Substrate recognition and allosteric regulation of synaptic vesicle glutamate transporter VGLUT2

The concentration of neurotransmitters inside synaptic vesicles (SVs) underlies the quantal nature of synaptic transmission. Uptake of glutamate, the principal excitatory neurotransmitter, 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. To elucidate the mechanism, we determined high-resolution structures of rat VGLUT2 with a cyclic analog of glutamate. We propose a mechanism of substrate recognition in which a positively charged cytoplasmic vestibule electrostatically attracts the negatively charged substrate. We also identify modification of VGLUT2 by palmitoylation and find that this promotes retrieval of the transporter after exocytosis. The structure also reveals an extensive network of electrostatic interactions that forms the cytoplasmic gate. Functional analysis of a mutant that disrupts the network shows how this cytoplasmic gate confers the allosteric requirement for lumenal H⁺ required to restrict VGLUT activity to SVs.