Dec. 27, 2005 A wilting, water-starved houseplant and flood-covered crops have something in common. That knowledge, gleaned from spinach and researchers on two continents, potentially could open the gate to advances in both plant and human health.
The research, which appeared online this month in advance of regular publication by the journal Nature, involved a tandem of basic-science firsts that offer immediate real world applications, the scientists say.
First, Swedish plant biochemists and crystallographers at Lund University and Chalmers University of Technology, studying membrane proteins of spinach, solved the structure of a water-protein channel -- an aquaporin that opens and closes a gate that regulates water movement in and out of cells.
Not only was the discovery the first for a plant-water channel, it was the first plant-membrane channel for which an atomic resolution structure has been determined. "By recovering an X-ray structure of a plant-membrane channel from over-expression in yeast, this work also lays down key technical foundations for future studies on other plant- and human-membrane proteins," said co-author Richard Neutze of Chalmers University.
Taking that structure, scientists at the University of Illinois at Urbana-Champaign used advanced molecular dynamics simulations to study the mechanics of how such proteins respond to cellular signals such as altering pH (acidity and alkalinity) or phosphorylation, a common cellular chemical process that controls protein activity.
Surprisingly, the simulations clearly showed that specific residues that sit far away from a water pore control the opening of the channel, said biophysics professor Emad Tajkhorshid (pronounced uh-MOD tazh-CORE-shid).
The residues, they found, latch onto an intracellular loop of the protein, blocking the water channel when not phosphorylated. When the residues undergo phosphorylation and become charged, they release the loop, opening the channel for water to pass through.
The researchers theorize the gating activity is universal in all plant aquaporins, because sequencing has shown the gating loop to be conserved in them.
"Plant cells close their water channels in response to drought stress in order to preserve their water content," said Tajkhorshid, who also is assistant director of research of the Theoretical and Computational Biophysics Group at the U. of I. Beckman Institute for Advanced Science and Technology. "It is interesting that they can also protect themselves against water overflow under flooding conditions by closing the very same water channels. It is amazing that although distinct cellular signals are involved in these two types of closing events, both mechanisms are mediated through the same structural elements of the protein -- that is by plugging the cytoplasmic entrance of the channel with an eolongated intracellular loop."
By atomic scale, the structural difference between open and closed channels is telling. When open, the protein allows water to go through; when closed, its pore size is reduced by 2 angstroms, effectively closing the channel, the Swedish researchers reported.
The work taken together, Tajkhorshid said, is a landmark study in membrane channels. It addresses the mechanism of gating and regulation of plasma membrane proteins in full detail. "We could see how every single atom moves and how collective motions of a large number of atoms resulted in the opening of the channel," he said.
Aquaporins -- discovered in 1991 by Peter Agre, now vice chancellor for science and technology at Duke University Medical Center -- help cells adjust water content. So far 13 forms of aquaporins have been found in animals and another 35 in plants.
Previous work in Tajkorshid's lab has identified how water molecules line up in single file and move as if dancing through open water channels. In less than a second, billions of water molecules can move through a channel.
The growing knowledge of how aquaporins regulate water passage eventually could help agricultural producers boost survivability of drought-stricken and flood-ridden crops. It also could lead to new pharmaceuticals that specifically target these proteins.
"The kidneys are responsible for maintaining a water balance in the body," said Swedish co-author Per Kjellbom in a Lund University news release. "If we can identify a chemical compound that can close the aquaporins in the kidneys, this can be developed into a diuretic drug. By the same token, compounds that stabilize the closed structure could be used in cancer treatment."
Tajkhorshid and Yi Wang, a biophysics graduate student at Illinois, conducted the molecular simulations using VMD and NAMD software developed in their Beckman lab, which is an NIH (National Institutes of Health) Resource for Macromolecular Modeling and Bioinformatics. The simulations were performed at both the National Center for Supercomputing Applications at Illinois and the Pittsburgh Supercomputer Center. Swedish co-authors were Kristina Hedfalk, Urban Johanson, Maria Karlsson, Kjellbom, Neutze and Susanna Tornroth-Horsefield.
The research was funded in the U.S. by the National Institutes of Health and in Sweden by Formas, a Swedish governmental research-funding agency, the Research School of Pharmaceutical Sciences, Swegene, the Swedish Research Council, the Swedish Strategic Research Foundation, the European Commission Integrated Projects EMEP and SPINE, and the Chalmers Bioscience Programme.
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