Activating EAAT4 Anion Channels

The excitatory amino acid transporter 4 (EAAT4) is an unusual beast. Expressed predominantly in cerebellum Purkinje cells rather than glia, the molecule also functions as an anion-selective ion channel. The current model for transport is that a molecule of glutamate is transported independently by each subunit in the homotrimeric transporter. The crystal structure of glutamate transporters suggests that the extracellular face is a water-filled bowl with glutamate binding sites sitting at the base. This week, Torres-Salazar et al. provide data suggesting a cooperative interaction of subunits in the generation of EAAT4 anion currents. The authors expressed EAAT4 in Xenopus oocytes and heterologous cells and reported sigmoidal glutamate concentration dependence. Two mutations close to the glutamate binding site altered the dissociation constant and cooperativity of the anion current, unlike glutamate transport itself. Thus, glutamate binding sites on different subunits must be occupied to generate the anion current, indicative of intersubunit interactions.

Delany Torres-Salazar and Christoph Fahlke

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Activating EAAT4 Anion Channels