Transport dependent on context
Transporter proteins move substrates across a membrane, often coupling this activity to cellular ion concentration gradients. For neurotransmitter transporters, which reside in synaptic vesicles that fuse with the plasma membrane after an action potential, transport activity needs to be regulated so that they do not pump out neurotransmitters after vesicle fusion. Using cryo–electron microscopy, Li et al. determined the structure of a vesicular glutamate transporter from rat that unveils some of the distinctive features that enable it to function properly in two distinct cellular environments. An allosteric pH sensor, proposed to be a glutamate residue, gates binding of the substrate glutamate and simultaneously permits binding and counterflow of chloride ions. This molecular traffic light allows for a single ion channel to behave appropriately in different contexts.
Science, this issue p. 893
Synaptic vesicles accumulate neurotransmitters, enabling the quantal release by exocytosis that underlies synaptic transmission. Specific neurotransmitter transporters are responsible for this activity and therefore are essential for brain function. The vesicular glutamate transporters (VGLUTs) concentrate the principal excitatory neurotransmitter glutamate into synaptic vesicles, driven by membrane potential. However, the mechanism by which they do so remains poorly understood owing to a lack of structural information. We report the cryo–electron microscopy structure of rat VGLUT2 at 3.8-angstrom resolution and propose structure-based mechanisms for substrate recognition and allosteric activation by low pH and chloride. A potential permeation pathway for chloride intersects with the glutamate binding site. These results demonstrate how the activity of VGLUTs can be coordinated with large shifts in proton and chloride concentrations during the synaptic vesicle cycle to ensure normal synaptic transmission.