Primary structure, genomic organization, and functional and electrogenic characteristics of human system N 1, a Na+- and H+-coupled glutamine transporter

We have cloned the human Na+- and H+-coupled amino acid transport system N (hSN1) from HepG2 liver cells and investigated its functional characteristi cs. Human SN1 protein consists of 504 amino acids and shows high homology t o rat SN1 and rat brain glutamine transporter (GlnT). When expressed in mam malian cells, the transport function of human SN1 could be demonstrated wit h glutamine as the substrate in the presence of LiCl (instead of NaCl) and cysteine. The transport activity was saturable, pH-sensitive, and specific for glutamine, histidine, asparagine, and alanine. Analysis of Li+ activati on kinetics showed a Li+:glutamine stoichiometry of 2:1. When expressed in Xenopus laevis oocytes, the transport of glutamine or asparagine via human SN1 was associated with inward currents under voltage-clamped conditions. T he transport function, monitored as glutamine- or asparagine-induced curren ts, was saturable, Na+-dependent, Li+-tolerant, and pH-sensitive. The trans port cycle was associated with the involvement of more than one Na+ ion. Up take of asparagine was directly demonstrable in these oocytes by using radi olabeled substrate, and this uptake was inhibited by membrane depolarizatio n. In addition, simultaneous measurement of asparagine influx and charge in flux in the same oocyte yielded an asparagine:charge ratio of 1. These data suggest that SN1 mediates the influx of two Na+ and one amino acid substra te per transport cycle coupled to the efflux of one H+, rendering the trans port process electrogenic.