AbstractThe glutamate transporter (GLT1) regulates glutamate concentrations in glutamatergic synapses and it is expressed in at least two isoforms, GLT1a and GLT1b. In this work, we show that the C‐terminus of GLT1b is able to interact with the PDZ domains of a number of proteins. Notably, one of them might be the scaffold protein post‐synaptic density (PSD‐95). GLT1b formed co‐immunoprecipitable complexes with PSD‐95 in solubilizated rat brain extracts, complexes that also contained NMDA receptors. Co‐transfection of GLT1b, PSD‐95, and NMDA receptor subunits in heterologous expression systems recapitulated in vitro the interactions among these proteins that had been observed in the rat brain extracts and revealed the importance of the GLT1b C‐terminal PDZ binding motif in tethering this transporter to PSD‐95. Significantly, co‐expression of GLT1b and PSD‐95 increased the Vmax of the transporter by decreasing the rate of GLT1b endocytosis. Moreover, GLT1b transfected into primary cultured neurons or glia formed protein clusters that co‐localized with co‐transfected PSD‐95, clusters that in these neurons accumulated preferentially in dendritic spines. We hypothesize that the GLT1b/PSD‐95 interaction, characterized here in vitro, might anchor this transporter close to the post‐synaptic glutamate receptors, thereby permitting the fine regulation of glutamate concentrations in this microenvironment. This tight association might also facilitate the regulation of GLT1b through the signaling pathways initiated by the activation of glutamate receptors.