|Kinesin Molecular Motors Can Exhibit Spiral Motion|
|SciMed - Biology|
|TS-Si News Service|
|Monday, 21 May 2012 14:00|
München, Germany. Kinesins, molecular motors key to cellular transport, can exhibit spiral motion, challenging assumptions that kinesins move only on straight paths.
Kinesin movements are important to critical cellular functions including mitosis (cell division), meiosis (cell division necessary for sexual reproduction) and the transport of cellular cargo.
Molecular motors are fundamental to the development of higher forms of life. They transport proteins, signal molecules and even entire chromosomes down long protein fibers, components of the so-called cytoskeleton, from one location in the cell to another. Similar to round-the-clock vehicle traffic on a highway, thousands of these small motor proteins comprise a highly coordinated and extremely fast mode of transport. This highly efficient infrastructure is a prerequisite for the formation of large, complex cells and multicellular organisms. Bacteria, for example, lack this foundation, because they possess neither molecular motors nor cytoskeletons.
Kinesin-1-type: Straight Movement. This film shows the straight movement of kinesin-1 type kinesins.
Kinesin-2 type Movement: Lane-change.. This film shows the spiraling movement of kinesin-2 type kinesins.
Videos courtesy of Z. Oekten, M. Brunnbauer, and Molecular Cell.Kinesins represent one class of such molecular motors, running along microtubules comprising 13 individual fibers arranged in a tube form. Made up of a twisted pair of protein chains, each kinesin chain comprises a head that can dock to the surface of the microtubules and a neck domain, as well as a stalk and tail domain attached to the cargo.
Kinesins move forward by placing one head in front of the other in alternation, resembling a human walking.
Kinesin-1, which performs numerous mechanistic steps in succession without detaching from the microtubule, was the first to be scrutinized. In the process it moves ahead in a perfectly straight path on its long journey, always remaining on a single fiber of the microtubule.
Scientists led by Zeynep Oekten, group leader at the Biophysics Department of the Technische Universität München (TUM), and Melanie Brunnbauer, a doctoral candidate at the Biophysics Department, have now for the first time demonstrated that kinesins also switch lanes during transport. The findings appear in the journal Molecular Cell.
The scientists identified a structural element in the neck domain of the kinesin protein that determines whether a given kinesin type moves on a straight path or in a spiral fashion.
"If the neck region is stable, the two kinesin heads have only limited reach. The kinesin cannot make any sidesteps and thus moves straight ahead," says Oekten. "However, if the responsible area becomes destabilized, the reach of the heads is increased and the motor protein can jump fibers and spiral around the microtubule."
To confirm this, the scientists integrated specific amino acids into the responsible areas a kind of molecular switch that allowed them to regulate the reach of the two heads.
The result left no doubt: destabilizing the neck region of the Kinesin-1 motor increases the reach of the two heads, which in turn causes the Kinesin-1 to depart from its normally perfectly straight path and move along a spiral-shaped path. When they mimicked a stable neck region using a chemical crosslinker, they coerced the protein into running straight again.
Oekten and Brunnbauer arrived at their new insight using a unique experimental setup. They placed two 3-micron large synthetic beads in a solution and trapped each using a laser beam, a pair of optical tweezers. They then placed a piece microtubule between the beads. In a final step, again using a laser beam, they trapped a third bead coated with a specific type of kinesin and carefully placed it onto the microtubule.
As soon as they deactivated the third laser beam, the motor protein started marching forward and the scientist could follow the path of the molecule under the microscope. "In this way we were able, for the first time ever, to directly observe the spiraling movement of a motor type," explains Oekten. "When we saw the teetering movement of a Kinesin-2 protein for the first time, we all laughed. The motion was so clear and obvious, you just had to look at it and all doubt vanished."
The experimental setup allows the molecular motors to move freely, thereby emulating real-life conditions in the cell much better than previous methods of investigation. Oekten and Brunnbauer investigated a whole series of different Kinesin-2 proteins from various organisms with an unexpected result: Contrary to the hitherto prevalent assumption that kinesins typically move only on straight paths, almost all kinesins displayed some form of spiral movement, in manifold variations.
"This shows us that spiral motion is not an exception in nature, but rather the rule," explains Oekten. "In fact, the more relevant question is why evolution has brought about the straight-line movement as we observe with the Kinesin-1. That is truly unusual considering the nano-scale precision it requires to confine a kinesin transporter on an exclusively straight path." Oekten and Brunnenbauer hope to more closely investigate the reasons for the various kinds of motion in the future.
FundingThe research was funded by the Deutsche Forschungsgemeinschaft (DFG).
CitationTorque Generation of Kinesin Motors Is Governed by the Stability of the Neck Domain. Melanie Brunnbauer, Renate Dombi, Thi-Hieu Ho, Manfred Schliwa, Matthias Rief, Zeynep Ökten. Molecular Cell 2012; 46(2): 147-158. doi:10.1016/j.molcel.2012.04.005
● Trajectories of kinesins on suspended microtubules are analyzed
● Kinesin-2 motors display a broad range of spiraling around microtubules
● The stability of the neck region is the molecular determinant of spiraling
In long-range transport of cargo, prototypical kinesin-1 steps along a single protofilament on the microtubule, an astonishing behavior given the number of theoretically available binding sites on adjacent protofilaments. Using a laser trap assay, we analyzed the trajectories of several representatives from the kinesin-2 class on freely suspended microtubules. In stark contrast to kinesin-1, these motors display a wide range of left-handed spiraling around microtubules and thus generate torque during cargo transport. We provide direct evidence that kinesin's neck region determines the torque-generating properties. A model system based on kinesin-1 corroborates this result: disrupting the stability of the neck by inserting flexible peptide stretches resulted in pronounced left-handed spiraling. Mimicking neck stability by crosslinking significantly reduced the spiraling of the motor up to the point of protofilament tracking. Finally, we present a model that explains the physical basis of kinesin's spiraling around the microtubule.
|Last Updated on Sunday, 20 May 2012 18:44|