Cloning and functional characterization of a new subtype of the amino acidtransport system N

We have cloned a new subtype of the amino acid transport system N2 (SN2 or second subtype of system N) from rat brain. Rat SN2 consists of 471 amino a cids and belongs to the recently identified glutamine transporter gene fami ly that consists of system N and system A. Rat SN2 exhibits 63% identity wi th rat SN1. It also shows considerable sequence identity (50-56%) with the members of the amino acid transporter A subfamily. In the rat, SN2 mRNA is most abundant in the liver but is detectable in the brain, lung, stomach, k idney, testis, and spleen. When expressed in Xenopus laevis oocytes and in mammalian cells, rat SN2 mediates Na+-dependent transport of several neutra l amino acids, including glycine, asparagine, alanine, serine, glutamine, a nd histidine. The transport process is electrogenic, Li+ tolerant, and pH s ensitive. The transport mechanism involves the influx of Na+ and amino acid s coupled to the efflux of H+, resulting in intracellular alkalization. Pro line, alpha-(methylamino) isobutyric acid, and anionic and cationic amino a cids are not recognized by rat SN2.