with tiny magnetic bars. If the motors spin the bars, the researchers will be able to measure precisely how strong the motors are by applying an outside magnetic field: By increasing the field until the motors can no longer spin, they will be able to probe the limit of the motor's power.

What's more, the spinning bars should generate an electrical current that might eventually be used to power devices, such as chip-based drug delivery pumps or chemical weapons sensors implanted in the body. But these uses, Montemagno says, are just the beginning. "There's 100,000 different things you could do with these motors," he says.

Washington's Vogel says much the same thing about her team's contraption, a nanoscale monorail in which a collection of molecular motors all lined up on a surface pass a tiny tube hand over hand down the line. Vogel based her monorail on one of the cell's own transport systems, which consists of tracks made of microtubules, tube-shaped assemblies of a protein called tubulin, and small motors made of another protein, kinesin. In cells, the kinesin motors latch onto the fixed microtubules and churn like steam engines from one end of the line to the other, ferrying molecular cargo such as proteins and lipids. But for their experiment, Vogel and her colleagues John Dennis and Jonathan Howard reversed these roles, fastening kinesin motors to a surface and having them shuttle microtubules down the line from one motor to the next.

Biophysicists studying kinesin motors had done related experiments in the past. But in those, Vogel says, the kinesins were in random locations on surfaces. When microtubules and ATP were then added, the kinesins shuttled microtubules in all directions. To control the transport, the Washington team had to line up the kinesins. Here, the researchers took a low-tech approach. They simply rubbed a block of polytetrafluoroethylene, or PFTE, across a glass slide, causing molecules of the chainlike polymers to rub off and coat it. The scraping acted something like a hair brush, getting all the PFTE chains to line up on the surface, creating a series of grooves running for micrometers along the slide.

After submerging the slides in water and coating them with a small protein called casein, to protect overlying proteins, they added the kinesin motors, which settled into the grooves. They then sprinkled on a few microtubules, which were tagged with fluorescent compounds so they could be seen, and dropped some ATP fuel into the solution.

By turning on a xenon lamp to set the microtubules aglow and letting their cameras roll, Vogel and her colleagues could see the kinesins push their tubular cargo in one direction, moving it hand over hand down the parallel grooves. "Even though kinesins move on the nanoscale, we could watch the microtubules move on the micron scale," says Vogel.

For now, the team is using the monorail to study the performance of their motors. But down the road, Vogel says that the tiny rail lines could be used to transport replacement components for self-healing biomaterials for medical implants. If this and other efforts to motorize the nanoworld are successful, those microscope slides may soon see their first traffic jams.