Ultraviolet (UV) radiation from the sun can damage proteins and DNA inside cells, leading to poor growth and even death (as well as carcinogenesis in humans). But plants have evolved some powerful adaptive defenses, including a complex array of protective responses orchestrated by a UV-sensing protein molecule known as UVR8. Now, scientists from The Scripps Research Institute and the University of Glasgow have put together a detailed picture of UVR8's structure and inner workings.
Scientists from Scripps Research and the University of Glasgow have put together a detailed picture of a UV-sensing protein known as UVR8, helping to explain mysteries such as how plant growth varies with sunlight. Credit: Image courtesy of the Getzoff lab, the Scripps Research Institute.
"It's an ancient molecule that seems to play a fundamental role in plants," said Scripps Research Professor Elizabeth Getzoff. "Knowing how it works helps us to understand better how plant growth varies with changes in sunlight, for example due to climate shifts; it's also important that we understand its basic light-switch mechanism."
Getzoff was a principal investigator for the study, which is reported February 9 in the journal Science's early online edition, Science Express.
Molecules similar to UVR8 have been found in more ancient plant species such as algae and mosses, suggesting that UVR8 represents a primordial adaptation to UV light, possibly originating before Earth's atmosphere developed a UV-absorbing ozone layer.
John Christie of the University of Glasgow was a visiting scientist in Getzoff's lab in 2010, and suggested a collaboration to find out more about UVR8. Getzoff's lab specializes in finding and analyzing the detailed atomic structures of proteins, while Christie and his colleague Gareth I. Jenkins, a professor of plant cell and molecular biology at the University of Glasgow, are experts on UVR8 biology.
UVR8 was known to be a "dimer" made of two identical protein subunits. The Getzoff lab's structural analysis revealed that these subunits are doughnut-shaped, and normally stick together weakly, like a couple of balloons that have become electrically charged after being rubbed. The interface connecting these two subunits is spanned by the component amino acids of the protein—including pyramidal structures made of tryptophan amino acids, which serve as the primary sensors of UV-B light. "The absorption of UV-B photons by these tryptophan pyramids leads to the weakening of the electrostatic force that holds the two UVR8 subunits together," said Christie.
As a result of this weakening, the subunits can separate, move singly to the cell nucleus, and begin their orchestration of gene activity. The team's analysis also suggested that within a few hours, the subunits can reassemble into UV-B-sensitive dimers again.
"Other light-sensing proteins require a chemical modification or helper molecule to detect light, but UVR8 is unique in that it has these inbuilt UV-B-sensing tryptophan pyramids—structures that no one has seen before," said Christie.Molecule with Its Own Light Sensor
Getzoff and her colleagues now intend to find out more precisely how the absorption of UV-B causes the disassociation of the UVR8 dimer, and then how the separated subunits interact with other proteins and chromosomes in the nucleus to switch on protective responses in the plant. Both UVR8 and a similar, but not UV-controlled, protein in humans bind to chromosomes to control gene activity.
In addition to Getzoff, Christie, and Jenkins, the other co-authors of the paper, "Plant UVR8 Photoreceptor Senses UV-B by Tryptophan-mediated Disruption of Cross-Dimer Salt Bridges," were Andrew S. Arvai, Ashley J. Pratt, and Kenichi Hitomi, of the Getzoff lab at Scripps Research; Katherine J. Baxter, Monica Heilmann, and Andrew O'Hara of the Jenkins lab at the University of Glasgow; Brian Smith and Sharon M. Kelly of the Institute of Molecular, Cell and Systems Biology at the University of Glasgow; and Michael Hothorn of the Salk Institute for Biological Studies in La Jolla, California.
The study was supported by funds from the National Institutes of Health and the National Science Foundation; the Skaggs Institute for Chemical Biology at Scripps Research; the Royal Society and the Leverhulme Trust.About The Scripps Research Institute
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