Substrate and Cation Binding Mechanism of Glutamate Transporter Homologs

Glutamate transporters and their homologs are membrane proteins that transport glutamate and aspartate together with sodium ions and/or protons. Human glutamate transporters remove the neurotransmitter glutamate after signal transmission. Therefore, glutamate transporters play a great role in processing signals from our environment. The same type of transporters can be found in prokaryotes (Bacteria and Archaea), where they enable the uptake of glutamate and aspartate as nutrients. This thesis describes substrate and cation binding in the prokaryotic glutamate transporter homologs GltPh from Pyroccoccus horikoshii and GltTk from Thermococcus kodakarensis. Using biochemical methods we show that sodium and substrate binding are strongly interlinked, and sodium binding has to precede substrate binding. Measurements on GltPh reveal that sodium binding is slow and involves quite large structural rearrangements. Differently, substrate binding is fast with only minor structural rearrangements. The biochemical characterization of GltTk shows that this transporter is, similar to GltPh, a sodium dependent aspartate transporter that binds one aspartate together with three sodium ions. Even though this transporter shows a slightly different substrate range, affinity, and transport speed, both transporters share many structural and functional properties. The structures of GltTk in unloaded and fully substrate and sodium ion loaded state show that substrate and all sodium ions bind in great proximity, while none of the binding sites is present in the empty transporter. The structures together with the functional data show how sodium and substrate binding influence each other, and give new insights into the transport mechanism of the glutamate transporter family.