Molecular and functional characterization of an amphibian urea transporter

We report the characterization of a frog (Rana esculenta) urea transporter (fUT). The cloned cDNA is 1.4 kb long and contains a putative open reading frame of 1203 bp. In frog urinary bladder, the gene is expressed as two mRNAs of 4.3 and 1.6 kb. The fUT protein is 63.1 and 56.3% identical to rat UT-A2 and UT-B1, respectively. The internal duplication of UT-A2 and UT-B, as well as the double LP box urea transporter signature sequence were found in this amphibian urea transporter. When expressed in Xenopus oocytes, fUT induced a 10-fold increase in urea permeability, which was blocked by both phloretin and mercurial reagents. The fUT protein did not transport thiourea, but the fUT-mediated urea transport was strongly inhibited by this compound. Thus, this amphibian urea transporter displays transport characteristics in between those of UT-A2 and UT-B.