Water channel required to promote glycerol permeability and water transport across cell membranes. Acts as a glycerol transporter in skin and plays an important role in regulating SC (stratum corneum) and epidermal glycerol content. Involved in skin hydration, wound healing, and tumorigenesis. Provides kidney medullary collecting duct with high permeability to water, thereby permitting water to move in the direction of an osmotic gradient. Slightly permeable to urea and may function as a water and urea exit mechanism in antidiuresis in collecting duct cells. It may play an important role in gastrointestinal tract water transport and in glycerol metabolism. Detected in principal cells in collecting ducts in kidney medulla (at protein level) (PubMed:16641094). Renal medulla and colon. Predominantly in the inner medulla. Expressed in basal layer of epidermal keratinocytes. Aquaporins contain two tandem repeats each containing three membrane-spanning domains and a pore-forming loop with the signature motif Asn-Pro-Ala (NPA). Mice have dry skin with reduced SC (stratum corneum) hydration, decreased elasticity and impaired biosynthesis. The glycerol content of SC and epidermis is reduced, whereas that of dermis and serum is normal. The dry, relatively inelastic skin is probably related to the humectant properties of glycerol, and the impaired SC repair to impaired epidermal biosynthetic function. Belongs to the MIP/aquaporin (TC 1.A.8) family. Truncated N-terminus.