Traffic Signals in the Cell

I saw a film a while back where a key part of the plot involved the main characters establishing control over the traffic lights in a major city. By slightly “adjusting” the action of the lights, they were able to create utter chaos in a way they desired. The plot revealed how dependent we are on those “blasted” traffic lights to maintain order in our life.

In a new study of the “traffic flow” within the body’s cells, biologist Ram Dixit and his colleagues examined how the functions of molecular motors dynein and kinesin are affected by the presence of other proteins called MAPs (microtubule-associated proteins). As they noted in their paper, the active transport of cell material along microtubules, the “highways” of cell structure, is critical for cell organization and operation. Defects in this process can be related to various dysfunctions and disease.

Much of the transportation of materials in cells depends on dynein and kinesin, which move cargo toward the cell nucleus and toward the cell boundary, respectively. As reviewed in an earlier post, dynein and kinesin have very different structures and mechanisms for accomplishing motion. Kinesin is a compact motor that walks along a single filament. In contrast, dynein is much larger and more complex, and is able to step across filaments. A wonderful animation of this activity is available on the Studio Daily website.

Dixit and colleagues conducted a series of studies where they followed the movement of the two types of motors along a microtubule filament. Collections of MAPs, such as tau, were bound to various points of the filament. The dynein and kinesin compete with the MAPs for binding to the microtubule surface. In this circumstance, the MAPs provided a form of regulation of the motor proteins, serving as a kind of traffic signal.

The researchers were able to directly observe individual encounters between single motors and the tau protein on the microtubule tracks. This helped them to determine how the motors responded to the obstacles in their path. The dynein tended to reverse direction, whereas kinesin tended to detach. In either case there was some dependency on the concentration of the tau protein. The team concluded that the MAPs regulate the motion of the molecular motors to maintain a balance of their transport function. It’s a traffic signal that works!

In my previous post on molecular motors I noted their superior design compared to those designed and manufactured by the best human engineers. As we see from this study, the molecular motors do not just randomly perform their task with some statistical benefit, but work in the presence of regulatory proteins to achieve greater efficiency of function. Such elegance provides more evidence for the hand of a master Designer.