Na+-dependent neutral amino acid transporter ATB(0) is a rabbit epithelialcell brush-border protein

System B-o activity accounts for the majority of intestinal and kidney lumi nal neutral amino acid absorption. An amino acid transport system, called A TB(o) (also known as ASCT2), with functional characteristics similar to tho se of system B-o, has been recently cloned. We generated polyclonal antibod ies to human and rabbit ATB(o) COOH-terminal peptides and used Western blot analysis to detect ATB(o) protein in rabbit tissues, rabbit ileal brush-bo rder membrane vesicles (BBMV), and HeLa cells transfected with plasmids con taining ATB(o) cDNAs. Immunohistochemistry was used to localize ATB(o) in r abbit kidney and intestine. In Western blots of rabbit tissues, ATB(o) was a broad smear of 78- to 85-kDa proteins. In transfected HeLa cells, ATB(o) appeared as a smear consisting of 57- to 65-kDa proteins. The highest expre ssion was found in the kidney. ATB(o) was enriched in rabbit ileal BBMV and in HeLa cells transfected with ATB(o) cDNAs. In the kidney and in the inte stine, ATB(o) was confined to the brush-border membrane (BBM) of the proxim al tubular cell and of the enterocyte, respectively. Tissue and intracellul ar distribution of ATB(o) protein parallels that of system B-o activity. AT B(o) protein could be the transporter responsible for system B-o in the BBM of epithelial cells.