Progress through Mechanics: Biomolecular Motors

Vivek Verma

Graduate Student, Department of Engineering Science and Mechanics, The Pennsylvania State University
188 MRI, 230 Innovation Blvd, University Park. PA 16803
Phone 814-865-0803 Fax 814-865-7173 Email vzv103@psu.edu

All living creatures are made up of cells that have the ability to replicate themselves in a repetitive process called cell division. As these cells mature and divide into two there is an extensive movement of cellular components. In order to perform this essential task that sustains life, cells have evolved machines composed of proteins to perform this function.

Biological motors, such as kinesin and dynein[1-3], transport intracellular cargo and position organelles in eukaryotic cells via unidirectional movement on cytoskeletal tracts called microtubules. Microtubules are made up of cylindrical polymers of the protein tubulin that typically have a diameter of 25 nm and are several micrometers in length[4]. In order to transport cargo or reorganize organelles along these microtubules, molecular motors convert chemical energy from the hydrolysis of adenosine triphosphate(ATP)[1] into mechanical energy. For example, kinesin can exert a maximal force of 6 pN[2] and can be immobilized on surfaces with densities of ~10³ motors/μm² [5] delivering a cumulative force on the order of nN/μm². These motors have been purified from cell cultures and quantified for standards such as velocity and power stroke per cycle. The ability to use biological motors in vitro makes them strong candidates for high-efficiency nanoengines. These are nature-made motors with a certain degree of robustness and a very high specificity to the types of cargos to which they bind. These motors are complete in themselves with many opportunities to use them for making different devices and structures. Some applications of biomolecular systems are envisioned below.

(a) Powering switchable devices: Nanofabrication processes have enabled the production of devices and structures with dimensions less than 10 nm. Motor technologies, based on silicon or other material systems, useful for powering such devices are not available, thus directing attention towards hybrid systems integrating synthetic molecules or biological motors [6-8]. Motors can easily be purified from cell cultures and have a high-energy conversion efficiency (~50%). Earlier studies have proved that the forces produced by the biomolecular motors are sufficient to power micro/nano electro-mechanical system (MEMS/NEMS) devices. For instance, if a micro gear with microtubules oriented on the cogs of the gear is placed on a bed of motors, it will start to rotate. Similarly, micropumps and microcantilevers powered by molecular motors have also been envisioned.

Microtubules have directional polarity, the fast polymerizing side is called the plus end and the other side is called the minus end. Kinesin moves towards the fast growing side of microtubules and dynein walks towards the minus end. These motors can be patterned juxtaposed on microfluidic channels, so that a device patterned with microtubules can be in contact with only one kind of motor. Changing pressure in the channels will allow a second type of motor to come in contact with a microtubule and the device will change the direction of motion. This will result in a switchable device.

(b) Directed self assembly: Microtubule based biomotors can be used for directed assembly of synthetic nanostructures. The fact that motor proteins walk unidirectionally on microtubules can be used for such self assembly. For this kind of self assembly all microtubules should be aligned with the plus end on one side and minus end on the other to the substrate. Motor proteins functionalized with the synthetic molecules can then be incubated on the substrate. Motors, e.g. kinesin, attached to synthetic molecules in solution can bind to the microtubules and walk along to the plus end. This way, kinesin will carry the synthetic molecule to the other end. The atomic force microscope (AFM) can also be used to assemble molecules but the process is very slow because the AFM tip can position only one particle at a time. On the other hand, there will be millions of motors working in concert to assemble a nanostructure, resulting in a faster process.

(c) Protein separation and identification: Kinesin and related motor proteins can be mutated so that they bind to specific cargos. This property can be employed to separate a particular protein from a protein mixture. Modified motors can be incubated in a mixture of proteins so that they bind to the desired protein. The motors will then bind to microtubules, walk along them and carry out the separation.

The biggest challenge in using biological motors is the lack of interdisciplinary expertise. To engineer systems using biological proteins it is necessary to pattern proteins and align microtubules at specific locations. To facilitate this complicated process, combined efforts from cell-biology, physics, chemistry, microelectronics and nanotechnology are needed. The lack of communication amongst the different disciplines is one of the main issues slowing the application of biomolecular motors in practical life.

Molecular motors offer several applications from devices to molecular self-assembly. There are innumerable ways these motors can be used, from transporting molecular shuttles to organizing molecules on surfaces. These motors can also help us understand certain cell functions in a better way. For example, the cell’s ability to transport a specific cargo to a particular location using different biomolecular motors encourages us to make devices that will separate different molecules. Understanding how cells orient microtubules during cell division and the various motors responsible for different tasks that eventually lead to cell division may one day hold the key to fighting cancer. It is well known that cancer cells divide in an uncontrolled manner. And just like any other cell division, biomotors are also required here. Research is in progress to investigate the role of each type of kinesin and dynein so that cell division is better understood. This type of research will eventually help in discovering the motors critical for cancer cell growth. This way, specific motors can be targeted within cancer cells which will restrict cancer cell division and hence the growth of cancer.

With increasing demands for innovative ideas in micro/nanoscale devices, nature has presented us with a wide opportunity to utilize these motors in vitro, to study their role in important cellular processes such as mitosis, and to understand their implications in certain human diseases such as cancer.

References:
[1] J. Howard, "The movement of kinesin along microtubules," Ann. Rev Physiol, vol. 58, pp. 703-29, 1996.
[2] R. D. Vale and R. J. Fletterick, "The design plan of kinesin motors," Ann. Rev Cell Dev Biol, vol. 13, pp. 745-77, 1997.
[3] F. D. Warner and J. R. McIntosh, Cell Movement, Volume 2, Kinesin, Dynein and Microtubule Dynamics. New York: Alan R. Liss, Inc., 1999.
[4] E. Nogales, Wolf, S. G., Downing, K. H., "Structure of the alpha beta tubulin dimer by electron crystallography," Nature, vol. 391, pp. 199-203, 1998.
[5] W. O. Hancock and J. Howard, "Processivity of the motor protein kinesin requires two heads," J Cell Biol, vol. 140, pp. 1395-405, 1998.
[6] G. D. Bachand and C. D. Montemagno, "Constructing Organic/Inorganic NEMS Devices Powered by Biomolecular Motors," Biomedical Microdevices, vol. 2, pp. 179-184, 2000.
[7] C. D. Montemagno and G. D. Bachand, "Constructing nanomechanical devices powered by biomolecular motors," Nanotechnology, vol. 10, pp. 225-231, 1999.
[8] M. D. Wang, M. J. Schnitzer, H. Yin, R. Landick, J. Gelles, and S. M. Block, "Force and velocity measured for single molecules of RNA polymerase," Science, vol. 282, pp. 902-907, 1998.