Thursday, October 15, 2015

One Small Step for Kinesin

Adenosine triphosphate, or ATP for short, is the universal currency of energy among living things. It’s the gasoline that drives our cellular motors, the necessary intermediate step between chemical and kinetic energy. By and large we’re still figuring out the details of how that conversion process works, but a new result from the Polish Academy of Sciences, slated for publication in PRL, brings us one step closer to understanding the mechanics of motor proteins called kinesins.

Inside every one of your body’s trillions of cells, there’s a network of structures called microtubules, tiny fibers that connect the organelles to one another and to the envelope of the cell. This series of tubes, though considered part of the cytoskeleton, provides more than just support; it acts as the cell’s transportation infrastructure, acting as a series of “roads” that guide proteins and other cellular components to where they’re needed.

If the microtubules are roads, kinesins are the freight trucks, carting around proteins and larger intracellular structures. If you recall the complex spindle-and-chromosome ballet of mitosis, you’ve got an idea of the kind of intricate operations these proteins perform. On a physical level, though, kinesins are more like the AT-ATs from Star Wars than actual trucks; they transport their cargo one literal step at a time.

A simplified illustration of a kinesin and a microtubule.
Kinesins are comprised of two “feet”, which bind to certain sites along the length of the microtubule, joined by a string of protein that also links them to their cargo. For each step it takes, kinesin uses one molecule of ATP, tearing off a phosphate group and turning the molecule into ADP. For a long time, it was assumed that it’s the splitting of this ATP molecule that provides the motive force to put one foot in front of the other. However, this new result makes it look like the stepping process is less active than was previously supposed.

A kinesin can ordinarily apply a force of a few picoNewtons—enough, for example, to tear apart a hydrogen bond and unzip a strand of DNA. By increasing the viscosity of the fluid surrounding it, the scientists tested the protein’s response to drag force, to see if it could still “walk” through a thicker medium.

Surprisingly, rather than displaying a linear decrease in speed or efficiency as the viscosity of the fluid increased, the protein completely stopped functioning, even when there was a drag force of less than a femtoNewton—a thousand times less than the kinesin’s towing capacity. To explain why this happens, the researchers refer to a diffusion-based model of kinesin motion, suggesting that, as one foot remains on the microtubule, the other is allowed to drift forward, pushed primarily by Brownian motion, until an ATP molecule binds to it. When that happens, a sort of “molecular seesaw” drives protein chains from the foot to bind to the next site on the microtubule, until the ATP is split into ADP and the foot can unhook itself from the binding site.
A kinesin "walks" along a microtubule, putting its foot down 

Ultimately, this guided-diffusion-based transportation is more energy-efficient than a propulsion-based method, but only works under the right environmental conditions, as the strong viscosity dependence demonstrates. In addition to clearing up some questions about the economy of energy inside our cells, this research will hopefully inform future attempts at biomimicry, as molecular engineers in coming years design proteins and other nanoscale structures that harness the elegance of nature to do more with less.

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