Molecular characterization of substrate-binding sites in the glutamate transporter family.

Glutamate transporters are essential for terminating synaptic excitation and for maintaining extracellular glutamate concentrations below neurotoxic levels. These transporters also mediate a thermodynamically uncoupled chloride flux that is activated by two of the molecules that they transport - sodium and glutamate. Five eukaryotic glutamate transporters have been cloned and identified. They exhibit approximately 50% identity and this homology is even greater in the carboxyl terminal half, which is predicted to have an unusual topology. Determination of the topology shows that the carboxyl terminal part of the molecule contains several transmembrane domains that are separated by at least two re-entrant loops. In these structural elements, we have identified several conserved amino acid residues that play crucial roles in the interaction with the transporter substrates sodium, potassium and glutamate.