Motor proteins may be vehicles for dru...( Specialized motor proteins that tran...),Health

Specialized motor proteins that transport cargo within cells could be turned into nanoscale machines for drug delivery, according to bioengineers. Chemical alteration of the proteins' function could also help inhibit the growth of cancerous tumors.

Each cell in the body contains motor proteins that ferry cargo such as chromosomes, mitochondria or bundles of proteins, either from the center of the cell to its outskirts or from the periphery toward the nucleus. Most motor proteins contain two motor domains, or heads, that are attached to a shared cargo-binding domain, or tail.

"Think of it as a freight train at the molecular level," said William Hancock, associate professor of bioengineering, Penn State. "And it runs on cylindrical tracks -- or microtubules -- made of many protein subunits meshed together into a long polymer that is one ten thousandth the diameter of hair."

Hancock and his colleagues are studying a particular motor protein known as kinesin-2. They are trying to understand the molecular mechanics of how these nanometer-scale proteins move within the cell.

"Kinesin motor proteins move by changing their shape," explains Hancock. "The two motor domains alternately bind to the microtubule, generate force and then detach, and the resulting displacement drags the cargo forward."

To power this hand-over-hand motion, the proteins convert the chemical energy of ATP molecules -- a common energy source in cells -- into mechanical work. But there is a problem if the proteins fall off their tracks.

"When a motor binds to the microtubule, it 'walks' about 100 steps -- each step being eight nanometers -- before detaching," said Hancock, whose findings appeared in a recent issue of Current Biology. "And the proteins are so small that if both motor domains let go, the proteins and their cargo would diffuse away within a few milliseconds. This profound effect of diffusion is one of the places where the nano
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