Duke Researchers Discover Power Behind Molecular Motors

After having demonstrated how “molecular motors” move within cells, a team of researchers led by a Duke University Medical Center scientist now believe they have discovered the power stroke that drives these motors.

Molecular motors are proteins made up of amino acids like any other protein in a cell. Unlike other proteins, however, they move along cellular highways of tiny filaments, called microtubules, as they transport nutrients around the cell or herd chromosomes during cell division.

Malfunctioning molecular motors might be responsible for some diseases such as Down’s syndrome caused by incorrect distribution of chromosomes during cell division. By understanding how motors work, how they organize chromosomes and how they lead the cell through the division process, researchers hope to be able to understand what causes these diseases and how to prevent them.

“I believe the findings of this study represent a breakthrough in our understanding of molecular motors and how they function,” said Duke cell biologist Sharyn Endow, Ph.D., who published the results of the Duke research today (Oct. 16, 2003) in the European Molecular Biology Organization (EMBO) journal. She collaborated with Hee-Won Park, Ph.D., a crystallographer at St. Jude Children’s Research Hospital, Memphis to obtain the new results.

“One of the major problems facing us in the field of molecular motor research is figuring out how the motor converts chemical energy into work or movement along microtubules,” she continued. “We believe we have found the mechanism for the force-producing stroke that directs the motor.”

In her experiments, Endow focused on a particular motor molecule called Ncd (nonclaret disjunctional), which she discovered more than a dozen years ago. Ncd belongs to a family of molecular motors called kinesins. The Ncd motor consists of a coiled-coil “neck/stalk” region that connects two “heads,” making up the molecular motor.

The researchers used two techniques — x-ray crystallography and cryo-electron microscropy — to visualize the structure of the Ncd motor at different stages of its movement along the microtubules. Endow explained that this movement occurs during the breakdown of ATP (adenosine triphosphate) that occurs in all cells.

ATP is a storage repository of energy for the cell – liberating energy when the chemical bonds that holds one of the phosphates on the molecule is broken by a process known as hydrolysis.

Endow found that during this process of ATP hydrolysis, the coiled-coil region of the motor changed in angle, or conformation. As a result of this conformational change, the coiled-coil region rotates relative to one of the two heads, amplifying the force produced by the motor, resulting in the working stroke of the motor.

“We were able to come up with a new crystal structure which showed that the coiled-coil domain undergoes a large rotational movement that could represent the force-producing stroke of the motor,” Endow explained. “The stalk appears to be rigid and may act like a lever. This is in contrast to models for other kinesin motors, whose movement appears to be more rachet-like than lever-like.”

From the time Endow first discovered Ncd in fruit flies, the little motor has been an enigma. At the time, it was the first molecular motor of its kind that moved toward the more stable, or “minus” end of microtubules. The other kinesin motor proteins moved toward the fast-growing, or “plus” end.

Endow previously showed that the normal Ncd motor moves only toward the minus end of the microtubule and that it also rotates to the right around the tubule. She made Ncd mutations that disrupted the sense of direction and created for the first time a motor that is equally likely to move to the plus end as to the minus end, and it rotates either to the right or left.

While Ncd was discovered in fruit flies, similar motors operate in all animals, including people, Endow said.

“Our hope is that by understanding how these molecular motors work, we will be able to identify why sometimes things go wrong in the reproductive process,” she said. “Right now it is very difficult to do these experiments with animals more advanced than flies because they make eggs internally. That makes it difficult to observe the process. But using flies, in which the process is thought to be closely related to higher animals, we can identify the components and learn how they work.”

The research was supported by the National Institutes of Health, the Human Frontiers Science Program, the St. Jude Children’s Research Hospital Cancer Center and the American Lebanese Syrian Associated Charities.