CLONING AND MOLECULAR CHARACTERIZATION OF THE ONTOGENY OF A RAT ILEALSODIUM-DEPENDENT BILE-ACID TRANSPORTER

Sodium-dependent bile acid transport in the rat ileum is abruptly expr essed at weaning. Degenerate oligonucleotides, based on amino acid seq uence identities between the rat liver and hamster ileal transporters, were used to amplify a rat ileal probe. A 1.2-kb cDNA clone, which co ntains the full coding region (348 amino acids, 38 kD), was isolated b y hybridization screening. In vitro translation yielded a 38-kD protei n which glycosylated to 48 kD. Sodium-dependent uptake of taurocholate was observed in oocytes injected with cRNA. Northern blot analysis re vealed a 5.0-kb mRNA in ileum, kidney, and cecum. A 48-kD protein was detected in ileal brush border membranes and localized to the apical b order of villus ileal enterocytes. mRNA and protein expression, which were negligible before weaning, increased dramatically at weaning. Nuc lear transcription rates for the transporter increased 15-fold between postnatal days 7 and 28. The apparent molecular weight of the transpo rter also increased between days 19 and 28. In summary, the developmen tal regulation of the rat ileal sodium-dependent bile acid cotransport er is characterized by transcriptionally regulated increases in mRNA a nd protein levels at the time of weaning with changes in apparent mole cular weight of the protein after weaning.