Figure 10. Proposed transport cycle for GltPh and EAATs.
Ion binding to the Na1 site of the outward-facing apo transport domain triggers isomerization into bound-like conformation, formation of the L-asp and Na2 binding sites and HP2 opening, impeding translocation of the domain. Closure of HP2, coupled to L-asp and Na2 binding, allows translocation. After the release of the ligands into the cytoplasm by as yet an unknown gating mechanism, the domain is in a compact apo state, and returns to the extracellular side. Notably, binding of cations to the inward-facing state does not lead to a crystallographically observed gate opening that would impede translocation. However, Na+ affinity in this state is only ∼250 mM (Reyes et al., 2013), and it will remain largely unbound when facing the cytoplasm. Hence, uncoupled Na+ transport should be limited. In EAATs, an open conformation of the gates might be more favored in the apo state, and K+ binding at the Ct site might be required to stabilize translocation-competent conformation of the apo transport domain.