The neuronal and epithelial human high affinity glutamate transporter: Insights into structure and mechanism of transport

High affinity transport of glutamate across plasma membranes of brain neurons and epithelia is mediated by a Na⁺- and uncoupled electrogenic transporter. Here we report the primary structure and functional characterization of the human high affinity glutamate transporter (HEAAC1). A unique characteristic of HEAAC1-mediated transport is that the affinity for glutamate and the maximal transport rate are strongly dependent on membrane potential. Our data provide new insights into individual steps of high affinity glutamate transport and show that the transport mechanism is distinct from that of the γ-aminobutyric acid transporter GAT-1 and the Na⁺/glucose transporter SGLT1. Under voltage clamp condition, HEAAC1 mediated large substrate-evoked inward currents (up to 1 μA). The substrate specificity, stereospecificity, the K_m value (30 ± 3 μM at -60 mV) of the L-glutamate-evoked current, and Northern analysis all agree with previously reported characteristics of high affinity glutamate transport in brain. In contrast to SGLT1 and GAT-1, voltage jump studies of HEAAC1 yielded only minor relaxation currents. Classic inhibitors of brain glutamate uptake such as DL-threo-β-hydroxyaspartate, L-trans-pyrrolidine 2,4,-dicarboxylic acid (PDC), and dihydrokainate were found to be either transport substrates or to have no significant effect on glutamate transport. We also found that the maximal transport rate for PDC was markedly reduced compared to that for L-glutamate. We propose that PDC most likely reduces the turnover rate of the transporter. A search of the sequence data bases revealed weak homology of HEAAC1 to the H⁺-coupled vesicular monoamine transporter, suggesting an evolutionary link between plasma membrane and vesicular transporters.