Rush Hour of LATs towards Their Transport Cycle

Abstract

The mammalian SLC7 family comprises the L-amino acid transporters (LATs) and the cationic amino acid transporters (CATs). The relevance of these transporters is highlighted by their involvement in several human pathologies, including inherited rare diseases and acquired diseases, such as cancer. In the last four years, several crystal or cryo-EM structures of LATs and CATs have been solved. These structures have started to fill our knowledge gap that previously was based on the structural biology of remote homologs of the amino acid-polyamine-organocation (APC) transporters. This review recovers this structural and functional information to start generating the molecular bases of the transport cycle of LATs. Special attention is given to the known transporter conformations within the transport cycle and the molecular bases for substrate interaction and translocation, including the asymmetric interaction of substrates at both sides of the plasma membrane.

Keywords: APC; LATs; SLC7; structure; substrate binding; substrate translocation; transport cycle.

Figure 1

Conformational states of the APC family transport cycle. The transition between inward- and outward-facing states involves the occluded substrate-bound state (bold arrows), except for the transporters with a nonobligatory exchange activity, which could also explore the occluded apo state (dashed arrows). Each conformational state is shown by a sliced-surface representation of the transporter indicated in bold, colored gold for LATs and white for other APC transporters. All transporters with known structures in each state are indicated. PDB codes for the structures depicted here are the following: ApcT occluded apo (3GIA), AdiC outward-facing open apo (3NCY), AdiC outward-facing open substrate-bound (5J4N), human LAT1 and GkApcT outward-facing substrate-bound occluded (7DSK and 5OQT, respectively), human LAT1 inward-facing open substrate-bound (6IRT), and BasC inward-facing open apo (6F2G). The two occluded substrate-bound structures shown (hLAT1 and GkApcT) might represent small variations within the occluded substrate-bound state of LATs/CATs. Protein representations were created with PyMOL software (The PyMOL Molecular Graphics System, Version 2.3, Schrodinger, LLC. 2010).

Figure 2

TM1 and TM6 structural rearrangements upon substrate occlusion. Structural superimposition of transmembrane domain 6 of (A) AdiC substrate-bound outward occluded (dark blue, PDB ID: 3L1L) and outward open (light blue, PDB ID: 5J4N) structures; (B) human LAT1 outward occluded, complexed with either JX-078 (pale green, PDB ID: 7DSL) and/or 3,5-diiodo-L-tyrosine (dark green PDB ID: 7DSQ); and (C) human LAT1 outward occluded with JX-078 (pale green), human LAT1 inward-open (lemon green, PDB ID: 6IRT), human b^0,+AT (orange, PDB ID: 6LI9), and BasC (salmon pink, PDB ID 6F2G). (D) Upper extracellular view of the structural superimposition of GkApcT substrate-bound inward-facing occluded conformation (solid colors, PDB ID: 5OQT) and human LAT1 outward occluded, complexed with JX-078 (light colors, PDB ID: 7DSL). (E) Detailed structural superimposition of transmembrane domains 1 (blue) and 6 (green) from (D). Protein representations were created with PyMOL software (The PyMOL Molecular Graphics System, Version 2.3, Schrodinger, LLC. 2010).

Figure 3

Substrate-binding site design in APC transporters. (A) View of the bacterial alanine–serine–cysteine exchanger substrate-binding site (PDB ID 6F2W). Distances between atoms of the substrate (AIB, orange) and BasC residues compatible with H bonds are indicated (dashed lines). F199 acts as the external thin gate, sealing the binding site from the extracellular side. (B) Superimposition of AdiC outward-facing open apo (PDB ID 3J4I) and outward-facing arginine-bound and occluded (PDB ID 3L1L). Distances between atoms of the substrate side chain (L-arginine, orange) and AdiC residues compatible with H bonds are indicated (dashed lines). Tyr 239 (equivalent to Tyr 236 in BasC) is also depicted. Trp 202, which acts as the external thin gate, tilts away from the binding site in the outward-facing open conformation, allowing substrate release. Protein representations were created with PyMOL software (The PyMOL Molecular Graphics System, Version 2.3, Schrodinger, LLC. 2010).