This peptide is a fragment of the human aquaporin-2 (AQP2) phosphorylated at Ser256. Protein phosphorylation plays a key role in vasopressin signaling in the renal-collecting duct. AQP2 residues including S256 and S261, are altered in response to vasopressin. In the presence of vasopressin, AQP2 monophosphorylated at Ser256 and diphosphorylated AQP2 (pSer256/261) increase in abundance, whereas AQP2 monophosphorylated at Ser261 decreases, suggesting that both sites are involved in vasopressin-dependent AQP2 trafficking. Aquaporins are water channels. They form pores in the membranes of cells and selectively pass water molecules through the membrane, and prevent the crossing of sodium, potassium and other ions and small molecules.