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

1. System L is the major Naf-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 mineralcorticoid 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. 2. 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. 3. 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. 4. It is concluded that the 4F2hc/LAT1 heterodimer regulates a closely associated cation channel or even constitutes a cation channel itself.