Structural conservation of an arginine residue at the interface of the transport domain and trimerization domain. (A) Cartoon representation of a locked R276S/M395R Glt_Ph protomer in the plane of the membrane in the outward-facing occluded state (PDB accession no. 4X2S). The trimerization domain (TM1, TM2, TM4, and TM5) is colored in light brown, and the transport domain (TM3, TM6, TM7, TM8, HP1, and HP2) is colored in light blue, with HP1 and HP2 highlighted in yellow and red, respectively. In this mutant Glt_Ph structure, the arginine has been moved from the native position of 276 to 395 to reflect the position of this positively charged residue in the EAATs. Residues 276 and 395 are shown in green stick representation and numbered according to Glt_Ph positioning, with EAAT1 numbering provided in parentheses. Na⁺ ions are represented as purple spheres. Aspartate is not present in this structure. The figure was made using the software PyMOL (The PyMOL Molecular Graphics System, version 1.3r1; Schrödinger, LLC). (B) Amino acid alignment of HP1 and TM8 of the SLC1 family. R276/S363 and M395/R477 are outlined in red.