089-AP-98
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UC IRVINE RESEARCHERS PINPOINT STRUCTURE OF PROTEIN THAT MAINTAINS ENERGY IN LIVING CELLS
Irvine, Calif. ó A UC Irvine research team has discovered the sharpest image to date of the three-dimensional structure of a protein that pumps hydrogen ions out of cells, a fundamental activity necessary for cells to stay alive.
Knowing the protein's structure will unlock a wealth of information on a wide variety of biological functions: how cells maintain an interior different from the outside environment, how nerve cells transmit impulses and how energy is used in plant and animal cells, according to an article in the June 19 issue of Science magazine.
The structure discovered by the UCI researchers is different from previous findings by other researchers.
The protein, a hydrogen ion pump called bacteriorhodopsin, is a blueprint for many other proteins in cell membranes that act as receptors as well as pumps, and play crucial roles in keeping cells alive, said Dr. Janos Lanyi, professor of biophysics and physiology at UC Irvine's College of Medicine.
For example, bacteriorhodopsin is closely related to receptors in the human eye that turn rays of light into images processed by the brain. "It's the mother of all ion pumps in biology," said Lanyi.
Lanyi and Dr. Hartmut Luecke, professor of molecular biology and biochemistry at UC Irvine, directed the research team that revealed bacteriorhodopsin's structure.
Further research on this and other pumps could reveal new ways to design drugs that fight disease, and possibly show other ways to keep cells in the body healthy.
For years, scientists looking at how cells utilized energy focused on the bacteriorhodopsin protein, which is housed in an unusual source, a bacterium called Halobacter that lives only in salt marshes and other saline environments.
Researchers knew that this protein was unusually small and simple, and that promised an easier way to understand how it pumps hydrogen ions and thereby transforms light waves into energy for its host cell.
But they needed a firm idea of what bacteriorhodopsin looked like in all its details. These details in the structure would reveal the pathway through which the ions travel, and that would help researchers understand how ion pumps function in general.
Bacteriorhodopsin keeps the cell alive by pumping the charged hydrogen ions through the cell's membrane. Detailing the protein's changes through this pumping action paves the way toward understanding how nerve cells conduct electrical impulses, how cells interact with hormones, and how animal cells maintain balances of essential nutrients and energy sources.
The pumping cycle begins when light hits a molecule called retinal, embedded in the bacteriorhodopsin protein, and gives the protein a purple color. When exposed to light, the retinal undergoes changes in shape and chemistry; these changes begin the bacteriorhodopsin pumping cycle and drive the passage of hydrogen ions from one side of the membrane to the other.
This process is very similar to the way green plants absorb light to create energy.
The protein consists of a coiled chain that loops through the width of the cell membrane seven times. Along the chain, amino acids lie at critical locations to perform the pumping action: binding, releasing and passing ions through a channel formed by the protein.
Luecke, Lanyi and other researchers on the UCI team determined the proteinÃs three-dimensional structure using X-ray crystallography. This highly sophisticated technology uses X-rays to peer into the precise atomic structure of chemicals, including proteins.
Researchers using this method have discovered the configurations of thousands of chemicals, from table salt to DNA. X-ray crystallography starts with the laborious process of turning the chemical into crystals, so the molecules of the chemical are arranged in a precise repeating pattern. The crystals are then bombarded with X-rays, and the pattern of the X-rays diffracted off the crystals is recorded. The process generates massive amounts of data, from which researchers reconstruct the chemicalsà three-dimensional structure.
Further tests will include using lasers to get instant images of the protein as it zips through phases of its pumping cycle. "It changes configuration through each part of this energy cycle, so we'll see how it changes shape as it binds and releases ions," Luecke said.
Knowing how the structure changes shape through this pumping cycle will expand understanding of how cells maintain energy needs and provide basic functions to living organisms. "This structure we've uncovered links up well with a number of hypotheses we want to test," Lanyi said.
Note: A graphic of the bacteriorhodopsin is available on the web at http://www.communications.uci.edu/~inform/98releases/089ap98.html
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