Molecular motors on the run

Researchers from the Max Planck Institute of Colloids and Interfaces in Germany have developed a theory stating that only a few motor molecules should be enough for directed transport over centimeters or even meters. It's probably a meaningless comparison, but it's like if you were able to walk to the moon and come back.

In the nanotech world, molecular 'motors' have been heavily investigated during the last decade. And you probably read that these nano-carriers will one day be able to move a useful drug right where it's needed inside your body. But think for a minute to the size gap between yourself and a molecule. It's pretty impressive! Now, according to this news release, researchers from the Max Planck Institute of Colloids and Interfaces in Germany have developed a theory stating that only a few motor molecules should be enough for directed transport over centimeters or even meters. It's probably a meaningless comparison, but it's like if you were able to walk to the moon and come back.

First, what are molecular motors?

Molecular motors are nano-tractors for all kinds of cargo within the cells of living beings. They move in a stepwise manner along filaments of the cytoskeleton, consuming energy provided by the hydrolysis of ATP, which can be considered the fuel of the cell. Kinesin and dynein motors move along microtubules and myosins move along actin filaments. The step sizes of these motors are of the order of 10 nm.

However, even if molecular motors have been actively studied, their behavior is still not perfectly known.

Molecular motors, unlike railways or cars, have a strong tendency to fall off their track and diffuse away into the surrounding aqueous solution. This is a direct consequence of their nanoscale size which makes them rather susceptible to thermal noise. Thus, a single molecular motor can only 'grab' onto the filament for a relatively short time, on the order of one second. During this time, a single motor covers about one micrometer, which represents only a tiny fraction, about 1/10000, of the long transport distances for cargo particles in axons. In other words, a single motor behaves much like a sprinter, whereas the whole cargo performs a marathon.

Below is a diagram showing the molecular motors carrying their load. "Each motor can unbind from and rebind to the filament, which implies that the number of motors that actually pull the cargo varies with time." (Credit: Max Planck Institute of Colloids and Interfaces). Here is a link to a larger version of this image.

Molecular motors carrying their load

And here is how the German scientists explain their theoretical model.

If the cargo is pulled by several motors as shown in [the figure above,] any motor that unbinds from the filament will stay close to that filament as long as the cargo and filament are still cross-linked by at least one bound motor. In such a situation, the unbound motor can rebind to the filament and then continue to pull the cargo – in contrast to human sprinters, molecular motors don't get tired.

The research work -- which is not currently available online -- will be published by the Proceedings of the National Academy of Sciences (Proceedings of the National Academy of Sciences) under the title "Cooperative cargo transport by several molecular motors."

And fore more information, you can visit the sites of the authors, Reinhard Lipowsky and Stefan Klumpp, who are both scientists at the Max Planck Institute of Colloids and Interfaces (MPI-KG).

Sources: Max Planck Institute of Colloids and Interfaces news release, via EurekAlert!, November 14, 2005; and various web sites

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