The SLC1A5 gene, located at 19q13.3, counts 28692 nucleotides with 8 exons. It has been found in 56 different organisms (NCBI). The gene encodes a protein involved in sodium-dependent neutral amino acid transport (Kekuda et al., 1996; Pingitore et al., 2013).

Three isoforms (transcripts) are reported either on NCBI and Ensembl databases for SLC1A5 human gene, deriving from different translation start. They differ in length, particularly at 5 extremity. The first variant NM_005628 represents the longest transcript, constituted by 2873 nucleotides and 8 exons. This transcript encodes a peptide of 541 amino acids. The second variant NM_001145144 is constituted by 1737 nucleotides and differs in the 5 UTR from the variant NM_005628. In NM_001145144 the translation starts downstream the third exon generating a shorter peptide of 313 aa. The third isoform NM_ 001145145 has 1927 nucleotides and lacks the first exon. It presents a different translation start at 5, coding a peptide of 339 amino acids. A longer transcript, XM_005259167, is reported only in NCBI database. It has been identified by automated computational analysis. More than 400 SNP(s), both in coding and non-coding regions of the SLC1A5 gene, are reported in dbSNP database (dbSNP). More than 40 are responsible of amino acid substitutions with unknown significance. Only the variant SLC1A5-P17A (rs3027956) is associated with breast cancer (Savas et al., 2006). A region constituted by 907 bp upstream of the ASCT2 gene possesses promoter activity (Bungard and McGivan, 2004). In this region the following putative elements have been identified: an amino acid-regulatory element, a consensus site for binding of the transcription factor activator protein 1 (AP1) and a consensus binding sites for nuclear and hepatocyte nuclear factors.

The gene is virtually present in all vertebrates. The better known orthologous of the human gene are those from rat, mouse and rabbit. Identity between the human and rat, mouse, rabbit are 79%, 82% and 85%, respectively.

541 amino acids; molecular mass 56598,34 Da.
Human SLC1A5 is a permease (membrane transporter). The 3D structure is not available. Homology modeling highlights a structure similar to that of the glutamate transporter of P. horikoshii (1XFH). N- and C-terminal ends are intracellular. Potential site of N-glycosylation and phosphorilation are predicted. In the structural model, at least one glycosylation site is extracellular and the phosphorilation sites are intracellular (Fig. 2).

Human SLC1A5 has been originally named ASCT2 from AlaSerCysTransporter2 or ATB0. The acronym ASCT2 is the most frequently used to designate this transport system. It is expressed in many tissues, including brain, (Bröer and Brookes, 2001; Deitmer et al., 2003; Gliddon et al., 2009). There is functional evidence of the expression of ASCT2 in kidney and intestine (Bode, 2001). Besides Caco-2 cells, apparently, also the HT-29 intestinal cell line functionally expresses ASCT2 (Kekuda et al., 1996; Kekuda et al., 1997). Poly(A)1 RNA isolated from several tissues of human origin revealed expression in placenta, lung, skeletal muscle, kidney, and pancreas (Kekuda et al., 1996).

The protein is localized in the plasma membrane.

Transport mediated by the human ASCT2 has been originally studied in intact cell systems over-expressing the transport protein (Kekuda et al., 1996; Kekuda et al., 1997). Recently, hASCT2 was over-expressed in the yeast P. pastoris, purified and reconstituted in artificial phospholipid vesicles (proteoliposomes), in absence of other interfering transporters. All experimental systems concur in demonstrating that hASCT2 is an obligate exchanger of neutral amino acid. This antiport requires the presence of extracellular Na⁺ which cannot be substituted by Li⁺ or K⁺. The Na⁺ ex:amino acidex stoichiometry of the human transporter is likely to be 1:1. Competition studies on ³H-glutamine, ³H-threonine or ³H-alanine transport performed in cells indicated that other potential substrates of hASCT2 are valine, leucine, serine, cysteine, asparagine, methionine, isoleucine, tryptophan, histidine, phenylalanine. While glutamate, lysine, arginine along with MeAIB [α-(methylamino)isobutyric acid] and BCH [2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid] are neither transported nor inhibit hASCT2. Experiments with radioactive compounds confirmed the competition data (Torres-Zamorano et al., 1998). In proteoliposomes, inhibition has been confirmed for most but not for all of the amino acids. Moreover, proteoliposome studies highlighted an asymmetric specificity for amino acids allowing to distinguish the amino acids inwardly transported (alanine, cysteine, valine, methionine) from those bi-directionally transported (glutamine, serine, asparagine, and threonine). The functional asymmetry was also confirmed by the kinetic analysis of [³H]glutamine/glutamine antiport: different Km values were measured on the external and internal sides of proteoliposomes, 0,097 and 1,8 mM, respectively. The SH reagents HgCl2, mersalyl and pOHMB potently inhibited hASCT2 mediated transport (Pingitore et al., 2013).
The physiological role of hASCT2 consists in providing cells with some neutral amino acids exporting others on the basis of the metabolic need of cells consistently with the intra and extracellular amino acid concentrations. In brain, particularly, hASCT2 contributes to glutamine homeostasis of neurons and astrocytes. On the basis of experiments performed with animal models, it was hypothesized that hASCT2 mediates efflux of glutamine from astrocytes, a process that is critical for the functioning of the glutamate-glutamine cycle to recover synaptically released glutamate in exchange with glutamine efflux (Bröer et al., 1999). The glutamine-glutamate cycle has been shown also in placenta. Glutamine crosses the placenta and enters the fetal liver where it is deamidated to glutamate. About 90% of glutamate generated by the liver is taken up by the placenta and used in the metabolism. The glutamine-glutamate cycle between the placenta and the fetal liver is obligatory for the generation of NADPH in the placenta (Torres-Zamorano et al., 1998). Among other functions reported for hASCT2 there is the regulation of mTOR pathway, translation and autophagy. The transporter regulates an increase in the intracellular concentration of glutamine which is then used by another plasma membrane transporter, named LAT1 (SLC7A5) (Galluccio et al., 2013) as efflux substrate to regulate the uptake of extracellular leucine with subsequent activation of mTORC1 (Nicklin et al., 2009). Moreover, it has been proposed that a group of retroviruses specifically uses the hASCT2 as a common cell surface receptor following a co-evolution phenomenon. The orthologous murine transporter mASCT2 is inactive as a viral receptor (Marin et al., 2003).