Vesicular glutamate transporters (VGLUTs) fill synaptic vesicles with glutamate andremove luminal Cl- via an additional anion channel mode. Both of these transportfunctions are stimulated by luminal acidification, luminal-positive membrane potential,and luminal Cl-. We studied VGLUT1 transporter/channel activation using acombination of heterologous expression, cellular electrophysiology, fast solutionexchange, and mathematical modeling. Cl- channel gating can be described with akinetic scheme that includes two protonation sites and distinct opening, closing, andCl--binding rates for each protonation state. Cl- binding promotes channel openingby modifying the pKa values of the protonation sites and rates of pore opening andclosure. VGLUT1 transports glutamate and aspartate at distinct stoichiometries:H+-glutamate exchange at 1:1 stoichiometry and aspartate uniport. Neurotransmittertransport with variable stoichiometry can be described with an alternating accessmodel that assumes that transporters without substrate translocate in the doubly protonatedstate to the inward-facing conformation and return with the bound amino acidsubstrate as either singly or doubly protonated. Glutamate, but not aspartate, promotesthe release of one proton from inward-facing VGLUT1, resulting in preferentialH+-coupled glutamate exchange. Cl- stimulates glutamate transport by making theglutamate-binding site accessible to cytoplasmic glutamate and by facilitating transitionsto the inward-facing conformation after outward substrate release. We concludethat allosteric modification of transporter protonation by Cl- is crucial for both VGLUT1transport functions.
Borghans et al. (Wed,) studied this question.