The uncoupled chloride conductance of a bacterial glutamate transporter homolog

Glutamate transporters (EAATs) are pivotal in mammalian synaptic transmission, tightly regulating synaptic levels of this excitatory neurotransmitter. In addition to coupled glutamate transport, the EAATs also show an uncoupled Cl- conductance, whose physiological importance has recently been demonstrated. Little is yet known about the molecular mechanism of chloride permeation. Here we show that Glt(Ph), a bacterial EAAT homolog whose structure has been determined, displays an uncoupled Cl- conductance that can determine the rate of substrate uptake. A mutation analogous to one known to specifically affect Cl movement in EAAT1 has similar effects on GltPh, suggesting that this protein is an excellent structural model for understanding Cl- permeation through the EAATs. We also observed an uncoupled Cl- conductance in another bacterial EAAT homolog but not in a homolog of the Na+/Cl--coupled neurotransmitter transporters.