Activation of system L heterodimeric amino acid exchangers by intracellular substrates

System L‐type transport of large neutral amino acids is mediated by ubiquitous LAT1‐4F2 hc and epithelial LAT2‐4F2 hc . These heterodimers are thought to function as obligatory exchangers, but only influx properties have been studied in some detail up until now. Here we measured their intracellular substrate selectivity, affinity and exchange stoichiometry using the Xenopus oocyte expression system. Quantification of amino acid influx and efflux by HPLC demonstrated an obligatory amino acid exchange with 1:1 stoichiometry. Strong, differential trans‐stimulations of amino acid influx by injected amino acids showed that the intracellular substrate availability limits the transport rate and that the efflux selectivity range resembles that of influx. Compared with high extracellular apparent affinities, LAT1‐ and LAT2‐4F2 hc displayed much lower intracellular apparent affinities (apparent K m in the millimolar range). Thus, the two system L amino acid transporters that are implicated in cell growth (LAT1‐4F2 hc ) and transcellular transport (LAT2‐4F2 hc ) are obligatory exchangers with relatively symmetrical substrate selectivities but strongly asymmetrical substrate affinities such that the intracellular amino acid concentration controls their activity.