Aquaporin

Aquaporins are membrane channel proteins that facilitate rapid, selective water movement across cell membranes while excluding most ions and other solutes.

Water-selective channel: A membrane pore that permits water passage while restricting other molecules or ions. Aquaporins achieve water selectivity through a narrow pore and precisely positioned chemical groups.
Cell membrane: A selectively permeable lipid bilayer that separates a cell from its surroundings and encloses internal compartments. Aquaporins are embedded in this barrier to provide a route for water.
Collecting duct: The final nephron segment that carries urine toward the renal pelvis and adjusts its water content. Aquaporin-2 insertion here allows the kidney to reclaim water under hormonal control.
Dehydration: A condition in which the body loses more water than it takes in, impairing normal physiological function. Impaired renal water reabsorption involving aquaporin-2 can contribute to excessive water loss.
Aquaglyceroporin: A member of the aquaporin family that transports water and small neutral solutes such as glycerol. Unlike classical water-selective aquaporins, these channels also pass certain small solutes.
NPA motif: A conserved asparagine-proline-alanine sequence found twice in the pore-forming region of many aquaporins. Its two copies help orient water molecules and prevent proton conduction.
Membrane protein: A protein associated with a biological membrane, either embedded in it or attached to its surface. Aquaporins are integral membrane proteins whose structure spans the lipid bilayer.
Aquaporin-2: A water channel regulated by vasopressin in collecting-duct cells of the kidney. This aquaporin is the principal regulated route for water reabsorption in the collecting duct.
Nephrogenic diabetes insipidus: A disorder in which the kidneys respond poorly to vasopressin, producing large volumes of dilute urine. Mutations in aquaporin-2 can cause this disorder by disrupting water reabsorption.
Ion channel: A membrane protein that forms a pore for selected ions to cross a biological membrane. Ion channels conduct charged particles, whereas aquaporins exclude ions from their water pores.

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