Structure and function of aquaporins: The membrane water channel proteins

Abstract

Aquaporins are integral membrane proteins which are also known as water channel proteins. They aid quick transportation of water across membranes and are important in controlling cell volume and transcellular water passage. Aquaporins are present in organisms, and they vary from archaea and bacteria to plants and animals. They are also found in insects and yeast. Presently, 13 mammalian aquaporins (AQP0 to AQP12) have been cloned and identified in every tissue in the body. These aquaporins are alike in basic structure with monomers containing six transmembrane and two short helical segments that enclose cytoplasmic and extracellular vestibules linked by aqueous pore. They have distinctive structures that define their functions, mode of action, and even their various control methods. Phylogenetic analysis of aquaporin consists of aquaporins, glycerol facilitators, plasma membrane integral proteins of plants, tonoplast integral proteins of plants, nodules of plants, and AQP8s. Aquaporins are structurally related due to their great similarity in their structural regions, mainly in the pore-forming domains, which accounts for the similarity in their transport mechanisms. The water movement by AQPs is controlled by a change in conformation or by modifying the AQP density in the membrane and at the transcriptional and translational levels. Aquaporins are important in several physiological processes and are also linked with several clinical disorders, such as brain edema, loss of vision, and kidney dysfunction.