Allosteric Regulation of a Synaptic Vesicle Glutamate Transporter

Concentration of neurotransmitter inside synaptic vesicles (SVs) underlies the quantal nature of synaptic transmission. In contrast to many transporters, SV uptake of the principal excitatory neurotransmitter glutamate is driven by membrane potential. To prevent nonquantal efflux of glutamate after SV exocytosis, the vesicular glutamate transporters (VGLUTs) are allosterically inhibited by the neutral pH of the synaptic cleft. We have now determined high-resolution structures of VGLUT2 with a cyclic analog of glutamate bound that defines the mechanism of substrate recognition, a positively charged cytoplasmic vestibule that electrostatically attracts the negatively charged substrate, and modification by palmitoylation that promotes retrieval of the transporter after exocytosis. The structure also incorporates an extensive, cytoplasmic network of electrostatic interactions that acts as a gate. Functional analysis shows how this cytoplasmic gate confers the allosteric requirement for lumenal H+ required to restrict VGLUT activity to SVs.