Abstract
L-amino acid transporters (LATs) play key roles in human physiology and are implicated in several human pathologies. LATs are asymmetric amino acid exchangers where the low apparent affinity cytoplasmic side controls the exchange of substrates with high apparent affinity on the extracellular side. Here, we report the crystal structures of an LAT, the bacterial alanine-serine-cysteine exchanger (BasC), in a non-occluded inward-facing conformation in both apo and substrate-bound states. We crystallized BasC in complex with a nanobody, which blocks the transporter from the intracellular side, thus unveiling the sidedness of the substrate interaction of BasC. Two conserved residues in human LATs, Tyr 236 and Lys 154, are located in equivalent positions to the Na1 and Na2 sites of sodium-dependent APC superfamily transporters. Functional studies and molecular dynamics (MD) calculations reveal that these residues are key for the asymmetric substrate interaction of BasC and in the homologous human transporter Asc-1.
Highlights
L-amino acid transporters (LATs) play key roles in human physiology and are implicated in several human pathologies
In the brain, Asc-1 is the major transporter of D-serine, which acts as a co-agonist of NMDA-glutamate receptors and is a novel target to treat schizophrenia[12]
The bacterial alanine-serine-cysteine exchanger (BasC)-nanobody 74 (Nb74) complex used for crystallization assays was purified by size exclusion chromatography (SEC) (Supplementary Fig. 1c)
Summary
L-amino acid transporters (LATs) play key roles in human physiology and are implicated in several human pathologies. Functional studies and molecular dynamics (MD) calculations reveal that these residues are key for the asymmetric substrate interaction of BasC and in the homologous human transporter Asc-1. LATs do not modify the overall amino acid concentration gradient between the two sides of the cell membrane, but rather act on the proportional distribution of particular amino acids, regulating intracellular amino acid pools[4]. The asymmetric functional interaction of LATs with the substrates at both sides of the plasma membrane allows the high concentration of intracellular substrates (mM range) to control the exchange with substrates in the extracellular medium (μM range)[5]. Consistent with the key role of these transporters in metabolism and physiology, dysfunctional HATs cause human disease. LAT1 and xCT are potential targets for cancer therapy because they are overexpressed in many tumors[13,14]
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