Expression of solute carrier 7A4 (SLC7A4) in the plasma membrane is not sufficient to mediate amino acid transport activity.
Member 4 of human solute carrier family 7 (SLC7A4) exhibits significant sequence homology with the SLC7 subfamily of human cationic amino acid transporters (hCATs) [Sperandeo, Borsani, Incerti, Zollo, Rossi, Zuffardi, Castaldo, Taglialatela, Andria and Sebastio (1998) Genomics 49, 230-236]. It is therefore often referred to as hCAT-4 even though no convincing transport activity has been shown for this protein. We expressed SLC7A4 in Xenopus laevis oocytes, but could not detect any transport activity for cationic, neutral or anionic amino acids or for the polyamine putrescine. In addition, human glioblastoma cells stably overexpressing a fusion protein between SLC7A4 and the enhanced green fluorescent protein (EGFP) did not exhibit an increased transport activity for l-arginine. The lack of transport activity was not due to a lack of SLC7A4 protein expression in the plasma membrane, as in both cell types SLC7A4-EGFP exhibited a similar subcellular localization and level of protein expression as functional hCAT-EGFP proteins. The expression of SLC7A4 can be induced in NT2 teratocarcinoma cells by treatment with retinoic acid. However, also for this endogenously expressed SLC7A4, we could not detect any transport activity for l-arginine. Our data demonstrate that the expression of SLC7A4 in the plasma membrane is not sufficient to induce an amino acid transport activity in X. laevis oocytes or human cells. Therefore, SLC7A4 is either not an amino acid transporter or it needs additional (protein) factor(s) to be functional.
Wolf
Sabine
S
Department of Pharmacology, Johannes Gutenberg University, Obere Zahlbacher Strasse 67, 55101 Mainz, Germany.
Janzen
Annette
A
Vékony
Nicole
N
Martiné
Ursula
U
Strand
Dennis
D
Closs
Ellen I
EI
D000595:Amino Acid Sequence; D027182:Amino Acid Transport System y+ / Q000235:genetics / Q000378:metabolism*; D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D001120:Arginine / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism*; D017931:DNA Primers; D005909:Glioblastoma; D049452:Green Fluorescent Proteins; D006801:Humans; D007700:Kinetics; D008164:Luminescent Proteins / Q000378:metabolism; D008322:Mammals; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D018243:Teratocarcinoma; D014407:Tumor Cells, Cultured; D014982:Xenopus laevis
D027182:Amino Acid Transport System y+; D000596:Amino Acids; D017931:DNA Primers; D008164:Luminescent Proteins; D011993:Recombinant Fusion Proteins; C420862:SLC7A4 protein, human; D049452:Green Fluorescent Proteins; D001120:Arginine