Molecular and functional characterization of the intestinal amino acid transporter B°

The neutral amino acid transport system B° in the intestine and kidney is believed to be the most likely site of defect in the genetic disorder Hartnup disease. We have cloned this transporter from cDNA libraries derived from the rabbit intestine and the human intestinal cell line Caco-2. The rabbit intestinal amino acid transporter B° (riATB°) cDNA is 2,578 bp long and codes for a protein consisting of 541 amino acids. The cDNA, when functionally expressed in HeLa cells or in Xenopus laevis oocytes, induces Na⁺-dependent transport of several neutral amino acids including glutamine. The cDNA-induced transport system also accepts acidic amino acids as substrates at acidic pH. The Caco-2 cell amino acid transporter B° (hiATB°) cDNA is 2,733 Bp long and codes for a protein consisting of 541 amino acids. riATB° and hiATB° are highly homologous in amino acid sequence (85% identity and 92% similarity). The molecular and functional characteristics of the hiATB° are identical to those of the ATB° recently cloned from a human choriocarcinoma cell cDNA library, indicating that both are the products of the same gene. Human intestine and a human kidney proximal tubular cell line also contain ATB°-specific transcripts as evidenced by RT-PCR and by restriction site analysis of the RT-PCR products.