Cell-Based Nano-machines Stronger Than Thought

Researchers have known for quite some time that a long, fibrous coil grown by a single-cell protozoan is, gram for gram, more powerful than a car engine. However, they have recently discovered that this coil is far stronger than once thought. Furthermore, the investigators have gleaned insights into the process behind this microscopic workhorse.

Scientists at Whitehead Institute (Boston, MA, USA), the Massachusetts Institute of Technology (MIT; Boston, MA, USA), Marine Biological Laboratory (Woods Hole, MA, USA), and University of Illinois, Chicago (USA) have conducted a study on a protozoan called Vorticella convallaria. "These findings are twofold,” stated Danielle France, a graduate student in the lab of Whitehead member Dr. Paul Matsudaira, and, along with Matsudaira, a member of MIT's division of biological engineering. "First, they give us an idea of how a cell can manage to generate such enormous force; and second, they provide clues for how engineers might reconstruct these mechanisms for nano-scale devices.”

Scientists have known about this nano-spring for approximately 300 years, ever since Anton van Leeuwenhoek first noticed the protozoan, Vorticella convallaria, through a hand-made microscope. The spring in the unicellular Vorticella is a contractile fiber bundle, called the spasmoneme, which runs the length of the stalk. When resting, the stalk is elongated similar to a stretched telephone cord. When it contracts, the spasmoneme recoils back in a flash, forming a tight coil. To find out how strongly Vorticella recoils, the researchers utilized a unique microscope to apply an extra load to the spring. The microscope, developed by Dr. Shinya Inoue and colleagues at the Marine Biological Laboratory, uses a spinning platform to increase the centrifugal force exerted against the protozoan.

In the past, researchers have measured Vorticella's ability to recoil its spring at 40 nano newtons of force and at a speed of 8 cm per second, units of measurement that are usually too large to be relevant for biologic processes. (These measurements, when scaled up to the size of a car engine, attest that the Vorticella to be the more powerful of the two.) However, when Ms. France used the centrifuge microscope, she discovered that the spring was able to recoil against as much as 300 nano newtons of force. "This is the maximum amount of power we can currently test,” remarked Ms. France. "We suspect the coil is even more powerful.”

Ms. France and coworkers also made a significant link between the engine's fuel source, calcium, and a key protein component of the stalk. This protein, centrin, belongs to a class of proteins that can be found in organisms ranging from green algae to humans. When the scientists introduced an antibody for the Vorticella centrin into the cell, the spring was no longer able to contract, indicating that the cell uses a powerful centrin-based mechanism, one that is unlike other known cellular engines.

"When it comes to creating nanodevices, this is a great mechanism for movement,” said Ms. France. "Rather than requiring electricity, this is a way to generate movement simply from a change in the chemical environment. Here, a simple change in calcium would power this spring.” The researchers are now developing techniques for replicating this process in the lab.

This research was funded by the U.S. Army. Ms. France presented her findings in December 2005 at the 45th Annual Meeting of the American Society for Cell Biology in San Francisco (CA, USA).