Cloning and functional expression of a urea transporter from human bone marrow cells

A rapid passive urea transport has been previously described in the mammalian renal inner medullary collecting duct epithelial cells and in mammalian erythrocytes. Recently, a vasopressin-regulated Urea Transporter (UT2) has been cloned from a rabbit kidney medullary cDNA library (You, G., Smith, C. P., Kanai, Y., Lee, W. S., Stelzner, M., and Hediger, M. A. (1993) Nature 365, 844-847). We now report the cloning and characterization of a complementary DNA (HUT11) encoding an Urea Transporter isolated from a human bone marrow library. It encodes a 43,000-Da polypeptide of 391 Amino acids that exhibited 63% sequence identity with the rabbit Urea Transporter and a similar membrane topology. HUT11 carries 2 putative glycosylation sites and 10 cysteines, of which only 7 are conserved at an equivalent position in UT2. HUT11 transcripts have been identified in human erythroid and renal tissues. Expression studies in Xenopus oocytes demonstrated that HUT11 mediates a facilitated urea transport that was inhibited, as described in mammalian erythrocytes, by very low concentrations of phloretin, p-chloromercuribenzene sulfonate, and urea analogues. No unidirectional movements of charged molecules, glycerol, or water were associated with HUT11 expression in oocytes. These findings suggest that HUT11 is most likely responsible for the facilitated urea transport in human red blood cells.