Aquaporin

Aquaporins are a family of integral membrane proteins that form channels specialized for water transport across cell membranes. They enable rapid, osmotically driven water movement with high selectivity and minimal solute permeation. Aquaporins were identified in the early 1990s; the discovery by Peter Agre and colleagues revealed water channels in red blood cells and kidney tubules, earning the Nobel Prize in Chemistry in 2003.

Structurally, aquaporins comprise six transmembrane alpha-helices that assemble as tetramers in the membrane. Each monomer forms a water-conducting pore with a narrow constriction that allows water molecules to pass in single file while excluding ions and protons. Selectivity is determined by two features: the conserved NPA (asparagine-proline-alanine) motifs and the aromatic/arginine (ar/R) constriction at the channel entrance.

Mammalian aquaporins include both water-selective members (classical aquaporins such as AQP1, AQP2, AQP4, and AQP5) and aquaglyceroporins (AQP3, AQP7, AQP9, AQP10) that can transport glycerol and other small solutes in addition to water. They are widely distributed, with key roles in kidney water reabsorption (AQP1 in proximal tubules and descending limb; AQP2 in collecting ducts regulated by vasopressin), brain water homeostasis (AQP4 in astrocytes), and other tissues such as skin and ocular structures.

Dysregulation or mutation of aquaporins can lead to disease, most notably AQP2 mutations causing nephrogenic diabetes insipidus. Aquaporins are investigated as drug targets in conditions involving edema, glaucoma, and cancer, and are also important in plant physiology where PIPs and TIPs regulate plant water relations.