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Tertiary active transport of amino acids reconstituted by coexpression of System A and L transporters in Xenopus oocytes

Baird, Fiona E., Bett, Kevin J., MacLean, Catherine, Tee, Andrew, Hundal, Harinder. S. and Taylor, Peter M. 2009. Tertiary active transport of amino acids reconstituted by coexpression of System A and L transporters in Xenopus oocytes. American Journal of Physiology. Endocrinology and Metabolism 297 (3) , E822-E829. 10.1152/ajpendo.00330.2009

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The System L transporter facilitates cellular import of large neutral amino acids (AAs) such as Leu, a potent activator of the intracellular target of rapamycin (TOR) pathway, which signals for cell growth. System L is an AA exchanger, proposed to accumulate certain AAs by coupling to dissipation of concentration gradient(s) of exchange substrates generated by secondary active AA transporters such as System A (SNAT2). We addressed the hypothesis that this type of coupling (termed tertiary active transport) acts as an indirect mechanism to extend the range of AA stimulating TOR to those transported by both Systems A and L (e.g., Gln) through downstream enhancement of Leu accumulation. System A overexpression enabled Xenopus oocytes to accumulate substrate AAs (notably Ser, Gln, Ala, Pro, Met; totaling 2.6 nmol/oocyte) from medium containing a physiological AA mixture at plasma concentrations. Net accumulation of System L (4F2hc-xLAT1) substrates from this medium by System L-overexpressing oocytes was increased by 90% (from 0.7 to 1.35 nmol/oocyte; mainly Leu, Ile) when Systems A and L were coexpressed, coincident with a decline in accumulation of specific System A substrates (Gln, Ser, Met), as expected if the latter were also System L substrates and functional coupling of the transport Systems occurred. AA flux coupling was confirmed as trans-stimulation of Leu influx in System L-expressing oocytes by Gln injection (0.5 nmol/oocyte). The observed changes in Leu accumulation are sufficient to activate the TOR pathway in oocytes, although intracellular AA metabolism limits the potential for AA accumulation by tertiary active transport in this system.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Medicine
Subjects: Q Science > QH Natural history > QH426 Genetics
R Medicine > R Medicine (General)
Uncontrolled Keywords: Nutrient transport ; Target of rapamycin pathway ; Glutamine ; Leucine ; Amino acid exchanger
Publisher: American Physiological Society
ISSN: 0193-1849
Last Modified: 04 Jun 2017 03:57

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