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-9 was identified from human leukocytes by homology cloning []. AQP9 has unusually broad solute permeability. It is expressed in hepatocyte plasma membranes and also in lung, small intestine and spleen cells []. Expression of AQP9 in liver was induced up to 20-fold in rats fasted for 24 to 96 hours, and the AQP9 level gradually declined after re-feeding []. AQP9 shares greater sequence identity with AQP3 and AQP7 than with other members of the family, suggesting that these 3 proteins belong to a subfamily.
The aquaporins (AQPs) are a family of integral membrane proteins composed of two subfamilies: the orthodox aquaporins, which transport only water, and the aquaglyceroporins, which transport glycerol, urea, or other small solutes []. AQP1 in pleural mesothelial cells participate in the formation of pleural fluid [], renal AQP2 is involved in levels of solutes []and changes in AQP9 in the hippocampus are related to delayed neuronal death []. Aquaporins contain two tandem repeats, each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA).Aquaporin-11 (AQP11) and aquaporin-12 belong to a new aquaporin subfamily termed superaquaporins []. AQP11 is found in the endoplasmic reticulum and has been connected to policystic kidney disease through knock-out experiments []. It has also been shown that although the characteristic NPA motif may not be fully conserved in both tandem repeats, the molecule nevertheless preserves its water-transport function [].
The aquaporins (AQPs) are a family of integral membrane proteins composed of two subfamilies: the orthodox aquaporins, which transport only water, and the aquaglyceroporins, which transport glycerol, urea, or other small solutes []. AQP1 in pleural mesothelial cells participate in the formation of pleural fluid [], renal AQP2 is involved in levels of solutes []and changes in AQP9 in the hippocampus are related to delayed neuronal death []. Aquaporins contain two tandem repeats, each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA).Aquaporins 11 and 12 are classified as members of a new AQP subfamily: the subcellular AQPs []. AQP 11 and 12 appear to be more distantly related to the other mammalian aquaporins and aquaglyceroporins. AQP11 is functionally distinct from other proteins of the aquaporin superfamily and could represent a new aquaporin subfamily []. In members of the subcellular AQPs, one of the two asparagine-proline-alanine (NPA) motifs are not completely conserved. They play a crucial role in selective water conduction and function as water channels []. This group represents an aquaporin types 11 and 12.
The aquaporins (AQPs) are a family of integral membrane proteins composed of two subfamilies: the orthodox aquaporins, which transport only water, and the aquaglyceroporins, which transport glycerol, urea, or other small solutes []. AQP1 in pleural mesothelial cells participate in the formation of pleural fluid [], renal AQP2 is involved in levels of solutes []and changes in AQP9 in the hippocampus are related to delayed neuronal death []. Aquaporins contain two tandem repeats, each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA).Aquaporin-11 and aquaporin-12 (AQP12) belong to a new aquaporin subfamily termed superaquaporins []. AQP12 is exclusively found in pancreatic acinar cells [], and is not localised in the plasma membrane. No specific localisation has yet been established for this subfamily, but its presence in the pancreas implies some role in the secretion of digestive enzymes []and fluids in the pancreatic cells []. A structural characteristic of AQP12 is the lack of a cytoplasmic N-terminal region, by contrast with all other known aquaporins.
