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Engineering the glutamate transporter homologue GltPh using protein semisynthesis.

ABSTRACT: Glutamate transporters catalyze the concentrative uptake of glutamate from synapses and are essential for normal synaptic function. Despite extensive investigations of glutamate transporters, the mechanisms underlying substrate recognition, ion selectivity, and the coupling of substrate and ion transport are not well-understood. Deciphering these mechanisms requires the ability to precisely engineer the transporter. In this study, we describe the semisynthesis of GltPh, an archaeal homologue of glutamate transporters. Semisynthesis allows the precise engineering of GltPh through the incorporation of unnatural amino acids and peptide backbone modifications. In the semisynthesis, the GltPh polypeptide is initially assembled from a recombinantly expressed thioester peptide and a chemically synthesized peptide using the native chemical ligation reaction followed by in vitro folding to the native state. We have developed a robust procedure for the in vitro folding of GltPh. Biochemical characterization of the semisynthetic GltPh indicates that it is similar to the native transporter. We used semisynthesis to substitute Arg397, a highly conserved residue in the substrate binding site, with the unnatural analogue, citrulline. Our studies demonstrate that Arg397 is required for high-affinity substrate binding, and on the basis of our results, we propose that Arg397 is involved in a Na+-dependent remodeling of the substrate binding site required for high-affinity Asp binding. We anticipate that the semisynthetic approach developed in this study will be extremely useful in investigating functional mechanisms in GltPh. Further, the approach developed in this study should also be applicable to other membrane transport proteins.

SUBMITTER: Focke PJ

PROVIDER: S-EPMC4511477 | biostudies-literature | 2015 Mar

REPOSITORIES: biostudies-literature

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Engineering the glutamate transporter homologue GltPh using protein semisynthesis.

Focke Paul J PJ Annen Alvin W AW Valiyaveetil Francis I FI

Biochemistry 20150217 8

Glutamate transporters catalyze the concentrative uptake of glutamate from synapses and are essential for normal synaptic function. Despite extensive investigations of glutamate transporters, the mechanisms underlying substrate recognition, ion selectivity, and the coupling of substrate and ion transport are not well-understood. Deciphering these mechanisms requires the ability to precisely engineer the transporter. In this study, we describe the semisynthesis of GltPh, an archaeal homologue of ...[more]

PMID: 25649707

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Project description:Excitatory amino acid transporters (EAATs) are membrane proteins responsible for reuptake of glutamate from the synaptic cleft to terminate neurotransmission and help prevent neurotoxically high, extracellular glutamate concentrations. Important structural information about these proteins emerged from crystal structures of GltPh, a bacterial homologue of EAATs, in conformations facing outward and inward. These remarkably different conformations are considered to be end points of the substrate translocation path (STP), suggesting that the transport mechanism involves major conformational rearrangements that remain uncharted. To investigate possible steps in the structural transitions of the STP between the two end-point conformations, we applied a combination of computational modeling methods (motion planning, molecular dynamics simulations, and mixed elastic network models). We found that the conformational changes in the transition involve mainly the repositioning the "transport domain" and the "trimerization domain" identified previously in the crystal structures. The two domains move in opposite directions along the membrane normal, and the transport domain also tilts by ?17° with respect to this axis. Moreover, the TM3-4 loop undergoes a flexible, "restraining bar"-like conformational change with respect to the transport domain. As a consequence of these conformational rearrangements along the transition path we calculated a significant decrease of nearly 20% in the area of the transport-to-trimerization domain interface (TTDI). Water penetrates parts of the TTDI in the modeled intermediates but very much less in the end-point conformations. We show that these characteristics of the modeled intermediate states agree with experimental results from residue-accessibility studies in individual monomers and identify specific residues that can be used to test the proposed STP. Moreover, MD simulations of complete GltPh trimers constructed from initially identical monomer intermediates suggest that asymmetry can appear in the trimer, consonant with available experimental data showing independent transport kinetics by individual monomers in the trimers.

Dataset Information

Engineering the glutamate transporter homologue GltPh using protein semisynthesis.

Publications

Engineering the glutamate transporter homologue GltPh using protein semisynthesis.

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