Scientists at Chalmers

Revealing secrets of water gates in cells

[051208]  By: Catharina  Hiort

Unique snapshots of the structure of a protein that function as a water channel through cell membranes have been taken by scientists in Göteborg together with their colleagues in Lund and Urbana, USA. Their results have just been published in the prestigious scientific journal Nature.

Schematic illustration demonstrating the function of aquaporins in plants. During drought and flooding the caps close in response to different types of chemical signals in the cell's environment.

From studies on the structure of the channel protein aquaporin from spinach, Richard Neutze and his colleagues can now explain how it opens and closes by putting a cap over the entrance to and from the interior of the cell.

"Our results provide new insight into how the flow of water into and out of plant cells is controlled", says Richard Neutze, associate professor of Biophysics at Chalmers.

"What we learn by studying these channels in plants also aids our understanding of similar water regulating mechanisms in humans."

Even though water is the medium of life, biological membranes have only limited water permeability. Therefore cell membranes have to assist the flow of water into and out of the cell using water-specific membrane protein channels called aquaporins (aqua = water; pore = hole). The discovery of these water channels that connect the outside and the inside of the cell earned Peter Agre the Noble Prize in Chemistry for 2003.

Very similar water channels are found throughout all kingdoms of life (eg. bacteria, yeast, animals and plants). In humans, aquaporins that do not function properly can be one of the underlying causes of serious conditions such as cancer, brain edema, cardiovascular complications, and diabetes.

Scientists at Chalmers have revealed the structure of the plant aquaporins in both the open and closed conformations and thereby the mechanism of how the plant aquaporins are opened and closed. By making and studying small crystals of the membrane protein using x-rays, the scientists can see that in the closed conformation a flexible loop of the protein's amino acid chain caps the channel from the inside of the cell and thereby occludes the access of water into the pore. In the open conformation, this loop is displaced and this movement opens the gate which otherwise blocks the channel entrance.

These structural results provide a deeper understanding of the how the flow of water is regulated within all land plants on earth. It helps us understand every-day phenomena such as a neglected pot-plant wilting or the flood irrigation of the rice paddies of Asia. Furthermore, a number of human aquaporins are also gated. As such, the structural results may also aid the understanding of the complex systems of regulation of water within the human body.

The authors of the article in Nature are:
Susanna Törnroth-Horsefield, Kristina Hedfalk and Richard Neutze, Department of Chemistry and Bioscience, Chalmers University of Technology, Göteborg, SWEDEN
Yi Wang, and Emad Tajkhorshid, Theoretical and Computational Biophysics Group, Beckman Institute, University of Illinois at Urbana-Champaign, Urbana, USA
Urban Johanson, Maria Karlsson, and Per Kjellbom
Department of Plant Biochemistry, Lund University, Lund, SWEDEN

Richard Neutze comes from New Zealand. He moved to Sweden eight years ago, first being at Uppsala University, and later moving to Chalmers in October 2000. He was promoted to Associate Professor in Biophysics at Chalmers University of Technology in 2005.

For further information please contact:
Richard Neutze
Telephone Monday to Thursday: +46 31 773 3974
Friday: +46 31 166 412, during normal working hours.