Molecular characterization of a novel UT-A urea transporter isoform (UT-5)in testis

Urea movement across plasma membranes is modulated by specialized transport er proteins that are products of two genes, termed UT-A and UT-B. These pro teins play key roles in the urinary concentrating mechanism and fluid homeo stasis. We have isolated and characterized a 1.4-kb cDNA from testes encodi ng a new isoform (UT-A5) belonging to the UT-A transporter family. For comp arison, we also isolated a 2.0-kb cDNA from mouse kidney inner medulla enco ding the mouse UT-A3 homologue. The UT-A5 cDNA has a putative open reading frame encoding a 323- amino acid protein, making UT-A5 the smallest UT- A f amily member in terms of molecular size. Its putative topology is of partic ular interest, because it calls into question earlier models of UT-A transp orter structure. Expression of UT-A5 cRNA in Xenopus oocytes mediates phlor etin-inhibitable urea uptake and does not translocate water. The distributi on of UT-A5 mRNA is restricted to the peritubular myoid cells forming the o utermost layer of the seminiferous tubules within the testes and is not det ected in kidney. UT-A5 mRNA levels are coordinated with the stage of testes development and increase 15 days postpartum, commensurate with the start o f seminiferous tubule fluid movement.