Reporting in the April 19 issue of Science, researchers at the University of Illinois Beckman Institute for Advanced Science and Technology and at the University of California at San Francisco say the orientation of water molecules moving through aquaporins assures that only water, not ions such as protons, permeates between cells. If the latter occurs, energy stored as electrical potential between the inside and outside of the cell wall is lost.
Aquaporins, a class of proteins, form transmembrane channels found in cell walls. Plants have 35 different proteins of this type. Mammals, including humans, have 10, with many of them in the kidney, brain and lens of the eye.
When working correctly, said Klaus Schulten, the Swanlund Professor of Physics at the UI, the transport of water between plant cells lets flowers bloom and leaves stand sturdily, for example. In mammals, the machinery processes water efficiently to help maintain optimum health. A breakdown in human kidneys, the busiest water-handling organ with 400 liters being pumped through daily, leads to diabetes insipidus, in which water is not reabsorbed and abnormally large volumes of dilute urine are produced. Breakdowns in other organs can lead to loss of hearing and cataracts.
The structure of aquaporins was determined two years ago by Robert M. Stroud and colleagues at UCSF, who determined the geometry of the protein in the bacterium E. coli (GlpF).
However, Shulten said, that work still could not resolve exactly how water is conducted in the channel, and how it prevents the conduction of ions. Crystall
Contact: Jim Barlow
University of Illinois at Urbana-Champaign