A Water Channel Network in Cell Membranes of the Filter Chamber of Homopteran Insects

Abstract

Water is the most ubiquitous molecule in the living cell and movement of water across the cell membrane accompanies fundamental cell functions. All biological membranes exhibit some water permeability as a result of diffusion through the lipid bilayer and osmotic gradients constitute the driving force for water flow. Osmotic water permeability is therefore of the highest relevance. However, some cells have the ability to transport water across their cell membrane at greatly accelerated rates, for example mammalian red blood cells, epithelial cells of the renal proximal tubules. Water permeability in such cells is simply too high to be accounted for by lipid-mediated diffusion, thus leading biophysicists to predict that water-selective channels must exist. The search for water channel began not surprisingly in tissues that had been already identified from physiological studies as having high water permeabilities. But the molecular basis of water channels remained elusive for a long time, since many attempts to determine its structure by biochemical approaches and expression cloning were unsuccessful. The reasons of this failure were linked to the inability of the water channel to be labeled by its substrate, the lack of highly specific inhibitors and the basal diffusional permeability of cell membranes.