by
Abstract
Biophysicist Jonathan Howard brings a mathematician's instincts to bear on questions involving biological motors, and plans to use them to build a molecular conveyer belt that could be the foundation for a true nanoscale factory.
When I was in graduate school, I used to study mutant genes in fibroblasts. In order to do this, you have to grow the cells in culture, and occasionally they become contaminated with bacteria. As aggravating as this sounds, it isn't all bad. Fibroblasts can be excruciatingly dull. They just sit there, day after day, always the same. But the bacteria move, twirling and spinning, racing around the field of the microscope like a ballet company on amphetamines. They are kinetic sculpture made small. I could watch them for hours.
Biophysicist Jonathon "Joe" Howard is lucky. He gets to watch things move under the microscope all the time. And what's more, studying cellular motors, his group is on the forefront of a field that has discovered that things are constantly moving around inside those fibroblasts, and inside all other cell types as well. It turns out that the cytoskeleton, which many of us were taught has little function except as a supporting scaffold, contains thousands of minute engines continually carrying things to where they need to go within the cell.
It also turns out that these molecular motors hold considerable potential for the future of nanotechnology - the manipulation of atoms and molecules to build stuff. Machines. Computer chips. Drug-delivery devices. Really small stuff.
The Labs of Tomorrow, Today
Howard is a professor of physiology and biophysics at the University of Washington at Seattle. His lab is also part of the university's newly created Center for Nanotechnology. The center was designed to "coordinate an interdisciplinary approach to research and education in nanoscale science and engineering."
Howard's lab doesn't look the way you'd expect a high tech "Lab of the Future" to look. In fact, it looks like the set from an old 1950s movie. This wing is in the oldest section of the UW School of Medicine and was, indeed, built back in the 1950s. The lab is filled with comfortable clutter, the wooden shelves and retro black bench tops crowded
with the stuff of good old-fashioned biology - flasks and test tubes, beakers and reagent bottles, Waring blenders filled with cow brains.
The central interest of Howard's group, kinesin, is altogether modern, however. Its very existence was only discovered about ten years ago. And the tools and techniques that they use to study it, such as an atomic force microscope that allows them to visualize and manipulate single molecules, are cutting edge.
As its name implies, kinesin is part of the molecular machinery that moves whatever needs to be moved, inside many different cell types. It is this machinery that ferries proteins, organelles, and ion channels to specific locations within the cell; that pulls chromosomes apart during mitosis; that makes muscle fibers contract; that propels sperm toward egg. Several molecular families, consisting of literally hundreds of related proteins, have been identified in various cell types, including myosin, actin, dynein, and tubulin microtubules, as well as kinesin.
It's the Wrong Trousers, Gromit
First discovered in neurons (hence the cow brains in the blender), kinesin is one of the "motor proteins." Kinesin is able to hydrolyze energy-rich ATP, and then to utilize that energy to propel itself along microtubules throughout the cell, carrying specific proteins or membrane vesicles along with it. The kinesin molecule has two "heads" that attach to and release from the microtubule in alternating steps, thus moving itself forward. An animated diagram of the kinesin molecule moving along a microtubule looks like nothing so much as the mechanical walking "Techno-Trousers" in the Wallace and Gromit "Wrong Trousers" cartoon.
Vroom, Vroom
We're watching long, wormlike shapes zip around on a video display of a microscope field. The entire field displayed on the screen is actually only the width of a human hair, and the worms are actually single microtubules, each only about 30 nanometers wide, sliding along molecules of kinesin. Howard and his group are able to measure the minute forces generated by a single kinesin molecule as it flexes and unflexes. By studying the physics of the kinesin motor, and by engineering mutant forms in order to observe alterations in function, they hope to identify and analyze the motor's moving parts, its "springs, shafts, and axles." They also hope to unravel how kinesin can convert the chemical energy released by ATP hydrolysis into mechanical work in order to drive the motor's forward motion.
A single molecule of kinesin can hydrolyze up to 100 ATP molecules per second, and it appears that the motor takes one "step" along the microtubule for each ATP hydrolyzed. That's 100 steps per second. As a motor, kinesin is about five times more efficient than an internal combustion engine. It's also about a hundred million times smaller. In addition, unlike many molecules that nanoscientists have been trying to manipulate recently, the microtubules along which the kinesin motors move actually become more stable at higher temperatures, making them easy to work with. It's not difficult to see how a researcher's thoughts might turn to nanotechnology.
Cells as NanoFactories
When you stop to think about it, then, those tedious fibroblasts are neither static nor dull, at least internally. "The interesting thing," muses Howard, "is that living cells are actually little nanofactories - everything is done on a very small scale. And even though the individual protein molecules are only a few nanometers in size, somehow those molecules make up [entire] cells . . . And there's no blueprint or anything like that."
Tubulin subunits polymerize into long microtubules, following very specific patterns in various cell types, as well as at different times in the cell cycle. Howard compares the tubulin subunits to "smart bricks" (building blocks containing microprocessors that tell them what to build). "Somehow, they know where to go in the cell. And not only do they know where to go, but they also know how to disassemble themselves as well. And each of our cells probably has 50 different motors, and they're all different."
"With regard to nanotechnology," he continues, "the way that I view it is that we need to be able to learn from cells how they organize themselves on that very, very small scale. You see, when we make things, we always make things from the top down - we have a huge factory that makes smaller things. And it seems like the smaller we want to get, we have to make these very, very large instruments in order to get down to that small scale. But cells do it the other way around. They start off with very, very small components, and those components know what they're doing."
The Little Engine That Could
Not content with passive theorizing, Howard is following his own directive. In a first step toward actually designing something useful using cellular motors, he and his colleagues plan to build a "molecular train set."
To make the "train tracks," they plan to etch a specific linear pattern on a silicon chip, and then to coat the pattern with purified kinesin molecules. When ATP hydrolysis is activated, microtubules will glide along these kinesin-coated tracks. "Then what we'd like to do," explains Howard, "is to load things onto the microtubules. So you'd have, say, lots of red fluorescent-labeled microtubules over here, then on another part of the surface you'd have green beads. And you'd send the microtubules over to the green beads, and load them on. And if you wanted to release them again somewhere else, you could."
Using the metaphor of a factory, then, the tracks and microtubules would function like molecular conveyor belts, and the various "stations," or stopping points, theoretically could be used to perform various assembly steps, chemical syntheses, or assays, on a molecular scale. Which is the whole idea behind nanotechnology.
Math and a New Path
Howard, a friendly and easygoing native of Australia, has an air of relaxed intensity about him. This immediately falls into context when he mentions that he received his undergraduate degree in pure mathematics. He became interested in the brain almost by accident, when he happened to read a description of action potentials in neurons by the 19th-century physicist Herman Helmholtz. Changing career paths, he decided to do his Ph.D. in biology, which in turn led to his research on motor proteins in neurons. "I just got really interested in how the brain works," he says. "It's the frontier of science. You know, the really deep questions . . ." After pondering some of those questions, Howard sighs
and grins wistfully, waving his hand toward the piles of paper on his desk and photographs of his three-year-old daughter. "At least, that was what I was thinking about back then. I don't have much time to think about it now."
As we're talking in Howard's office, anguished moans filter in from the next room. Will Hancock, a postdoctoral fellow who is analyzing kinesin's walking mechanism, is plotting data on the computer. He seems to be in such distress that we stop to see if perhaps he needs medical intervention. As it turns out, the results on the monitor screen have evoked sighs of euphoria from the tired biophysicist. The data fall into an almost perfect curve, which finally answers several complex questions regarding how the two heads of the molecule communicate with each other to coordinate their "steps" along the microtubule.
Howard studies the screen. "It is rare when things turn out that well," he says, and his face relaxes into an almost beatific smile.
Katherine Austin is a freelance writer who recently edited Nanotechnology: Emerging Trends and The Biotech Revolution for John Wiley & Sons Publishers.
Andrzej Krauze is an illustrator, poster maker, cartoonist, and painter who illustrates regularly for HMS Beagle, The Guardian, The Sunday Telegraph, Bookseller, and New Statesman.
Endlinks
Kinesin Home Page - an entire site devoted to kinesin. Contains short articles by various kinesin researchers explaining everything about kinesin, from its discovery to its role in human disease. Also contains movies of kinesin transport.
Home Page of the Molecular Motors Group - animations of the kinesin molecule "walking." Also contains videos, electron microscopy, and descriptions of kinesin chemistry. From the Marie Curie Research Institute, Oxted, United Kingdom.
MCRI Molecular Motors Group Video Microscopy Page - real-time videos of actin and kinesin/microtubules in motion.
Microtubules - basic facts on microtubules, cilia, etc. From the University of Texas Medical Branch.
Microtubule Models - computer models of microtubules and protofilaments, including theoretical nucleation of microtubule assembly. Page maintained by Harold Erickson of Duke University.
Steven M. Block - another well-known kinesin researcher at Princeton, with descriptions of optical tweezers and other techniques to study forces generated by single molecules.
Moving Traffic Inside the Cell - article from the Marine Biological Laboratory's "Lab Notes" about the discovery of an alternative cellular transport system based on actin filaments.