Structural Research of Amino Acid Secondary Transporters
Amino acid secondary transporters are a class of membrane proteins that play a crucial role in the transport of amino acids across the cell membrane. These transporters are essential for cellular processes such as protein synthesis, amino acid metabolism, and neurotransmitter signaling.
Glutamate acts as an excitatory neurotransmitter, and its precise control is critical for maintaining normal brain function and preventing excitotoxicity. Extracellular glutamate removal is achieved by plasma membrane-bound transporters that use an elevator mechanism to couple glutamate transport to potassium, sodium, and pH gradients. Glutamate transporters also conduct chloride ions through a channel-like process that is uncoupled from the thermodynamics of transport. Recent studies report cryo-EM structure of a glutamate transporter homologue in the open channel state, revealing the formation of an aqueous cavity during the glutamate transport cycle. Functional characterization of this cavity combined with molecular dynamics simulations revealed that it is an aqueous-accessible chloride permeation pathway gated by two hydrophobic regions.
Figure 1. GltPh can be trapped in an open channel conformation. (Chen I, et al., 2021)
| Protein | Organism | Method | Resolution | PDB Entry ID |
| LEUTAA Leucine transporter (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 1.65 Å | 2A65 |
| LeuT Leucine transporter bound to inhibitors (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 1.85 Å | 2QEI |
| LeuT Leucine transporter bound to Tryptophan and Sodium (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.00 Å | 3F3A |
| LeuT Leucine transporter bound to Desipramine (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.90 Å | 2QJU |
| LeuT Leucine transporter bound to octylglucopyranoside (OG) (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.00 Å | 3GJD |
| F177R1 mutant of LeuT (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.25 Å | 3MPN |
| LeuT mutant F259V bound to sodium and L-tryptophan (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.63 Å | 3QS4 |
| LeuT in the outward-open conformation in complex with Fab (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 3.10 Å | 3TT1 |
| LeuT bound to L-leucine in space group C2 from lipid bicelles (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.50 Å | 3USG |
| LeuT in outward-facing Na+-free state, pH 6.5 (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.50 Å | 5JAE |
| LeuT, substrate bound in inward-facing occluded conformation (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.60 Å | 6XWM |
| LeuBAT (delta13 mutant) in complex with paroxetine (expressed in E. coli) | Aquifex aeolicus | X-ray diffraction | 2.89 Å | 4MM4 |
| Glutamate transporter homolog (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 3.50 Å | 1XFH |
| GltPh K55C-A364C mutant crosslinked with divalent mercury (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 3.51 Å | 3KBC |
| Crosslinked GltPh V216C-M385C mutant (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 3.80 Å | 3V8F |
| GltPh in inward-facing apo form (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 3.25 Å | 4P19 |
| 276S/M395R-GltPh in inward-facing conformation (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 4.21 Å | 4X2S |
| GltPh L66C-S300C mutant crosslinked with divalent mercury (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 4.50 Å | 4IZM |
| GltPh, Outward-facing state, in complex with TBOA (expressed in E. coli) | Pyrococcus horikoshii | Cryo-EM single particle analysis | 3.66 Å | 6X17 |
| GltPh, outward-facing state of the substrate-free Na+-only state (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 2.50 Å | 7AHK |
| GltPh with L-Asp & Na+, outward-facing state (expressed in E. coli) | Pyrococcus horikoshii | Cryo-EM single particle analysis | 3.08 Å | 6UWF |
| GltPh mutant - Y204L A345V V366A (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 3.38 Å | 6V8G |
| GltPh V216C-A391C mutant cross-linked in outward-facing state (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 3.65 Å | 6X01 |
| GltPh mutant (S279E/D405N) in complex with aspartate and sodium ions (expressed in E. coli) | Pyrococcus horikoshii | Cryo-EM single particle analysis | 2.20 Å | 7RCP |
| Glutamate transporter homologue GltTk in complex with D-aspartate (expressed in E. coli) | Thermococcus kodakarensis | X-ray diffraction | 2.80 Å | 6R7R |
| Glutamate transporter homologue GltTk in sodium only condition (expressed in E. coli) | Thermococcus kodakarensis | Cryo-EM single particle analysis | 3.22 Å | 6XWR |
| GltTk in complex with a photo switchable compound (expressed in E. coli) | Thermococcus kodakarensis | X-ray diffraction | 2.80 Å | 6ZLH |
| GltPh in complex with L-Asp (expressed in E. coli) | Pyrococcus horikoshii | X-ray diffraction | 2.96 Å | 2NWL |
| Substrate-free glutamate transporter homologue (expressed in E. coli) | Thermococcus kodakarensis | X-ray diffraction | 3.00 Å | 4KY0 |
| Substrate-free glutamate transporter homologue GltTk (expressed in E. coli) | Thermococcus kodakarensis | X-ray diffraction | 2.70 Å | 5DWY |
| MhsT multi-hydrophobic amino acid transporter, occluded inward-facing state (expressed in Lactococcus lactis) | Alkalihalobacillus halodurans | X-ray diffraction | 2.10 Å | 4US3 |
Table 1. Structural Research of Amino Acid Secondary Transporters.
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References
- Gotfryd K, et al. X-ray structure of LeuT in an inward-facing occluded conformation reveals mechanism of substrate release. Nature Communications. 2020, 11(1): 1005.
- Chen I, et al. Glutamate transporters have a chloride channel with two hydrophobic gates. Nature. 2021, 591(7849): 327-331.
