Cloning of the sodium-dependent, broad-scope, neutral amino acid transporter B(o) from a human placental choriocarcinoma cell line

We have isolated a cDNA from a human placental choriocarcinoma cell cDNA library which, when expressed in HeLa cells, induces a Na⁺-dependent amino acid transport system with preference for zwitterionic amino acids. Anionic amino acids, cationic amino acids, imino acids, and N-methylated amino acids are excluded by this system. These characteristics are identical to those described for the amino acid transporter B(o). When expressed in Xenopus laevis oocytes that do not have detectable endogenous activity of the amino acid transporter B(o), the cloned transporter increases alanine transport in the oocytes severalfold and induces alanine-evoked inward currents in the presence of Na⁺. The cDNA codes for a polypeptide containing 541 amino acids with 10 putative transmembrane domains. Amino acid sequence homology predicts this transporter (hATB(o)) to be a member of a superfamily consisting of the glutamate transporters, the neutral amino acid transport system ASCT, and the insulin-activable neutral/anionic amino acid transporter. Chromosomal assignment studies with somatic cell hybrid analysis and fluorescent in situ hybridization have located the ATB(o) gene to human chromosome 19q13.3.