Walking proteins need to rock and roll, new study finds

one possible explanation of how a single, 8-nanometer step of kinesin might occur:

First, a molecule of ATP binds with Head 1, causing the head to become firmly attached to the microtubule. Head 2 then leapfrogs over Head 1 by the maximum allowable distance of 5 nanometers, but instead of becoming rigidly fixed, Head 2 rocks back and forth on the surface of the microtubule. The breakdown of ATP then causes Head 1 to wobble uncontrollably, giving Head 2 just enough slack to roll forward an additional 3 nanometers. At that point, Head 2 becomes firmly bound to the microtubule, while Head 1 leapfrogs another step forward.

Each 8-nanometer step takes only about 10 milliseconds to complete, says Moerner, noting that the alternating rigid and wobbly states of the two heads could allow kinesin to complete a typical 1,000-nanometer walk in a few seconds before separating from the microtubule.

He points out that some kinesins work in teams in order to transport their cargo very long distances.

"There are some nerve cells that extend from the spine to the foot," Moerner observes. "It takes one or two days for kinesins to carry vesicle containers loaded with neurotransmitters from end to end via microtubules embedded in the nerve."

He compares the movement to a relay race in which kinesin molecules hand off vesicles to one another all the way down the nerve axon, thus guaranteeing a constant supply of neurotransmitters at the synapse located in the foot.

"We don't know all of the implications of our findings," Moerner points out. "In the future, we hope to observe the orientation of kinesin as it takes multiple steps. That's one of the virtues of single-molecule measurements: You can look at biological systems in a native environment, and thus see movement rather than a static structure."


Contact: Mark Shwartz
Stanford University

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