Everyone needs a holiday. Even a drop of water. Especially if it has spent its life in urban surroundings. And that is exactly what three molecules of water do in "Aquaporin? Wet's it for?", a play written by Sylvie Déthiollaz and Vivienne Baillie Gerritsen of the Swiss-Prot group. The play was recently published on our site in French and the English version will follow shortly. Aquaporin is a protein that deals with the passage of water in organisms as diverse as dandelions and mosquitoes. Thanks to its diversity of expression, coupled with its capacity to offer privileged transport to water molecules, the three companions find their way from the slums of Geneva to the French Riviera.
The greater part of every living being is made up of water. On average, humans are made up of 65% water! We need it for everything our body does: to maintain our body temperature, to digest and to help in the transit of our bodily wastes for example. As a consequence, huge quantities of water have to circulate - continuously - in our body, our blood, across tissues and cells. Mere water molecule diffusion is not sufficient to sustain such traffic, which is why another system is needed.
Aquaporins are part of a huge family. They assemble in fours and span the cell membrane to form a pore. Each separate aquaporin acts as a channel. By way of a narrowing in the tetramer's centre, molecules larger than water molecules cannot pass through. The mechanism is ingenious. Furthermore, the molecules don't just drift through the pore but become part of a peculiar molecular ballet which takes them from one end of the channel to the other. At the pore's entrance, one molecule of water floats in 'oxygen-first', so to speak, while behind await millions of other molecules all just as eager to get through. Once it reaches the narrowing at the pore's centre, the amino acids positioned there make it perform a delicate pirouette. Consequently, the two atoms of hydrogen now face the exit.
Why the pirouette? Protons hitch a ride on the molecules' backs and the pirouette makes them lose hold, which is a very elegant way of getting rid of them. It may sound a tad selfish, or even cruel, but cells have to keep their reservoir of protons. Protons are a source of energy and if they just disappear through every pore, the cell would die. Aquaporins are therefore vital because in the space of a second, one billion molecules of water cross a cell membrane!
The first aquaporin was discovered in the 1990s and offered an understanding on how so much water passed from one side of a cell to the other. Since then, many more aquaporins have been identified and found in mammals, invertebrates, plants and microorganisms. It is hardly surprising then that Peter Agre, the man behind the discovery of such an ingenious and widespread biological process, was awarded the Nobel Prize in Chemistry in 2003.
Read also : "Aquaporins : A Nobel Prize for the water ferries" and "Liquid states"
And our play: "Aquaporin? Wet's it for?"