什么是水通道蛋白（发现、作用机制、研究现状）

什么是水通道蛋白（发现、作用机制、研究现状）
作用机制如何?
（最好有部分英文解释的,）

In the middle of the 1950s it was discovered that water can be rapidly transported into and out of cells through pores that admit water molecules only,known as the water channel.Because water is vital for life,can be said that the water channel is one of the most important channels in cell membranes.Although scientists found in the presence of water channel,but the water channel is what had been a mystery.
Agre tested his hypothesis that aquaporin might be a water channel protein in a simple experiment.He compared cells which contained the protein in question with cells which did not have it.When the cells were placed in a water solution,those that had the protein in their membranes absorbed water by osmosis and swelled up while those that lacked the protein were not affected at all.Agre also ran trials with artificial cell membranes,termed liposomes,which are a simple lipid bound droplets of water.He found that the liposomes became permeable to water only if the aquaporin protein was implanted in their artificial membranes.
Peter Agre’s experiment with cells containing or lacking aquaporin.The aquaporin is necessary for making the cell absorb water and swell[2].
In 1992 a "water channel" was identified and what its molecular machinery might look like was suggested; that is,proteins were identified that formed an actual channel in membranes that facilitated water movement
In 1999,together with other research teams,Agre reported the first high-resolution images of the three-dimensional structure of an aquaporin,namely,aquaporin-1
The pioneering discoveries and research on water channels by Peter Agre and Roderick MacKinnon resulted in the presentation of a Nobel Prize in Chemistry to Agre in 2003
Aquaporins (AQPs) are membrane water channels that play pivotal roles in physiological and pathophysiological processes in diverse mammalian organs.Thirteen mammalian AQP have been molecularly identified since AQP1 was first cloned from the membrane of erythrocyte in 1990.Recent studies uncovered an important role of aquaporins in cell migration,and migration-associated cell function such as angiogenesis,wound healing,and neutrophil motility,provided a new molecular mechanism of cell migration and opend a new field of aquaporin gene function
Structure
Aquaporin proteins are made up of six transmembrane α-helices arranged in a right-handed bundle,with the amino and the carboxyl termini located on the cytoplasmic surface of the membrane.The amino and carboxyl halves of the sequence show similarity to each other,in what appears to be a tandem repeat.Some researchers believe that this results from an early evolution event that saw the duplication of the half-size gene.
There are also five interhelical loop regions (A – E) that form the extracellular and cytoplasmic vestibules.Loops B and E are hydrophobic loops that contain the highly,although not completely conserved,asparagine–proline–alanine (NPA) motif,which overlap the middle of the lipid bilayer of the membrane forming a 3-D 'hourglass' structure where the water flows through.This overlap forms one of the two well-known channel constriction sites in the peptide,the NPA motif and a second and usually narrower constriction known as 'selectivity filter' or ar/R selectivity filter
Aquaporins form tetramers in the cell membrane,with each monomer acting as a water channel.The different aquaporins contain differences in their peptide sequence,which allows for the size of the pore in the protein to differ between aquaporins.The resultant size of the pore directly affects what molecules are able to pass through the pore,with small pore sizes only allowing small molecules like water to pass through the pore.
再问： 全英？？！！ 厉害！ 那它的机制是怎样的呢？最好有一点中文 谢谢
再答： In 2000, together with other research teams, Agre reported the first high-resolution images of the three-dimensional structure of the aquaporin. With these data, it was possible to map in detail how a water channel functions. How is it that it only admits water molecules and not other molecules or ions? The membrane is, for instance, not allowed to leak protons. This is crucial because the difference in proton concentration between the inside and the outside of the cell is the basis of the cellular energy-storage system. Selectivity is a central property of the channel. Water molecules worm their way through the narrow channel by orienting themselves in the local electrical field formed by the atoms of the channel wall. Protons (or rather oxonium ions, H3O+) are stopped on the way and rejected because of their positive charges[3].
再答： Aquaporins form tetramers in the cell membrane, and facilitate the transport of water and, in some cases, other small solutes, as glycerol, across the membrane. However, the water pores are completely impermeable to charged species, such as protons, a remarkable property that is critical for the conservation of membrane's electrochemical potential. Based on hydrophobicity plots of their amino acid sequences , the aquaporins are predicted to have six membrane-spanning segments, as depicted in the model of aquaporin-1 below. Aquaporins exist in the plasma membrane as homotetramers. Each aquaporin monomer contains two hemi-pores, which fold together to form a water channel[2].