Sequence homologies among intestinal and renal Na+/glucose cotransporters

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Abstract
Sodium-dependent glucose transport occurs in the intestine and kidney of most animal species. The cDNA encoding the Na+/glucose cotransporter from rabbit jejunum was used to examine the distribution of homologous mRNA in other rabbit tissues and in the intestines of other species. Northern blots of mRNA extracted from various tissues were probed with radiolabeled cDNA of the cloned rabbit transporter. The probe hybridized with mRNA of approximately 2.2 kb from rabbit jejunum, renal cortex, and renal medulla, indicating that related mRNA of the same size is found in these tissues. With the use of the same cDNA probe, a 1.6-kb partial-length clone encoding 484 amino acids was isolated from a rabbit renal cortex cDNA library. There was greater than 99% identity between the cDNA sequences, and 100% identity between the amino acid sequences, of the renal clone and the rabbit intestinal Na+/glucose cotransporter. The 2.2-kb transcript was seen in mRNA from duodenum, jejunum, and ileum, with a distribution that matched the Na+/glucose transport capacity in these tissues. A faint signal at 2.2 kb was also seen in colon mRNA. There was no detectable hybridization to blots of stomach and heart mRNA. The rabbit probe also hybridized to intestinal mRNA from a number of species from trout to humans. We conclude that a Na+/glucose cotransporter of rabbit renal cortex is very similar to that of the intestine and that the intestinal transporter has been conserved during evolution.