Aquaporin 9 (IPR015685)
Short name: Aquaporin_9
Overlapping homologous superfamilies
- Aquaporin-like (IPR023271)
- Major intrinsic protein (IPR000425)
- Aquaporin 9 (IPR015685)
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 [PMID: 15340377]. 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 [PMID: 16406529]. 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 [PMID: 11780053, PMID: 15377788, PMID: 14691544]. 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 [PMID: 10872456, PMID: 16650285].
Aquaporin-9 was identified from human leukocytes by homology cloning [PMID: 9514918]. AQP9 has unusually broad solute permeability and is expressed in hepatocyte plasma membranes. 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 [PMID: 12594337]. 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.