Cloning and Expression of a b0,+-like Amino Acid Transporter Functioning as a Heterodimer with 4F2hc Instead of rBAT

Abstract

We have cloned a transporter protein from rabbit small intestine, which, when coexpressed with the 4F2 heavy chain (4F2hc) in mammalian cells, induces a b^0,+-like amino acid transport activity. This protein (4F2-lc6 for the sixth member of the 4F2 light chain family) consists of 487 amino acids and has 12 putative transmembrane domains. At the level of amino acid sequence, 4F2-lc6 shows significant homology (44% identity) to the other five known members of the 4F2 light chain family, namely LAT1 (4F2-lc1), y⁺LAT1 (4F2-lc2), y⁺LAT2 (4F2-lc3), xCT (4F2-lc4), and LAT2 (4F2-lc5). The 4F2hc/4F2-lc6 complex-mediated transport process is Na⁺-independent and exhibits high affinity for neutral and cationic amino acids and cystine. These characteristics are similar to those of the b^0,+-like amino acid transport activity previously shown to be associated with rBAT (protein related to b^0,+ amino acid transport system). However, the newly cloned 4F2-lc6 does not interact with rBAT. This is the first report of the existence of a b^0,+-like amino acid transport process that is independent of rBAT. 4F2-lc6 is expressed predominantly in the small intestine and kidney. Based on the characteristics of the transport process mediated by the 4F2hc/4F2-lc6 complex and the expression pattern of 4F2-lc6 in mammalian tissues, we suggest that <i>4F2-lc6</i> is a new candidate gene for cystinuria.