Research into the mechanism of how cells transport nutrients, conducted at the Max Planck Institute of Colloids and Interfaces in Potsdam, has produced a model of dual molecular motors, special protein molecules of kinesin or dynein type that move along the microtubules, that compete in a tug-of-war over the direction to travel.
The kinesin motors move to the end of the microtubule that biologists call the positive end, while the dynein motors move to the minus end. The findings of the Potsdam-based scientists show that the stronger motor team determines the direction in which the cargo is moved. It involves a tug-of-war where opposing motors break off from the microtubule. It was previously assumed that there was a system of coordination that allowed for only one team of motors; it was believed this alternated between one team and the other.
"The tug-of-war is the simplest imaginable mechanism," says Melanie Müller, one of the scientists involved in the project. "But it is possible, if you consider the properties of the individual motors measured experimentally. They produce a strong non-linear reaction when they are pulled." A motor from the losing team is subject to a strong force and is quickly removed from the microtubule. The remaining motors must then take the force of the winning team alone and are also removed even more quickly. In a domino effect, the losing motors concede and are removed from the microtubule until no others remain. The winning team is then able to transport the cargo quickly, unopposed. "However, the cell does not leave it to chance to ensure that the cargo arrives at its destination. Regulatory proteins probably intervene," says Melanie Müller.
Researchers into the transport of fat particles in Drosophila embryos examined whether her model applied in reality. It is actually explained by experimental observations that took place previously on the transport mechanism. A cargo in a microtubule does not move directly from one end to the other. It is constantly pulled back in the opposite direction. The losing motors can, however, occasionally remove the winning ones from the microtubule as heat sometimes blows the winning motors away. The cargo particles therefore move in both directions.
Press release: Tug of war in the cells
Illustration: The competition between molecular motors: A blue cargo is transported by two teams of molecular motors moving along the yellow microtubule. The red team of motors is pulling to the right towards the positive end (+), while the green team is pulling to the left towards the minus end (−). When both teams pull (in the center), they cancel each other out so the cargo hardly moves forwards. As soon as one team gains the upper hand, it moves quickly as the opposing motors are removed from the microtubule. Credit: Melanie Müller