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. Identified in a complex with STOM (By similarity). Interacts with EPHB2; involved in endolymph production in the inner ear. Detected in erythrocytes (at protein level) (PubMed:12133842). In the kidney, expressed on luminal and basal borders of proximal tubules and in the thin limb of Henle's loop (at protein level) (PubMed:31605441). Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Mutant mice are born at the expected Mendelian rate and appear healthy and normal, but display strongly reduced urine osmolality. Besides, their erythrocytes show reduced water permeability. Mice lacking both Aqp1 and Slc14a1 are born at the expected Mendelian ratio, but do not thrive; half of them die within ten days after birth and none are alive after two weeks. Urine osmolality is somewhat lower than that observed with mice lacking only Aqp1. Besides, erythrocyte water permeability is significantly lower than in mice lacking only Aqp1. Pharmacologically inhibited by submillimolar concentrations of mercury. Belongs to the MIP/aquaporin (TC 1.A.8) family.