| Description: |
Aquaporins are water channels, present in both higher and lower organisms, that belong to the major intrinsic protein family. Most aquaporins are highly selective for water, though some also facilitate the movement of small uncharged molecules such as glycerol []. In higher eukaryotes these proteins play diverse roles in the maintenance of water homeostasis, indicating that membrane water permeability can be regulated independently of solute permeability. In microorganisms however, many of which do not contain aquaporins, they do not appear to play such a broad role. Instead, they assist specific microbial lifestyles within the environment, e.g. they confer protection against freeze-thaw stress and may help maintain water permeability at low temperatures []. The regulation of aquaporins is complex, including transcriptional, post-translational, protein-trafficking and channel-gating mechanisms that are frequently distinct for each family member.Structural studies show that aquaporins are present in the membrane as tetramers, though each monomer contains its own channel [, , ]. The monomer has an overall "hourglass"structure made up of three structural elements: an external vestibule, an internal vestibule, and an extended pore which connects the two vestibules. Substrate selectivity is conferred by two mechanisms. Firstly, the diameter of the pore physically limits the size of molecules that can pass through the channel. Secondly, specific amino acids within the molecule regulate the preference for hydrophobic or hydrophilic substrates.Aquaporins are classified into two subgroups: the aquaporins (also known as orthodox aquaporins), which transport only water, and the aquaglyceroporins, which transport glycerol, urea, and other small solutes in addition to water [, ].Aquaporin-3 (AQP3) forms a water-specific channel that renders the kidney medullary collecting duct highly permeability to water. It may function as a water and urea exit mechanism in antidiuresis in collecting duct cells, and is also slightly permeable to urea and glycerol. It may also play an important role in gastrointestinal tract water transport and in glycerol metabolism. It is widely expressed in epithelial cells of kidney (collecting ducts []) and airways, in keratinocytes, immature dendritic cells and erythrocytes. Expression studies have shown that cells transfected with AQP3 cDNA react strongly with anti-GIL antibodies []. These studies hence report the first cases of human AQP3 deficiency and offer the molecular basis of a new blood group system, GIL, believed to be encoded by AQP3 []. |
