P-Glt _Ph at equilibrium does not have mobility in extracellular helices. Transport domains were superimposed on HP1 and TM7a (residues 258–309). Superimposition of transport domains from Data S2. OFS_out (D390 down) is purple, OFS_out (D390 up) wheat, OFS_mid green, and OFS_in blue. The views are from the intracellular (left) and extracellular (right) sides of the transport domain.