cDNA cloning of a rat small-intestinal Na+/SO42- cotransporter

We have isolated a cDNA (ileal NaSi-1) from rat small intestine by homology screening with a cDNA (renal NaSi-1) encoding rat kidney cortex Na+-SO42-cotransport. Ileal NaSi-1 cRNA specifically stimulates Na+-dependent SO42-uptake in a time- and dose-dependent manner in Xenopus laevis oocytes, with kinetic parameters almost identical to those of the renal NaSi-1. Ileal NaSi-1 cDNA contains 2722 base pairs (bp), almost 500bp more than the renal NaSi-1 cDNA; however, it encodes a protein of 595 amino acids identical to the renal NaSi-1 protein. Northern blot analysis shows strong signals in rat lower small intestine and kidney cortex (2.9×103 and 2.3×103 bases), with the ileal NaSi-1 corresponding to the longer transcript. We conclude that we have identified a rat ileal cDNA that encodes a membrane protein most likely involved in brush-border Na+-SO42-cotransport. It differs to the renal NaSi-1 only in the length of the 3′ untranslated region, suggesting that the major difference lies in the differential use of polyadenylation signals.