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Scientists Catch a Biological Light Sensor in the Act

March 7, 1997, La Jolla, CA -- A team of scientists led by Elizabeth D. Getzoff, Ph.D., at The Scripps Research Institute (TSRI) has succeeded in determining the structure of a biological photosensor in the very instant in which it responds to a light stimulus. These observations show, in atomic detail, how a photoreceptor protein transforms a single light particle into a biological signal. The results have implications for such areas as the development of optical transistors and intracellular signal transduction.

The article entitled, Structure of a Protein Photocycle Intermediate by Millisecond Time-Resolved Crystallography, was published in today's issue of Science.

Traditionally, scientists have used x-ray crystallography to determine the three-dimensional structure of proteins. However, the traditional methods of protein structure determination have one severe setback, according to Ulrich Genick, a graduate student working on the project. "They are much slower than the primary events in almost all protein mediated biological processes allowing structures to be determined only for the very long lived resting states of proteins, precluding what has been termed, 'molecular action photography.'"

TSRI's team, working in collaboration with Keith Moffat, Ph.D., and his colleagues at the University of Chicago, used a technique known as time-resolved Laue x-ray diffraction to reduce the time required for data collection from several hours to 10 milliseconds, thus allowing structure determination of a photoreceptor in its short-lived action state. This increase in speed is made possible by the use of x-rays created in synchrotrons, a class of particle accelerators. By using the broad spectrum of very intense x-rays produced by the National Synchrotron Light Source at Brookhaven National Laboratory, the researchers were able to collect a multitude of data in a single brief exposure to the x-ray beam while the traditional approach would require the collection of multiple exposures as the sample is rotated.

The result of more than 10 years of scientific inquiry, the technical advances made during this project have opened the door for many other applications of time-resolved diffraction techniques. Other scientists in the field describe the study as "the coming of age for the Laue diffraction technique."

Photoactive Yellow Protein (PYP), the protein under investigation by TSRI's team, is a small biological photoreceptor isolated from a species of purple bacterium that lives in evaporating bodies of salt water. When the level of near-ultraviolet light becomes too high, PYP sends a signal that causes the bacterium to swim away from the harmful radiation. The work shows in atomic detail how the initial steps of this process work.

Within PYP, the light-capturing chromophore changes its shape after catching a photon. This causes a rotation around the otherwise rigid, central double bond of this molecule. As a result, the way the chromophore fits into its nest is changed. The PYP protein responds by adapting its structure to the new geometry of the chromophore. According to Dr. Getzoff, these findings are considered a breakthrough that could serve as a paradigm for the field of biological light sensing.

The findings also are relevant to the field of material science in terms of the color changes that go along with these structural rearrangements. When PYP adopts its activated structure it loses its brilliant yellow color, which only recovers as it returns to the resting state. This in essence makes PYP an optical transistor in which one beam of light could be used to alter the state of a PYP-containing sample so that a second beam of light could either pass or would be blocked. Getzoff said, "It is our hope that knowledge of the molecular structure of PYP in different functional states will help guide our efforts to engineer variants of PYP with defined optical properties that might someday be used to develop protein-based optical computers."

This research is supported by the National Institute of General Medical Sciences, National Institutes of Health under Program Director John Norvell.


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