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Time-resolved mechanism of extracellular gate opening and substrate binding in a glutamate transporter.

ABSTRACT: Glutamate transporters, also referred to as excitatory amino acid transporters (EAATs), are membrane proteins that regulate glutamatergic signal transmission by clearing excess glutamate after its release at synapses. A structure-based understanding of their molecular mechanisms of function has been elusive until the recent determination of the x-ray structure of an archaeal transporter, Glt(Ph). Glt(Ph) exists as a trimer, with each subunit containing a core region that mediates substrate translocation. In the present study a series of molecular dynamics simulations have been conducted and analyzed in light of new experimental data on substrate binding properties of EAATs. The simulations provide for the first time a full atomic description of the time-resolved events that drive the recognition and binding of substrate. The core region of each subunit exhibits an intrinsic tendency to open the helical hairpin HP2 loop, the extracellular gate, within tens of nanoseconds exposing conserved polar residues that serve as attractors for substrate binding. The NMDGT motif on the partially unwound part of the transmembrane helix TM7 and the residues Asp-390 and Asp-394 on TM8 are also distinguished by their important role in substrate binding and close interaction with mediating water molecules and/or sodium ions. The simulations reveal a Na+ binding site comprised in part of Leu-303 on TM7 and Asp-405 on TM8 and support a role for sodium ions in stabilizing substrate-bound conformers. The functional importance of Leu-303 or its counterpart Leu-391 in human EAAT1 (hEAAT1) is confirmed by site-directed mutagenesis and Na+ dependence assays conducted with hEAAT1 mutants L391C and L391A.

SUBMITTER: Shrivastava IH

PROVIDER: S-EPMC2568915 | biostudies-literature | 2008 Oct

REPOSITORIES: biostudies-literature

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Time-resolved mechanism of extracellular gate opening and substrate binding in a glutamate transporter.

Shrivastava Indira H IH Jiang Jie J Amara Susan G SG Bahar Ivet I

The Journal of biological chemistry 20080804 42

Glutamate transporters, also referred to as excitatory amino acid transporters (EAATs), are membrane proteins that regulate glutamatergic signal transmission by clearing excess glutamate after its release at synapses. A structure-based understanding of their molecular mechanisms of function has been elusive until the recent determination of the x-ray structure of an archaeal transporter, Glt(Ph). Glt(Ph) exists as a trimer, with each subunit containing a core region that mediates substrate trans ...[more]

PMID: 18678877

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Project description:20110318_GluC: Cell culture supernatants from Balb/c 3T3 mouse fibroblasts that had been incubated with the endoproteinase GluC (Staphylococcus aureus protease V8) as test protease for 1, 2, 4, 8, and 16 hours or buffer alone (0, 12, and 16 hours controls) were subjected to 8plex-iTRAQ-TAILS analysis. Labeling scheme: 113: 0h control, 114: 1h, 115: 2h, 116: 4h, 117: 8h, 118: 12h control, 119: 16h, 121: 16h control. 20130214_MMP10: Cell culture supernatants from matrix metalloproteinase (MMP) 10 deficient murine embryonic fibroblasts that had been incubated with recombinant human MMP10 as test protease for 1, 2, 4, 8, and 16 hours or buffer alone (0 and 16 hours controls) were subjected to 8plex-iTRAQ-TAILS analysis. Labeling scheme: 113: 0h control, 114: 1h, 115: 2h, 116: 4h, 117: 8h, 118: 12h, 119: 16h, 121: 16h control.MS data analysis: Mascot Distiller (Matrix Science) was used to extract peak lists from raw files and for merging of corresponding CID/HCD spectra pairs. Peak lists (mgf) were searched by Mascot v2.3 search engine against a mouse UniProtKB database (release 2012_03; 54232 entries), to which reversed decoy sequences as well as sequences for common contaminants and either for GluC or for human MMP10, respectively, had been added and with following parameters: semi-Arg-C for enzyme specificity allowing up to 2 missed cleavages; carbamidomethyl(C), iTRAQ(K) as fixed modifications; acetyl (N-term), iTRAQ(N-term), oxidation(M), and deamidation(NQ) as variable modifications; parent mass error at 10 ppm, fragment mass error at 0.8 Da. For GluC experiments an additional search was performed using the same parameters but with semi-GluC as enzyme and allowing up to 5 missed cleavages. The Trans-Proteomic Pipeline (TPP v4.6, rev 1, Build 201212051643)46 was used to secondary validate Mascot search results and to compile a single peptide list from all peptide fractions obtained from both the pre-pullout and the pullout samples. First, data were processed by PeptideProphet setting the ‘minimum peptide length’ to 1, using ‘accurate mass binning’ and omitting the ‘NTT model’. Next, iProphet was employed for additional validation and for combining PeptideProphet results from multiple searches, and only peptides with an iProphet probability of >=0.9 (corresponding to false discovery rate (decoy) of <1%) were included in subsequent analyses. For relative quantification iTRAQ reporter ion intensities were extracted from mgf files using a modified version of i-Tracker47 with a mass tolerance of 0.1 Da and purity corrections supplied by the iTRAQ manufacturer and assigned to filtered peptides.

Dataset Information

Time-resolved mechanism of extracellular gate opening and substrate binding in a glutamate transporter.

Publications

Time-resolved mechanism of extracellular gate opening and substrate binding in a glutamate transporter.

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