The dual role of chloride in synaptic vesicle glutamate transport

The transport of glutamate into synaptic vesicles exhibits an unusual form of regulation by Cl^- as well as an associated Cl^- conductance. To distinguish direct effects of Cl^- on the transporter from indirect effects via the driving force Δψ, we used whole endosome recording and report the first currents due to glutamate flux by the vesicular glutamate transporters (VGLUTs). Chloride allosterically activates the VGLUTs from both sides of the membrane, and we find that neutralization of an arginine in transmembrane domain four suffices for the lumenal activation. The dose dependence suggests that Cl^- permeates through a channel and glutamate through a transporter. Competition between the anions nonetheless indicates that they use a similar permeation pathway. By controlling both ionic gradients and Δψ, endosome recording isolates different steps in the process of synaptic vesicle filling, suggesting distinct roles for Cl^- in both allosteric activation and permeation.