Human aquaporin 2 (AQP2) is a water channel found in the kidney collecting duct, where it plays a key role in concentrating urine. Water reabsorption is regulated by AQP2 trafficking between intracellular storage vesicles and the apical membrane. This process is tightly controlled by the pituitary hormone arginine vasopressin and defective trafficking results in nephrogenic diabetes insipidus (NDI). Here we present the X-ray structure of human AQP2 at 2.75 Å resolution. The C terminus of AQP2 displays multiple conformations with the C-terminal α-helix of one protomer interacting with the cytoplasmic surface of a symmetry-related AQP2 molecule, suggesting potential protein-protein interactions involved in cellular sorting of AQP2. Two Cd 2+ -ion binding sites are observed within the AQP2 tetramer, inducing a rearrangement of loop D, which facilitates this interaction. The locations of several NDI-causing mutations can be observed in the AQP2 structure, primarily situated within transmembrane domains and the majority of which cause misfolding and ER retention. These observations provide a framework for understanding why mutations in AQP2 cause NDI as well as structural insights into AQP2 interactions that may govern its trafficking.membrane protein | X-ray crystallography | water channel protein W ater is the major ingredient of the human body, constituting 55-65% of our total body weight (1). Water homeostasis is maintained by the kidneys, which filter ∼180 L of primary urine every day. Although most water is constitutively reabsorbed in the proximal tubules and descending limbs of Henle (2), the body's water balance is fine-tuned by regulated water reabsorption, which takes place in the kidney collecting duct. Water reabsorption is mediated by aquaporins, membranebound water channels, of which seven of the 13 human isoforms have been located in the human kidney (3). Of these, human aquaporin 2 (AQP2) is present in the principal cells of the collecting duct and is responsible for regulated water reabsorption.AQP2 is stored in intracellular vesicles under water-saturating conditions. When the levels of the pituitary antidiuretic hormone arginine vasopressin (AVP) are elevated in response to dehydration or hypernatremia, AVP binding to the vasopressin 2 receptor (V2R) in the basolateral membrane stimulates an increase in intracellular cAMP. This triggers the phosphorylation of Ser256 in the AQP2 C terminus by protein kinase A (PKA) and flags the protein for trafficking from storage vesicles to the apical membrane (4-6). AVP also triggers additional phosphorylation at Ser264 and Ser269 (7,8), with all three sites being phosphorylated in AQP2s targeted to the plasma membrane (9). The resulting redistribution of AQP2 increases transcellular water permeability and concentrates urine (Fig. S1). Once correct water balance is restored, AQP2 is internalized through ubiquitinmediated endocytosis and redirected to storage vesicles or targeted for degradation (10-12).Because of its central role in water homeostasis, dysregula...