(a) moving barrier, rocker switch, exemplified by the fructose transporter GLUT5 with two protein domains (blue shades) rotating around substrate-binding site (orange circle) changing the barrier position (red bars) (PDB IDs for outward and inward states: 4YBQ and 4YB9).(b) moving barrier, rocking bundle, exemplified by the leucine transporter LeuT with transport domain (blue) moving relative to the scaffold domain (yellow). The substrate-binding site does not change its position relative to the membrane plane during the transition from outward to the inward state, but the barrier (red bar) does change (PDB IDs: 3TT1 and 3TT3). (c) the mitochondrial ADP/ATP carrier represents the moving-barrier, diaphragm mechanism, where three protein domains (blue shades) rotate around substrate-binding site changing the barrier position, indicated by the red bars (PDB IDs: 6GCI and 4C9H).