Researchers at Howard Hughes Medical Institute's Janelia Farm Research Campus, the National Institutes of Health, and Florida State University have developed and applied a new light microscopy technique that will allow them to determine the arrangement of proteins that make up the individual organelles, or structures, within a cell.
The microscope and the technology that make it possible are described in an article appearing on-line in the August 10 issue of "Science Express." The technique was conceived by Eric Betzig, Ph.D., and Harald Hess, Ph.D. while working as independent inventors and later as investigators at Janelia Farm, which subsequently supported their effort on the project. Funding for the project was also provided by the NIH. Drs. Betzig and Hess built the microscope and demonstrated the method at the NIH, while working with Jennifer Lippincott-Schwartz, Ph.D. and her colleagues in the Cell Biology and Metabolism Branch of the National Institute of Child Health and Human Development. Also working on the project was Michael Davidson of the National High Magnetic Field Laboratory at Florida State University.
"This is a major advance that will allow us to understand the fundamental organization of the key structures within a cell," said Elias A. Zerhouni, M.D., Director of the NIH. "What researchers learn from the new microscopy technique will provide a broad foundation for understanding the complexity of how proteins, the building blocks of cells, interact in health and disease."
The new technique is known as photo-activated localization microscopy (PALM). It relies on the earlier pioneering effort of Dr. Lippincott-Schwartz and NIH Staff Scientist George Patterson, Ph.D. to develop a new class of molecules, called photo-activated fluorescent proteins, which emit green or yellow light when exposed to a laser, but only after being activated by brief exposure to violet light. The cell itself is coaxed to produce these molecules, which are then bound to specific proteins of interest, thereby optically marking the molecular constituents of specific cellular structures.
In a conventional optical microscope, objects less than about 200 nanometers apart cannot be distinguished from one another. The trick of the new technique is to control the violet light to activate only a few molecules at a time, so that they are statistically likely to be well separated. Even though each fluorescing molecule still appears as an approximately 200 nanometer diameter spot, the center of the spot, and hence the location of the molecule, can be determined to within 2 to 25 nanometers, depending on its brightness.
"It's important to activate only a few fluorescent proteins at a time, or else you'd only see one bright blur of light, without being able to distinguish the individual position of the protein," Dr. Lippincott-Schwartz said.
Repeating this process many thousands of times, a computer image is eventually created in which the positions of all the molecules are determined, often with near-molecular precision. Currently, the main tool researchers use to produce high resolution images of the structures within a cell is an electron microscope. Although electron microscopes produce a detailed image of very small structures, they cannot provide an image of the proteins that make up those structures.
With the new technique, the researchers were able to study several cellular subsystems, including the mitochondria, the structures within a cell that provide energy for the cell's activities. The researchers were able to visualize the distribution of the proteins involved in the assembly and budding of the AIDS virus from a host cell.