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.
The Colton antigen system (Co) is present on the membranes of red blood cells and in the tubules of the kidney and helps determine a person's blood type. The Co antigen is found on a protein called aquaporin-1 which is responsible for water homeostasis and urine concentration.
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.
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).
Spectrin beta chain, erythrocyte is a protein that in humans is encoded by the SPTB gene.
Alpha-1-syntrophin is a protein that in humans is encoded by the SNTA1 gene. Alpha-1 syntrophin is a signal transducing adaptor protein and serves as a scaffold for various signaling molecules. Alpha-1 syntrophin contains a PDZ domain, two Pleckstrin homology domain and a 'syntrophin unique' domain.
Plasma membrane calcium-transporting ATPase 1 also known as Plasma membrane calcium pump isoform 1 is a plasma membrane Ca2+
ATPase, an enzyme that in humans is encoded by the ATP2B1 gene. It's a transport protein, a translocase, a calcium pump EC 7.2.2.10.
Rh-associated glycoprotein (RHAG) is an ammonia transporter protein that in humans is encoded by the RHAG gene. RHAG has also recently been designated CD241. Mutations in the RHAG gene can cause stomatocytosis.
Lens fiber major intrinsic protein also known as aquaporin-0 is a protein that in humans is encoded by the MIP gene.
Anion exchange protein 2 (AE2) is a membrane transport protein that in humans is encoded by the SLC4A2 gene. AE2 is functionally similar to the Band 3 Cl−/HCO3− exchange protein.
Aquaporin-5 (AQP-5) is a protein that in humans is encoded by the AQP5 gene.
Aquaporin-9 (AQP-9) is a protein that in humans is encoded by the AQP9 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.
The channelome, sometimes called the "ion channelome", is the complete set of ion channels and porins expressed in a biological tissue or organism. It is analogous to the genome, the metabolome, the proteome, and the microbiome. Characterization of the ion channelome, referred to as channelomics, is a branch of physiology, biophysics, neuroscience, and pharmacology, with particular attention paid to gene expression. It can be performed by a variety of techniques, including patch clamp electrophysiology, PCR, and immunohistochemistry. Channelomics is being used to screen and discover new medicines.
Gheorghe Benga is a Romanian physician and molecular biologist. He is professor and chairman in the Department of Cell and Molecular Biology of the Iuliu Hațieganu University of Medicine and Pharmacy in Cluj-Napoca, Romania, and a titular member of the Romanian Academy.
Aquaporin-6, (AQP-6) also known as kidney-specific aquaporin is a protein in humans that is encoded by the AQP6 gene.
Aquaglyceroporins are recognized as a subset of the aquaporin family of proteins which conduct water, glycerol and other small, uncharged solutes. They are the only mammal proteins which are able to permeate glycerol through the plasma membrane. Aquaglyceroporins are found in many species including bacteria, plants and humans. Because of their ubiquitous nature they are important in agriculture as well as medicine. They have been identified as a possible source of metalloid contamination in agriculture as aquaglyceroporins were have been shown to conduct As(III) and Sb(III) in yeast and could be a possible source of metalloids entering the food sources of humans.
