Forms a water-specific channel that provides the plasma membranes of red cells and kidney proximal tubules with high permeability to water, thereby permitting water to move in the direction of an osmotic gradient. Homotetramer. Interacts with EPHB2; involved in endolymph production in the inner ear (By similarity). Identified in a complex with STOM. Detected in erythrocytes (at protein level). Expressed in a number of tissues including erythrocytes, renal tubules, retinal pigment epithelium, heart, lung, skeletal muscle, kidney and pancreas. Weakly expressed in brain, placenta and liver. Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). AQP1 is responsible for the Colton blood group system [MIM:110450]. Approximately 92% of Caucasians are Co(A+B-) (Ala-45), approximately 8% are Co(A+B+), and only 0.2% are Co(A-B+) (Val-45). Co(A-B-) which is very rare, is due to a complete absence of AQP1. Pharmacologically inhibited by submillimolar concentrations of mercury. Belongs to the MIP/aquaporin (TC 1.A.8) family. Blood group antigen gene mutation database Liquid states - Issue 36 of July 2003