Figure S1.

P-Glt_Ph (S279E/D405N) has partial proton dependence. (a) Structural model of substrate-bound Glt_Ph (PDB accession no. 2NWX). (b) Sequence alignment of Na⁺-coupled (Glt_Ph and Glt_Tk) and H⁺-coupled (GltP_Ec, GltT_Bc, and DctA_Bs) transporters; structural elements are indicated below the alignment. Residues mutated in P-Glt_Ph are in bold. (c) pH-dependent aspartate uptake of P-Glt_Ph. Proteoliposomes were loaded with 50 mM potassium phosphate buffer, pH 7, and 100 mM potassium acetate and diluted into the following 50 mM buffers containing 1 μM [³H]L-Asp: MES/NMDG, pH 6 (triangles); HEPES/Tris, pH 7 (diamonds); or HEPES/Tris, pH 8 (squares). Buffers contained either 100 mM KCl (filled symbols) or 100 mM NaCl (empty symbols). Solid lines are shown to guide the eye, and error bars (SD) not displayed represent errors smaller than the size of the symbol.