por Lynn Shapiro
, Writer | September 30, 2009
The researchers' technique is analogous to looking at a dark highway from the window of an airplane. The dark, invisible lanes of roads are microtubules, the skeleton of the cell. The nerve growth factor molecules ride in the cars that are illuminated by their quantum dot headlights. One thing Cui and her colleagues have observed, which has never been seen before, is that packages in transport can jump from one microtubule to another as they move along -- like cars switching lanes as they roll down the highway. They also discovered that the majority of those cars are single "passenger"; they only contain a single nerve growth factor molecule.
Scientists have known for a long time that these proteins are essential, since they help the nerve cells survive by regulating gene expression. But Cui and her colleagues showed that even a single molecule of nerve growth factor is enough to trigger the transport process and sustain signaling during axonal transport to the cell body. (Paper LSThB3, "Single Molecule Imaging of Axonal Transport in Live Neurons" is at 9 a.m. Thursday, Oct. 15).
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WATCHING PROTEINS FOLD
One of the most important biological actions is the folding and unfolding of protein molecules. But getting hold of single protein molecules is difficult, and monitoring their gymnastic gyrations is even more so. Scientists at Harvard University have produced new video-based "optical tweezers" techniques for doing just this, enabling ultra-precise measurements to be made in a way that is simple and effective. The current U.S. secretary of energy, Steven Chu, won a Nobel Prize for his contribution toward controlling atoms with laser beams inside an enclosed trap; he later pioneered the use of laser beams for actually holding tiny objects--even biological molecules--in place. The Harvard device is among the latest and most versatile use of this optical tweezers approach.
Wesley Wong of the Rowland Institute at Harvard and his colleagues have developed a unique optical tweezers system that uses a combination of interference imaging, light modulation and custom software algorithms to achieve the necessary resolution and stability to watch proteins fold. This system, which employs already-existing optical technology components, utilizes 3-D video tracking to measure the lengths of short molecular tethers with angstrom resolution (less than 1 billionth of a meter) and active feedback control for a force stability of femtoNewtons (10-15 Newtons). Fluctuations can be glimpsed at rates faster than 100,000 frames per second -- all with inexpensive video imaging. The act of protein folding is quantified by measuring the end-to-end distance of a single molecule while the strength of the tweezers' grip is varied.