Aquaporins, also called water channels, are channel 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. Aquaporins have six membrane-spanning alpha helical domains with both carboxylic and amino terminals on the cytoplasmic side. Two hydrophobic loops contain conserved asparagine–proline–alanine which form a barrel surrounding a central pore-like region that contains additional protein density. Because aquaporins are usually always open and are prevalent in just about every cell type, this leads to a misconception that water readily passes through the cell membrane down its concentration gradient. Water can pass through the cell membrane through simple diffusion because it is a small molecule, and through osmosis, in cases where the concentration of water outside of the cell is greater than that of the inside. However, because water is a polar molecule this process of simple diffusion is relatively slow, and in tissues with high water permeability the majority of water passes through aquaporin.
Aquaporin-4, also known as AQP-4, is a water channel protein encoded by the AQP4 gene in humans. AQP-4 belongs to the aquaporin family of integral membrane proteins that conduct water through the cell membrane. A limited number of aquaporins are found within the central nervous system (CNS): AQP1, 3, 4, 5, 8, 9, and 11, but more exclusive representation of AQP1, 4, and 9 are found in the brain and spinal cord. AQP4 shows the largest presence in the cerebellum and spinal cord grey matter. In the CNS, AQP4 is the most prevalent aquaporin channel, specifically located at the perimicrovessel astrocyte foot processes, glia limitans, and ependyma. In addition, this channel is commonly found facilitating water movement near cerebrospinal fluid and vasculature.
Aquaporin-2 (AQP-2) is found in the apical cell membranes of the kidney's collecting duct principal cells and in intracellular vesicles located throughout the cell. It is encoded by the AQP2 gene.
Aquaporin 3 (AQP-3) is the protein product of the human AQP3 gene. It is found in the basolateral cell membrane of principal collecting duct cells and provides a pathway for water to exit these cells. Aquaporin-3 is also permeable to glycerol, ammonia, urea, and hydrogen peroxide. It is expressed in various tissues including the skin, respiratory tract, and kidneys as well as various types of cancers. In the kidney, aquaproin-3 is unresponsive to the antidiuretic hormone vasopressin, unlike aquaporin-2. This protein is also a determinant for the GIL blood group system.
Aquaporin 1 (AQP-1) is a protein that in humans is encoded by the AQP1 gene.
Major intrinsic proteins comprise a large superfamily of transmembrane protein channels that are grouped together on the basis of homology. The MIP superfamily includes three subfamilies: aquaporins, aquaglyceroporins and S-aquaporins.
- The aquaporins (AQPs) are water selective.
- The aquaglyceroporins are permeable to water, but also to other small uncharged molecules such as glycerol.
- The third subfamily, with little conserved amino acid sequences around the NPA boxes, include 'superaquaporins' (S-aquaporins).
Lens fiber major intrinsic protein also known as aquaporin-0 is a protein that in humans is encoded by the MIP gene.
Solute carrier family 22 member 2 is a protein that in humans is encoded by the SLC22A2 gene.
Solute carrier family 23 member 2 is a protein that in humans is encoded by the SLC23A2 gene.
High affinity copper uptake protein 1 (CTR1) is a protein that in humans is encoded by the SLC31A1 gene.
Oncomodulin 2, also known as OCM2, is a human gene that is similar to oncomodulin.
Aquaporin-5 (AQP-5) is a protein that in humans is encoded by the AQP5 gene.
Potassium-transporting ATPase alpha chain 2 is a protein that in humans is encoded by the ATP12A gene.
Sodium–hydrogen exchanger 2 is a protein that in humans is encoded by the SLC9A2 gene.
Chloride transport protein 6 is a protein that in humans is encoded by the CLCN6 gene.
Aquaporin-8 is a protein that in humans is encoded by the AQP8 gene.
Aquaporin-7 (AQP-7) is a protein that in humans is encoded by the AQP7 gene.
Urea transporter 2 is a protein that in humans is encoded by the SLC14A2 gene.
Chloride intracellular channel protein 3 is a protein that in humans is encoded by the CLIC3 gene. This protein is a chloride channel.
Aquaporin-6, (AQP-6) also known as kidney-specific aquaporin is a protein in humans that is encoded by the AQP6 gene.
