K55C/C321A/A364C mutation was introduced within a heptahistidine mutant of GltPh, used in earlier crystallographic studies8 (link),9 (link), to which we refer as “wild type” for brevity. Purified protein was cross-linked in the presence of 10 fold molar excess of HgCl2, dialyzed against buffer containing 10 mM HEPES/NaOH, 7 mM n-decyl-β-D-maltopyranoside, 100 mM NaCl and 100 μM L-asp, diluted to the final concentration of 2–4 mg/ml and supplemented with 0.5 mM E. coli total polar lipid extract and 100 mM NaBr. Protein solution was mixed at 1:1 (v:v) ratio with the reservoir solution, containing 100 mM MES, pH 5.0, 18–20 % PEG 350 MME and 200 mM CaCl2, and crystallized at 4 °C by hanging drop vapour diffusion. Crystals were cryoprotected by allowing the drop to dry until its volume was reduced by 50 %. Selenomethionine-substituted protein was expressed as described previously8 (link) and crystallized as above. Diffraction data were indexed, integrated and scaled using HKL-2000 package41 . Further analyses were performed using CCP4 programs42 (link). Initial phases were obtained using Phaser43 (link), and the protein model built manually in Coot44 and refined using REFMAC42 (link) with TLS45 (link) and three fold NCS restrains.