Max Planck researchers determine optimal parameters for biomimetic transport systems based on molecular motors
Molecular motors are nanoscale engines which move along very thin rod-like filaments and, in this way, drive the heavy traffic of molecular cargo within biological cells. Both motors and filaments can be isolated from the cells and used to construct biomimetic transport systems. In order to increase the flux of the cargo transport, it would be necessary to increase the number of motors that contribute to this transport but, at the same time, avoid the build-up of traffic jams. Scientists from the Max Planck Institute of Colloids and Interfaces in Potsdam and from the University of Amsterdam have now modelled and simulated the motor traffic for different compartment geometries and filament arrangements, and have determined the optimal conditions for the transport of nanocargo in these systems (Biophysical Journal, 88, 3118-3132, May 2005).
Each cell of our body contains a huge number of small vesicles which exhibit complex patterns of intracellular traffic: some vesicles travel from the cell center to the periphery and vice versa, some shuttle between different organelles or cellular compartments. An extreme case is provided by the long-ranged transport of vesicles and organelles along the axons between our nerve cells, which can be as long as half a meter. All of these movements are based on two molecular components: very thin rod-like filaments, which form a complex network of rails, and molecular motors, which move along those filaments and carry vesicles and other nanocargo along. When bound to the filaments, the motors are able to transform the chemical energy of a single ATP molecule into mechanical work. In this way, they can utilize the smallest possible amount of fuel.
Both filaments and motors can be isolated from biological cells and used to construct biomimetic transport systems. A relatively simple example for such a system consists of filaments which are aligned on a substrate surface as shown in Figure 1. The filaments are polar and have two different ends, a ‘plus’ end and a ‘minus’ end. In Figure 1, the filaments are arranged in such a way that all ‘plus’ ends point into the same direction. Such an arrangement provides many parallel tracks for the molecular motors and, thus, represents a multi-lane highway in the nanoregime. Using such a biomimetic model system, scientists can study the transport properties in a quantitative manner, identify useful control parameters, and determine the functional dependence of the transport properties on these parameters. This is the only possible strategy to obtain the basic knowledge that is necessary to improve the system design and to optimize its performance.
We now have a basic understanding of the behavior of single motors. These motors are dimeric proteins with two legs, which make discrete steps along the filament. Each step corresponds to a motor displacement of about 10 nanometers, comparable to the size of its legs. In one second, the motor makes about 100 steps which leads to a velocity of about one micrometer per second. The absolute value of this velocity is not very impressive, but relative to its size, the motor molecule moves very fast: indeed, on the macroscopic scale, its movement would correspond to an athlete who runs 200 meters in one second! This is even more surprising if one realizes that the motor moves in a very viscous environment since it steadily undergoes many collisions with a large number of water molecules. Because of these collisions, the molecular motor has a finite run length: After a few seconds, it unbinds from its track and performs random Brownian motion in the surrounding water until it rebinds to the same or another filament.
