Walking proteins need to rock and roll, new study finds

ovides energy for all cellular activity in the body. From different lines of evidence, scientists have proposed a model in which chemical changes in ATP cause kinesin to alternately bind each of its two heads to a microtubule. This "head-over-head" leapfrogging action propels kinesin forward, much as a child moves along a horizontal rope by alternately placing one hand in front of the other.

Unlike kids on a playground, kinesin activity is measured in nanometers. One nanometer is one-billionth of a meter - or about 50,000 times smaller than the width of a human hair.

Stanford biophysicist Steven Block recently discovered that kinesin heads walk in steps that are exactly 8 nanometers apart. But X-ray crystal analysis of kinesin shows that its two heads are separated by only 5 nanometers of space. So where do the additional 3 nanometers come from that are necessary to complete an 8-nanometer step?

To solve the puzzle, Moerner and his colleagues used a novel technique that allowed them to observe the movement of a single kinesin molecule - an object only one ten-millionth of an inch long.

"We use a fluorescing label molecule - or fluorophore - to measure orientation," says Moerner. "The fluorophore, which is bound to the kinesin head with two chemical bonds, absorbs and emits light in a special pattern as the head turns."

By measuring different levels of brightness and darkness, Moerner and his co-workers were able to determine the orientation of the head in relation to a microtubule in the presence of the different chemical forms of ATP.

The results were surprising.

"If the head were rigidly attached to a microtubule, we'd expect to see a light-dark-light-dark fluorescent pattern," notes Moerner. "But what if the head is rocking around? Then all of the images would have equal brightness. That's what we observed: With one of the forms of ATP, kinesin rocks!"


Moerner and his team offer

Contact: Mark Shwartz
Stanford University

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