Scientists Figure Out How Cells' Tiniest Motors Work

Led by the UC San Francisco scientist who discovered this protein motor called kinesin in 1985, the researchers have now scrutinized and measured how the motor changes shape, and they have deduced the crucial mechanism behind a leapfrog motion that allows the motors to transport material throughout cells.

While scientists have known for 15 years that kinesin motors pull their cargo by moving along the cells' internal trackways known as microtubules, the new research reveals how the tiny motors, each only about one ten-millionth of an inch across, generate the force to haul objects up to a thousand times their own size.

The scientists report their discoveries in the December 16 issue of the journal Nature. Senior author is Ronald Vale, PhD, an investigator in the Howard Hughes Medical Institute and professor of cellular and molecular pharmacology at UCSF. First author is Sarah Rice, a graduate student in Vale's lab. Altogether, 15 researchers took part in the project, including Ronald Milligan, PhD, professor of cell biology at Scripps Research Institute who led the effort to obtain images of a key component of the microscopic motor.

The mechanism appears to be fundamental to all cells of higher organisms, but with slight modifications engineered to produce different types of motion. There are likely to be 50 or more different kinds of kinesin motors in humans, their discoverer estimates, and that degree of specificity suggests that the motors might make good drug targets. Inhibition of kinesins involved in cell division, for example, might provide a means of stopping the growth of cancer cells, and stimulation of kinesin motors involved in nerve transport may improve neurodegenerative diseases. Research along these lines is already under way in private biomedical labs.

Using four different techniques to visualize and measure changes in the motor's shape, the researchers showed that a small region of the protein called the "neck linker" thrusts forward when powered by ATP, the chemical that serves as a nearly universal energy source inside cells.

Two kinesin motors work as a coordinated team to pull their cargo, one motor just ahead of the other on the tracks. In the crucial, repeating step described in the Nature report, the thrusting movement of the lead motor's neck pulls the rear motor free of the track, flinging it on ahead to become the new leader of this two-engine train. The leapfrog action repeats, advancing the pair along the track. As kinesin progresses along its track, it pulls forward its cargo - whether a chromosome or an organelle destined for a new site in the cell.

The energy that powers kinesin motors comes from breakdown of ATP in the motor's core, similar to the fueling route in the far-more-studied cellular motor, myosin which enables muscles to move. Both protein motors convert the chemical energy into work with an impressive 50 percent efficiency, roughly five times better than car engines.

"This kinesin motor movement is quite amazing," said UCSF's Ronald Vale. "Myosin produces motion using a long, rigid lever arm that swings in response to changes in its fuel core, a model proposed about 30 years ago that has dominated thinking of how molecular motors work.

Kinesin, a much smaller motor, has no long lever. We now know that kinesin uses a very differently designed and 10-times smaller mechanical element to produce the thrust that propels motion."

"It has become clear in the past 10 or 15 years that molecular motors are ubiquitous in living cells," Vale said. "The kinesin motor is another way Nature has devised to move cellular material along tracks."

Kinesin and myosin motors operate on different kinds of tracks - kinesin on microtubules and myosin on actin - but the cores of their motors are very similar, observes Sarah Rice, lead author on the paper and a UCSF graduate student in cellular and molecular pharmacology.

"The two motors handle the ATP chemical energy in a similar way," Rice says, "but the two types of motors evolved different solutions to create movement along their tracks."

The researchers used several methods to develop the new understanding of kinesin motility. Using genetic engineering techniques, they attached a variety of "reporter" chemicals to specific regions of the protein. If the region changed its position by even one hundred millionth of an inch, instrumentation could sense the change by detecting the reporter molecule's position or degree of motion.

Ronald Milligan, co-author on the paper, was able to directly obtain images of the different positions of the neck using electron microscopy. In one technique, the motion of the neck was visualized by observing tiny gold particles chemically bonded to it.

"Each technique has its own unique problems which could affect interpretation of the data," says Vale. "But when all of the techniques pointed to the same result, then we became convinced and our understanding of the mechanism began to take shape."

Almost all kinesin motors move in the same direction, Vale noted, but one special type moves the opposite way.

"It uses a different mechanical process, and we would like to visualize the internal motions of this protein too to understand how it drives motion in reverse."

Collaborators in the research and co-authors of the paper, along with Vale, Rice and Milligan are Abel W. Lin and Shane M. Cain, Elizabeth M. Wilson-Kubalek and Michael Whittaker, all in the cell biology department, Scripps Research Institute; Daniel Safer in the physiology department of the University of Pennsylvania School of Medicine; Cynthia L. Hart in the Howard Hughes Medical Institute at UCSF; Nariman Naber, PhD, a post-doctoral researcher and Roger Cooke, PhD, a professor in biochemistry and biophysics at UCSF.

Also collaborating were Bridget O. Carragher, professor of biology at the Beckman Institute, University of Illinois at Urbana-Champaign; Elena Pechatnikova, postdoctoral researcher and Edwin W. Taylor, professor, both in molecular genetics and cell biology, University of Chicago; and Edward Pate, professor of pure and applied mathematics, Washington State University.

The research was funded by the National Institutes of Health, the National Science Foundation and the UCSF Graduate Group in Biophysics.

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NOTE: Schematic and actual images of kinesin and microtubules can be found at http://Motorhead.ucsf.edu/valelab/ Click on "Nature Article." For caption information, please call Wallace Ravven at UCSF: (415) 476-2557. Related information can also be found at this above website address.