Cysteine Scanning Mutagenesis of TM4b-4c Loop of Glutamate Transporter EAAT1 Reveals Three Conformationally Sensitive Residues

Synthetic Biology: Bottom-Up Assembly of Molecular Systems

The bottom-up assembly of biological and chemical components opens exciting opportunities to engineer artificial vesicular systems for applications with previously unmet requirements. The modular combination of scaffolds and functional building blocks enables the engineering of complex systems with biomimetic or new-to-nature functionalities. Inspired by the compartmentalized organization of cells and organelles, lipid or polymer vesicles are widely used as model membrane systems to investigate the translocation of solutes and the transduction of signals by membrane proteins. The bottom-up assembly and functionalization of such artificial compartments enables full control over their composition and can thus provide specifically optimized environments for synthetic biological processes. This review aims to inspire future endeavors by providing a diverse toolbox of molecular modules, engineering methodologies, and different approaches to assemble artificial vesicular systems. Important technical and practical aspects are addressed and selected applications are presented, highlighting particular achievements and limitations of the bottom-up approach. Complementing the cutting-edge technological achievements, fundamental aspects are also discussed to cater to the inherently diverse background of the target audience, which results from the interdisciplinary nature of synthetic biology. The engineering of proteins as functional modules and the use of lipids and block copolymers as scaffold modules for the assembly of functionalized vesicular systems are explored in detail. Particular emphasis is placed on ensuring the controlled assembly of these components into increasingly complex vesicular systems. Finally, all descriptions are presented in the greater context of engineering valuable synthetic biological systems for applications in biocatalysis, biosensing, bioremediation, or targeted drug delivery.

Critical amino acids in the TM2 of EAAT2 are essential for membrane-bound localization, substrate binding, transporter function and anion currents

Excitatory amino acid transporter 2 (EAAT2), the gene of which is known as solute carrier family 1 member 2 (SLC1A2), is an important membrane-bound transporter that mediates approximately 90% of the transport and clearance of l-glutamate at synapses in the central nervous system (CNS). Transmembrane domain 2 (TM2) of EAAT2 is close to hairpin loop 2 (HP2) and far away from HP1 in the inward-facing conformation. In the present study, 14 crucial amino acid residues of TM2 were identified via alanine-scanning mutations. Further analysis in EAAT2-transfected HeLa cells in vitro showed that alanine substitutions of these residues resulted in a decrease in the efficiency of trafficking/targeting to the plasma membrane and/or reduced functionality of membrane-bound, which resulted in impaired transporter activity. After additional mutations, the transporter activities of some alanine-substitution mutants recovered. Specifically, the P95A mutant decreased EAAT2-associated anion currents. The Michaelis constant (Km ) values of the mutant proteins L85A, L92A and L101A were increased significantly, whereas R87 and P95A were decreased significantly, indicating that the mutations L85A, L92A and L101A reduced the affinity of the transporter and the substrate, whereas R87A and P95A enhanced this affinity. The maximum velocity (Vmax) values of all 14 alanine mutant proteins were decreased significantly, indicating that all these mutations reduced the substrate transport rate. These results suggest that critical residues in TM2 affect not only the protein expression and membrane-bound localization of EAAT2, but also its interactions with substrates. Additionally, our findings elucidate that the P95A mutant decreased EAAT2-related anion currents. Our results indicate that the TM2 of EAAT2 plays a vital role in the transport process. The key residues in TM2 affect protein expression in the membrane, substrate transport and the anion currents of EAAT2.

Chemical Targeting of Membrane Transporters: Insights into Structure/Function Relationships

Chemical modification of proteins is a vintage strategy that is still fashionable due to the information that can be obtained from this approach. An interesting application of chemical modification is linked with membrane transporters. These proteins have peculiar features such as the presence of hydrophobic and hydrophilic domains, which show different degree of accessibility to chemicals. The presence of reactive residues in the membrane transporters is at the basis of the chemical targeting strategy devoted to investigating structure/function relationships; in particular, information on the substrate binding site, regulatory domains, dimerization domains, and the interface between hydrophilic loops and transmembrane domains has been obtained over the years by chemical targeting. Given the difficulty in handling membrane transporters, their study experienced a great delay, particularly concerning structural information. Chemical targeting has been applied with reasonable success to some membrane transporters belonging to the families SLC1, SLC6, SLC7, and SLC22. Furthermore, some data on the potential application of chemical targeting in pharmacology are also discussed.

The 4b-4c loop of excitatory amino acid transporter 1 containing four critical residues essential for substrate transport

In the mammalians, the 4b-4c loop of excitatory amino acid transporters (EAATs) spans more than 50 amino-acid residues that are absent in glutamate transporter homologue of Pyrococcus horikoshii (Glt_Ph). This part of insertion is unique for metazoans and indispensable to the localization of EAATs. The excitatory amino acid transporter (EAAT) 1 is one of the two glial glutamate transporters, which are responsible for efficiently clearing glutamate from the synaptic cleft to prevent neurotoxicity and cell death. Although the crystal structure of EAAT1_cryst (a human thermostable EAAT1) was resolved in 2017, the structure-function relationship of the 4b-4c loop has not been elucidated in EAAT1_cryst. To investigate the role of the 4b-4c loop, we performed alanine-scanning mutagenesis in the mutants and observed dramatically decreased transport activities in T192A, Y194A, N242A, and G245A mutants. The surface expression of T192A and Y194A mutants even decreased by more than 80%, and most of them were detained in the cytoplasm. However, when T192 and Y194 were substituted with conservative residues, the transport activities and the surface expressions of T192S and Y194F were largely recovered, and their kinetic parameters (K_m values) were comparable to the wild-type EAAT1 as well. In contrast, N242 and G245 substituted with conservative residues could not rescue the uptake function, suggesting that N242 and G245 may play irreplaceable roles in the glutamate uptake process. These results indicate that the 4b-4c loop of EAAT1 may not only affect the glutamate uptake activity, but also influence the surface localization of EAAT1 by T192 and Y194.Communicated by Ramaswamy H. Sarma.

Paired-Cysteine Scanning Reveals Conformationally Sensitive Proximity between the TM4b-4c Loop and TM8 of the Glutamate Transporter EAAT1

Excitatory amino acid transporters (EAATs) take up the neurotransmitter glutamate from the synaptic cleft and maintain glutamate concentrations below neurotoxic levels. Recently, the crystal structures of thermostable EAAT1 variants have been reported; however, little is understood regarding the functional mechanism of the transmembrane domain (TM) 4b-4c loop, which contains more than 50 amino acids in mammalian EAATs that are absent in prokaryotic homologues. To explore the spatial position and function of TM4 during the transport cycle, we introduced pairwise cysteine substitutions between the TM4b-4c loop and TM8 in a cysteine-less version of EAAT1, CL-EAAT1. We observed pronounced inhibition of transport by Cu(II)(1,10-phenanthroline)₃ (CuPh) for doubly substituted V238C/I469C and A243C/I469C variants, but not for corresponding singly substituted CL-EAAT1 or for more than 20 other double-cysteine variants. Dithiothreitol treatment partially restored the uptake activity of the CuPh-treated V238C/I469C and A243C/I469C doubly substituted variants, confirming that the effects of CuPh on these variants were due to the formation of intramolecular disulfide bonds. Glutamate, KCl, and d,l-threo-β-benzyloxy-aspartate weakened CuPh inhibition of the V238C/I469C variant, but only KCl weakened CuPh inhibition of the V243C/I469C variant, suggesting that the TM4b-4c loop and TM8 are separated from each other in the inward-facing conformations of EAAT1. Our results suggest that the TM4b-4c loop and TM8 are positioned in close proximity during the transport cycle and are less closely spaced in the inward-facing conformation.

Substrate-Induced Motion between TM4 and TM7 of the Glutamate Transporter EAAT1 Revealed by Paired Cysteine Mutagenesis

To maintain efficient synaptic communication, glutamate transporters reuptake glutamate from the synaptic cleft and prevent glutamate concentrations from reaching neurotoxic levels. The number of amino acid residues of the transmembrane (TM) domain 4b-4c loop of mammalian excitatory amino acid transporters (EAATs) is 50 amino acids more than that of the prokaryotic homolog. To investigate the spatial proximity and functional significance of residues in glutamate transporters, cysteine pairs were introduced at positions A243 of the TM4b-4c loop and T396 or A414 of TM7, respectively. The transport activity of double mutants A243C/T396C and A243C/A414C was inhibited by Cu(II) (1,10-phenanthroline)₃ [copper phenanthroline (CuPh)] and cadmium ions, but the uptake activity of corresponding single mutants remained unchanged. Treatment with dithiothreitol after CuPh restored much of the transport activity. The inhibitory effects of CuPh and cadmium could only be detected when cysteine pairs are in the same polypeptide. Therefore, we suggest that the formation of these disulfide bonds occurs intramolecularly. Glutamate, potassium, and DL-threo-β-benzyloxyaspartate facilitated crosslinking in the A243C/T396C transporter and this suggests that the TM4b-4c loop and β-bridge region in TM7 were drawn into close proximity to each other in the inward- and outward-facing conformation of EAAT1. Thus, these data provide evidence that substrate-induced structural rearrangements occur between the TM4b-4c loop and TM7 during the transport cycle.