Fig. 5.

Alternating access model of substrate translocation. Transmembrane (TM) helices 1, 3, 6 and 8 that define the substrate translocation pore are displayed as cylinders. The minimum required conformations of MATs during substrate translocation are shown. An outward-facing conformation allows entry of substrate and ions (S and Na+, respectively) into the transporter from the extracellular space. The occluded conformation restricts access from both the extracellular and intracellular environments. Conversion to the inward-facing conformation allows the release of the substrate and ion into the cytoplasm