The heterodimeric amino acid transporter 4F2hc/LAT1 is associated in Xenopus oocytes with a non-selective cation channel that is regulated by the serine/threonine kinase sgk-1.
System L is the major Na(+)-independent amino acid transporter of mammalian cells. It is constituted of the type II membrane protein 4F2hc (CD98) which is covalently linked to the polytopic membrane protein LAT1 via a disulfide bridge. The transporter is known to be regulated by the mineral corticoid aldosterone in Xenopus A6 cells. To understand the regulation of the transporter, the 4F2hc/LAT1 heterodimer was functionally expressed in Xenopus laevis oocytes and its transport properties were analysed using flux measurements and the two-electrode voltage-clamp technique. Expression of 4F2hc/LAT1 resulted in a rapid increase in a Na(+)-independent neutral amino acid antiport activity and simultaneously gave rise to a cation conductance. The cation channel was non-rectifying and non-selective, conducting Li(+) > Cs(+) = Na(+) > K(+). After replacement of Na(+) by NMDG, however, the currents were suppressed almost completely. The cation channel was not inhibited by amiloride, Ba2(+), TEA, Hoe293B, flufenamic acid or substrates of the system L amino acid transporter. Significant inhibition, however, was observed in the presence of La3(+), Gd3(+) and quinidine. Channel activity was upregulated by coexpression of 4F2hc/LAT1 with the aldosterone-regulated protein kinase sgk-1. The cation conductance was sensitive to changes in the redox potential, being inhibited following incubation of the oocytes with DTE for 30 min. Mutation of either of the disulfide bridge-constituting cysteines to serine resulted in a loss of ion channel activity whereas amino acid transport was unaffected. It is concluded that the 4F2hc/LAT1 heterodimer regulates a closely associated cation channel or even constitutes a cation channel itself.
Wagner
C A
CA
Physiologisches Institut, University of Tübingen, Gmelinstrasse 5, D-72076 Tübingen, Germany.
Bröer
A
A
Albers
A
A
Gamper
N
N
Lang
F
F
Bröer
S
S
D026905:Amino Acid Transport Systems; D000596:Amino Acids / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D015703:Antigens, CD / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D002352:Carrier Proteins / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D002412:Cations / Q000378:metabolism*; D002478:Cells, Cultured; D002794:Choline / Q000494:pharmacology; D019281:Dimerization; D004220:Disulfides / Q000378:metabolism; D027261:Fusion Regulatory Protein-1; D006801:Humans; D017874:Immediate-Early Proteins; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism; D017136:Ion Transport / Q000187:drug effects; D008536:Meglumine / Q000494:pharmacology; D008845:Microinjections; D016297:Mutagenesis, Site-Directed; D009687:Nuclear Proteins; D009865:Oocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D018408:Patch-Clamp Techniques; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D018075:RNA, Complementary / Q000008:administration & dosage / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism; D014162:Transfection; D014981:Xenopus; D000099184:Serum-Glucocorticoid Regulated Kinases
D026905:Amino Acid Transport Systems; D000596:Amino Acids; D015703:Antigens, CD; D002352:Carrier Proteins; D002412:Cations; D002794:Choline; D004220:Disulfides; D027261:Fusion Regulatory Protein-1; D017874:Immediate-Early Proteins; D007473:Ion Channels; D008536:Meglumine; D009687:Nuclear Proteins; D017346:Protein Serine-Threonine Kinases; D018075:RNA, Complementary; D012964:Sodium; D000099184:Serum-Glucocorticoid Regulated Kinases