Atomic switch capable of discriminating color in a bacterial membrane protein

Researchers using extremely high resolution imaging have found an atomic switch capable of discriminating color in a bacterial membrane protein.

In a paper posted today on Science Express, the rapid advance publication page of Science, scientists from The University of Texas Medical School at Houston and the University of California , Irvine, describe the versatile light-sensing protein at levels of resolution smaller than a nanometer – one billionth of a meter.

“High-resolution X-ray crystallography revealed the light-absorbing part of the protein was present in two alternative positions, suggesting to us that light of different colors drives this protein back and forth between two differently colored states of the protein,” said corresponding author John L. Spudich, Ph.D., director of the Center for Membrane Biology in the UT Medical School Department of Biochemistry and Molecular Biology.

“Chemical analysis and spectroscopic methods then proved that the switch, buried in the middle of this membrane-embedded protein, similar in structure to our visual pigments, is controlled by blue versus orange photon absorption.” Spudich said.

That function makes the protein novel among its family of light-sensing proteins known as rhodopsins, which are present in microbes and higher animals. In human eyes, rhodopsin is the light-absorbing pigment of the rods, located in the retina.

The team studied a new-found rhodopsin on the surface membrane of the bacterium Anabaena, classified as “blue-green algae” or cyanobacteria, which rely on photosynthesis to generate energy.

Having a single sensory protein capable of distinguishing color would provide Anabaena with information about the color of light available in its environment, enabling more efficient harvesting of light for photosynthesis, Spudich said.

“Understanding rhodopsins helps us understand the large number of related membrane receptors involved in cell signaling that govern biological functions,” Spudich said. In the longer term, the novel protein found in Anabaena has the potential to be used in nano-machinery as a color-sensor; however the authors point out that this practical application is years in the future.

First author of the paper is Lutz Vogeley, a graduate student in the UC Irvine Department of Molecular Biology and Biochemistry. Senior authors are Dr. Spudich and Dr. Hartmut Luecke, Ph.D., professor of molecular biology and biochemistry and of physiology and biophysics at UC-Irvine. Co-authors include Oleg Sineshchekov, Ph.D., of Moscow State University in Russia, and visiting professor in the UT Center for Biology, and research fellow Vishwa Trivedi, Ph.D., and Jun Sasaki, Ph.D., assistant professor, both of the UT Center for Membrane Biology.

“One of the key frontiers of biomedical science in the genomic era is the crucial role of cell membranes in normal cell function and disease states,” said Spudich, who holds the Robert A. Welch Distinguished Chair in Chemistry and is a professor in the UT Graduate School of Biomedical Sciences. “Ask virtually any investigator and you'll find his or her research program bumps up against a membrane.”

Cell membrane surfaces and their exposed proteins are the most accessible targets to treat human tissue or destroy infectious microbes, he said. More than 60 percent of medications target membrane proteins on human cells and many antibiotics target membranes on pathogens.