Aquaporin 2 (AQP2) is a water transport protein important in regulating water movement in and out of cells. Whilst the kidney removes urine from the blood, Aquaporin 2 is responsible for readsorbing water from the urine. Aquaporin 2 has 6 transmembrane domains. Loops B and E are important in forming a pore in the middle of the membrane protein through which water can pass. Near the pore on the E loop is a cysteine residue which when bound by mercury will inhibit the transport of water molecules.
Aquaporin 2 is specifically found in the apical membrane of the collecting tube of the kidney. When vasopressin is released, aquaporin 2 is inserted into the membrane to allow the reabsorption of water into the body.