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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

Characterization of a human colonic cDNA encoding a structurally novel urea transporter, hUT-A6

¹School of Biological Sciences, University of Manchester, Manchester M13 9PT, United Kingdom; and ²Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1603

Submitted 27 August 2003 ; accepted in final form 5 June 2004

Two closely related genes, UT-A (Slc14a2) and UT-B (Slc14a1), encode specialized transporter proteins that modulate the movement of urea across cell membranes. In this article, we report the characterization of a cDNA isolated from human colonic mucosa encoding a novel UT-A urea transporter, hUT-A6. The encoded protein is 235 amino acids (aa) in length, making it the smallest UT-A member characterized. On the basis of previous structural predictions, hUT-A6 is structurally unique in that it consists of a single hydrophobic core flanked by hydrophilic NH₂- and COOH-terminal domains. The transcript encoding hUT-A6 contains a novel 129-bp exon, exon 5a, which, as a result of alternative splicing, introduces a unique 19-aa segment and a stop codon. Functionally, the protein transports urea, and this activity is inhibited by phloretin. Interestingly, despite the lack of a protein kinase A (PKA) consensus site {[RK](2)-X-[ST]}, transport of urea by hUT-A6 is stimulated by PKA agonists. Deletion of the two PKA consensus sites from murine UT-A3 (mUT-A3) did not affect the stimulatory response of PKA agonists, which, together with the lack of PKA consensus sites in hUT-A6, indicates that regulation of hUT-A6 and mUT-A3 is not mediated through a classic PKA phosphorylation consensus.

urea; transporter protein; colon; cAMP

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