Researchers find protein works as motor

By MELANIE LUTZ
Collegian Science Writer

Penn State researchers have proven how one protein works as a motor, carrying chemicals produced by nerve cells to their release sites on the cell surface.

Kenneth A. Johnson, Paul Berg Professor of biochemistry, said the protein, called kinesin, works like feet that carry little packages to the tips of nerves. His research was published in the journal, Nature, last Thursday.

Johnson explained that when a person thinks about moving a body part -- for example, an arm -- little packages containing neurotransmitters and other chemicals produced by the nerve cells are already sitting at the nerve tips, waiting for an electrical impulse to trigger the nerves and cause the arm to move.

Those packages were carried to the nerve tips along microtubule filaments that are strung through the long arm of nerve cells. The membrane packages were carried along the microtubules by kinesin. The nerve fibers that kinesin travels along can sometimes be up to a meter long, Johnson said.

Susan P. Gilbert, assistant professor of biological science at the University of Pittsburgh who worked on the research as a post-doctoral candidate at Penn State, explained that kinesin walks the length of the microtubule while pulling along the membrane package. Just as legs are permanently attached to a body, the kinesin is permanently attached to the package, she said.

Kinesin is also involved in nerve regeneration, bringing new membrane material to the tips of nerves so they can grow, Johnson said. Because it helps the regeneration, it may also be involved with diseases caused by the degeneration of nerve cells, such as Parkinson's and Lou Gehrig's diseases.

By measuring reactions on time scales of thousandths of a second, Johnson found how kinesin works to transport the membrane packages.

The kinesin molecule looks like two balls attached by strings that are twisted together, and it is anchored to the membrane package.

Johnson examined the way the round head structures of kinesin interacted with the microtubules and the compound ATP, a molecule used to store energy. The heads move in a "hand-over-hand motion," crossing over each other. They bind and then unbind to the surface of the microtubule, like feet taking steps. The microtubule surface is like a ladder, so the heads of kinesin can only bind to the surface at certain points, he said.

At the same time this binding process occurs, ATP breaks down to form its product pieces. The energy released from this breakdown is converted to energy used by kinesin to transport the membrane packages, Johnson said.

"The binding of the head and molecule is coupled to the binding of ATP hydrolysis," he said.

Gilbert said kinesin is the first cytoplasmic motor whose mechanism has been determined. Johnson said about 20 other cellular motors exist, but their functions are not known. He said it may be possible that one motor is involved in chromosome movement for cell division and another may be involved in the secretion of insulin.

Johnson's research has shown how chemical energy was converted to cause a force of movement. This basic understanding of how one motor works may lead to understanding how the other motors work, he said.