Engineering the glutamate transporter homologue GltPh using protein semisynthesis

Distinct roles of the Na+ binding sites in the allosteric coupling mechanism of the glutamate transporter homolog, GltPh

SignificanceGlutamate transporters harness ionic gradients present across the membrane for the rapid removal of glutamate from the synaptic space. Normal function of glutamate transporters is required for efficient synaptic transmission and preventing excitotoxicity. Central to the transport mechanism is the coupled binding of Na⁺ and the substrate. While structural studies have identified the Na⁺ and the substrate binding sites, the mechanism by which Na⁺ and substrate binding is coupled is not known. In this study, we developed assays to monitor Na⁺ binding and to track key conformational changes in Glt_Ph, an archaeal homolog of glutamate transporters. We use these assays along with previously developed assays to describe the specific roles of the Na⁺ sites in the coupling mechanism.

Efflux Transporters' Engineering and Their Application in Microbial Production of Heterologous Metabolites

Metabolic engineering of microbial hosts for the production of heterologous metabolites and biochemicals is an enabling technology to generate meaningful quantities of desired products that may be otherwise difficult to produce by traditional means. Heterologous metabolite production can be restricted by the accumulation of toxic products within the cell. Efflux transport proteins (transporters) provide a potential solution to facilitate the export of these products, mitigate toxic effects, and enhance production. Recent investigations using knockout lines, heterologous expression, and expression profiling of transporters have revealed candidates that can enhance the export of heterologous metabolites from microbial cell systems. Transporter engineering efforts have revealed that some exhibit flexible substrate specificity and may have broader application potentials. In this Review, the major superfamilies of efflux transporters, their mechanistic modes of action, selection of appropriate efflux transporters for desired compounds, and potential transporter engineering strategies are described for potential applications in enhancing engineered microbial metabolite production. Future studies in substrate recognition, heterologous expression, and combinatorial engineering of efflux transporters will assist efforts to enhance heterologous metabolite production in microbial hosts.

Na+-dependent gate dynamics and electrostatic attraction ensure substrate coupling in glutamate transporters

Excitatory amino acid transporters (EAATs) harness [Na⁺], [K⁺], and [H⁺] gradients for fast and efficient glutamate removal from the synaptic cleft. Since each glutamate is cotransported with three Na⁺ ions, [Na⁺] gradients are the predominant driving force for glutamate uptake. We combined all-atom molecular dynamics simulations, fluorescence spectroscopy, and x-ray crystallography to study Na⁺:substrate coupling in the EAAT homolog Glt_Ph A lipidic cubic phase x-ray crystal structure of wild-type, Na⁺-only bound Glt_Ph at 2.5-Å resolution revealed the fully open, outward-facing state primed for subsequent substrate binding. Simulations and kinetic experiments established that only the binding of two Na⁺ ions to the Na1 and Na3 sites ensures complete HP2 gate opening via a conformational selection-like mechanism and enables high-affinity substrate binding via electrostatic attraction. The combination of Na⁺-stabilized gate opening and electrostatic coupling of aspartate to Na⁺ binding provides a constant Na⁺:substrate transport stoichiometry over a broad range of neurotransmitter concentrations.

Chemoenzymatic Semisynthesis of Proteins

Protein semisynthesis-defined herein as the assembly of a protein from a combination of synthetic and recombinant fragments-is a burgeoning field of chemical biology that has impacted many areas in the life sciences. In this review, we provide a comprehensive survey of this area. We begin by discussing the various chemical and enzymatic methods now available for the manufacture of custom proteins containing noncoded elements. This section begins with a discussion of methods that are more chemical in origin and ends with those that employ biocatalysts. We also illustrate the commonalities that exist between these seemingly disparate methods and show how this is allowing for the development of integrated chemoenzymatic methods. This methodology discussion provides the technical foundation for the second part of the review where we cover the great many biological problems that have now been addressed using these tools. Finally, we end the piece with a short discussion on the frontiers of the field and the opportunities available for the future.

A facile approach for the in vitro assembly of multimeric membrane transport proteins

Membrane proteins such as ion channels and transporters are frequently homomeric. The homomeric nature raises important questions regarding coupling between subunits and complicates the application of techniques such as FRET or DEER spectroscopy. These challenges can be overcome if the subunits of a homomeric protein can be independently modified for functional or spectroscopic studies. Here, we describe a general approach for in vitro assembly that can be used for the generation of heteromeric variants of homomeric membrane proteins. We establish the approach using Glt_Ph, a glutamate transporter homolog that is trimeric in the native state. We use heteromeric Glt_Ph transporters to directly demonstrate the lack of coupling in substrate binding and demonstrate how heteromeric transporters considerably simplify the application of DEER spectroscopy. Further, we demonstrate the general applicability of this approach by carrying out the in vitro assembly of VcINDY, a Na⁺-coupled succinate transporter and CLC-ec1, a Cl^-/H⁺ antiporter.

Structural characterisation reveals insights into substrate recognition by the glutamine transporter ASCT2/SLC1A5

Cancer cells undergo a shift in metabolism where they become reliant on nutrients such as the amino-acid glutamine. Glutamine enters the cell via the alanine/serine/cysteine transporter 2 (ASCT2) that is upregulated in several cancers to maintain an increased supply of this nutrient and are therefore an attractive target in cancer therapeutic development. ASCT2 belongs to the glutamate transporter (SLC1A) family but is the only transporter in this family able to transport glutamine. The structural basis for glutamine selectivity of ASCT2 is unknown. Here, we identify two amino-acid residues in the substrate-binding site that are responsible for conferring glutamine selectivity. We introduce corresponding mutations into a prokaryotic homologue of ASCT2 and solve four crystal structures, which reveal the structural basis for neutral amino acid and inhibitor binding in this family. This structural model of ASCT2 may provide a basis for future development of selective ASCT2 inhibitors to treat glutamine-dependent cancers.

Cancer cells are reliant on nutrients such as glutamine, which enter the cell via the alanine/serine/cysteine transporter 2 (ASCT2). Here, authors use crystallography to show which amino-acid residues in the substrate-binding site are responsible for conferring glutamine selectivity to ASCT2.

Combining in Vitro Folding with Cell Free Protein Synthesis for Membrane Protein Expression

Cell free protein synthesis (CFPS) has emerged as a promising methodology for protein expression. While polypeptide production is very reliable and efficient using CFPS, the correct cotranslational folding of membrane proteins during CFPS is still a challenge. In this contribution, we describe a two-step protocol in which the integral membrane protein is initially expressed by CFPS as a precipitate followed by an in vitro folding procedure using lipid vesicles for converting the protein precipitate to the correctly folded protein. We demonstrate the feasibility of using this approach for the K(+) channels KcsA and MVP and the amino acid transporter LeuT. We determine the crystal structure of the KcsA channel obtained by CFPS and in vitro folding to show the structural similarity to the cellular expressed KcsA channel and to establish the feasibility of using this two-step approach for membrane protein production for structural studies. Our studies show that the correct folding of these membrane proteins with complex topologies can take place in vitro without the involvement of the cellular machinery for membrane protein biogenesis. This indicates that the folding instructions for these complex membrane proteins are contained entirely within the protein sequence.

The Hill analysis and co-ion-driven transporter kinetics

Interaction of multiple ligands with a protein or protein complex is a widespread phenomenon that allows for cooperativity. Here, we review the use of the Hill equation, which is commonly used to analyze binding or kinetic data, to analyze the kinetics of ion-coupled transporters and show how the mechanism of transport affects the Hill coefficient. Importantly, the Hill analysis of ion-coupled transporters can provide the exact number of transported co-ions, regardless of the extent of the cooperativity in ion binding.

Low Affinity and Slow Na+ Binding Precedes High Affinity Aspartate Binding in the Secondary-active Transporter GltPh

GltPh from Pyrococcus horikoshii is a homotrimeric Na(+)-coupled aspartate transporter. It belongs to the widespread family of glutamate transporters, which also includes the mammalian excitatory amino acid transporters that take up the neurotransmitter glutamate. Each protomer in GltPh consists of a trimerization domain involved in subunit interactions and a transport domain containing the substrate binding site. Here, we have studied the dynamics of Na(+) and aspartate binding to GltPh. Tryptophan fluorescence measurements on the fully active single tryptophan mutant F273W revealed that Na(+) binds with low affinity to the apoprotein (Kd 120 mm), with a particularly low kon value (5.1 m(-1)s(-1)). At least two sodium ions bind before aspartate. The binding of Na(+) requires a very high activation energy (Ea 106.8 kJ mol(-1)) and consequently has a large Q10 value of 4.5, indicative of substantial conformational changes before or after the initial binding event. The apparent affinity for aspartate binding depended on the Na(+) concentration present. Binding of aspartate was not observed in the absence of Na(+), whereas in the presence of high Na(+) concentrations (above the Kd for Na(+)) the dissociation constants for aspartate were in the nanomolar range, and the aspartate binding was fast (kon of 1.4 × 10(5) m(-1)s(-1)), with low Ea and Q10 values (42.6 kJ mol(-1) and 1.8, respectively). We conclude that Na(+) binding is most likely the rate-limiting step for substrate binding.