Aquaporin

Aquaporins, also called water channels, are integral membrane proteins from a larger family of major intrinsic proteins that form pores in the membrane of biological cells, mainly facilitating transport of water between cells. The cell membranes of a variety of different bacteria, fungi, animal and plant cells contain aquaporins through which water can flow more rapidly into and out of the cell than by diffusing through the phospholipid bilayer. Aquaporin has six membrane-spanning alpha helical domains with both carboxylic and amino terminals on the cytoplasmic side. Two hydrophobic loops contain conserved asparagine-proline-alanine NPA motif. Because Aquaporin is usually always open and is prevalent in just about every cell type, it causes water to almost always flow to down its concentration gradient. This leads to a misconception that water readily passes through the cell membrane down its concentration gradient. This is not true because only non-polar substances can diffuse directly through the lipid bilayer. The 2003 Nobel Prize in Chemistry was awarded jointly to Peter Agre for the discovery of aquaporins and Roderick MacKinnon for his work on the structure and mechanism of potassium channels. Genetic defects involving aquaporin genes have been associated with several human diseases including nephrogenic diabetes insipidus and neuromyelitis optica. The mechanism of facilitated water transport and the probable existence of water pores has attracted researchers since 1957. In most cells, water moves in and out by osmosis through the lipid component of cell membranes. Due to the relatively high water permeability of some epithelial cells, it was long suspected that some additional mechanism for water transport across membranes must exist. Solomon and his co-workers performed pioneering work on water permeability across the cell membrane in the late 1950s. In the mid-1960s an alternative hypothesis (the 'partition–diffusion model') sought to establish that the water molecules partitioned between the water phase and the lipid phase and then diffused through the membrane, crossing it until the next interphase where they left the lipid and returned to an aqueous phase. Studies by Parisi, Edelman,Carvounis et al. accented not only the importance of the presence of water channels but also the possibility to regulate their permeability properties. In 1990, Verkman's experiments demonstrated functional expression of water channels, indicating that water channels are effectively proteins. It was not until 1992 that the first aquaporin, 'aquaporin-1' (originally known as CHIP 28), was reported by Peter Agre, of Johns Hopkins University. 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. Further studies using supercomputer simulations identified the pathway of water as it moved through the channel and demonstrated how a pore can allow water to pass without the passage of small solutes. The pioneering research and subsequent discovery of water channels by Agre and his colleagues won Agre the Nobel Prize in Chemistry in 2003. Agre said he discovered aquaporins 'by serendipity.' He had been studying the Rh blood group antigens and had isolated the Rh molecule, but a second molecule, 28 kilodaltons in size (and therefore called 28K) kept appearing. At first they thought it was a Rh molecule fragment, or a contaminant, but it turned out to be a new kind of molecule with unknown function. It was present in structures such as kidney tubules and red blood cells, and related to proteins of diverse origins, such as in fruit fly brain, bacteria, the lens of the eye, and plant tissue. However the first report of protein-mediated water transport through membranes was by Gheorghe Benga in 1986, prior to Agre's first publication on the topic. This led to a controversy that Benga's work had been adequately recognized neither by Agre nor by the Nobel Prize Committee. Aquaporins are 'the plumbing system for cells'. Water moves through cells in an organized way, most rapidly in tissues that have aquaporin water channels. For many years, scientists assumed that water leaked through the cell membrane, and some water does. However, this did not explain how water could move so quickly through some cells. Aquaporins selectively conduct water molecules in and out of the cell, while preventing the passage of ions and other solutes. Also known as water channels, aquaporins are integral membrane pore proteins. Some of them, known as aquaglyceroporins, also transport other small uncharged dissolved molecules including ammonia, CO2, glycerol, and urea. For example, the aquaporin 3 channel has a pore width of 8–10 Ångströms and allows the passage of hydrophilic molecules ranging between 150 and 200 Da. However, the water pores completely block ions including protons, essential to conserve the membrane's electrochemical potential difference.