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 co-senior 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 unusual 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 invol
Contact: Scott Merville
University of Texas Health Science Center at Houston