Binding and transport of D-aspartate by the glutamate transporter homolog GltTk

Mammalian glutamate transporters are crucial players in neuronal communication as they perform neurotransmitter reuptake from the synaptic cleft. Besides L-glutamate and L-aspartate, they also recognize D-aspartate, which might participate in mammalian neurotransmission and/or neuromodulation. Much of the mechanistic insight in glutamate transport comes from studies of the archeal homologs GltPh from Pyrococcus horikoshii and GltTk from Thermococcus kodakarensis. Here, we show that GltTk transports D-aspartate with identical Na+: substrate coupling stoichiometry as L-aspartate, and that the affinities (Kd and Km) for the two substrates are similar. We determined a crystal structure of GltTk with bound D-aspartate at 2.8 Å resolution. Comparison of the L- and D-aspartate bound GltTk structures revealed that D-aspartate is accommodated with only minor rearrangements in the structure of the binding site. The structure explains how the geometrically different molecules L- and D-aspartate are recognized and transported by the protein in the same way.