Recycling of aromatic amino acids via TAT1 allows efflux of neutral amino acids via LAT2-4F2hc exchanger

Abstract

The rate of amino acid efflux from individual cells needs to be adapted to cellular demands and plays a central role for the control of extracellular amino acid homeostasis. A particular example of such an outward amino acid transport is the basolateral efflux from transporting epithelial cells located in the small intestine and kidney proximal tubule. Because LAT2-4F2hc (Slc7a8-Slc3a2), the best known basolateral neutral amino acid transporter of these epithelial cells, functions as an obligatory exchanger, we tested whether TAT1 (Slc16a10), the aromatic amino-acid facilitated diffusion transporter, might allow amino acid efflux via this exchanger by recycling its influx substrates. In this study, we show by immunofluorescence that TAT1 and LAT2 indeed colocalize in the early kidney proximal tubule. Using the Xenopus laevis oocytes expression system, we show that L-glutamine is released from oocytes into an amino-acid-free medium only when both transporters are coexpressed. High-performance liquid chromatography analysis reveals that several other neutral amino acids are released as well. The transport function of both TAT1 and LAT2-4F2hc is necessary for this efflux, as coexpression of functionally inactive but surface-expressed mutants is ineffective. Based on negative results of coimmunoprecipitation and crosslinking experiments, the physical interaction of these transporters does not appear to be required. Furthermore, replacement of TAT1 or LAT2-4F2hc by the facilitated diffusion transporter LAT4 or the obligatory exchanger LAT1, respectively, supports similar functional cooperation. Taken together, the results suggest that the aromatic amino acid diffusion pathway TAT1 can control neutral amino acid efflux via neighboring exchanger LAT2-4F2hc, by recycling its aromatic influx substrates.