Human Neutral Amino Acid Transporter ASCT1: Structure of the Gene (SLC1A4) and Localization to Chromosome 2p13-p15

Abstract

Screening for cDNAs encoding proteins similar to the sodium-coupled glutamate transporter GLAST1 led to the isolation of a cDNA clone coding for a protein that turned out to be identical to the recently described neutral amino acid transporter ASCT1. The new member of the GLAST-related transporter family does not transport glutamate or aspartate but alanine, serine, cysteine, and threonine instead. The expressed sequence tag EST02446, a short cDNA sequence found in the course of a large-scale sequencing project of human brain-derived cDNAs, showed significant similarity to the eukaryotic glutamate transporter GLAST1 and was therefore used as probe in the search for further glutamate transporter cDNAs. Fragments of the cDNA were used for the isolation and characterization of human ASCT1 genomic clones. The ORF of 1572 bp encoding 524 amino acid residues is distributed over 8 exons, which span at least 40 kb of human chromosomal DNA. The ASCT1 gene locus was assigned to chromosome 2p13-p15 by chromosomal in situ suppression (CISS) studies. The gene structure is not related to any other previously characterized transporter gene. In contrast to the genes of the sodium-coupled nonglutamate neurotransmitter transporters, it shows no obvious correspondence between intron/exon structure and transmembrane organization. The transcription start site in human liver tissue was determined by primer extension analysis to be located 291 bp upstream of the initiating ATG codon. The DNA region immediately upstream of the transcription start lacks any TATA or CAAT boxes but contains several binding sites for the transcription factors Sp1 and Egr-1 . The ASCT1 gene (SLC1A4) structure reported here will facilitate the characterization of the genes of the other members of the GLAST-related transporter family and might be useful in the elucidation of amino acid transport-related defects.