Amino acid and peptide transport

To other research topics:

ABC transport
Secondary active transport

ABC Transport (Prof. Dr. B. Poolman, Prof. Dr. D.J. Slotboom)

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Secondary active transport (Prof. Dr. D.J. Slotboom)

Glutamate is the main excitatory neurotransmitter in the brain. It is released into the synaptic cleft where it activates receptors in the post-synaptic membrane. To clear the neurotransmitter from the extracellular fluid after signal transmission, glutamate transporters are present in the plasma membranes of cells surrounding the synaps and catalyze the cellular uptake of glutamate. Glutamate transporters belong to a large superfamily of glutamate/aspartate transport proteins found in both eukaryotes and prokaryotes. We study prokaryotic members, which are involved in uptake of nutrients that serve as carbon and nitrogen sources.

Glutamate transport is stoichiometrically coupled to the transport of cations (sodium/potassium ions and/or protons) across the membrane. Pre-existing gradients of these cations allow accumulation of the amino acid substrate in cells. The type and number of co- or counter-transported cations varies among the members of the family. Mammalian transporters co-transport three sodium ions and a proton with glutamate, followed by translocation of a potassium ion in the opposing direction, which resets the empty transporter for a new cycle of glutamate transport. We recently found that the homologous aspartate transporter Glt_ph from Pyrococcus horikoshii couples aspartate transport to the symport of 3 Na⁺ ions. Glt_ph does not use potassium ions or protons. The bacterial glutamate transporter GltP (Escherichia coli) co-transports glutamate with an unknown number of protons and does not translocate sodium or potassium ions. The structural basis for the differences in cation selectivity in the homologous proteins is unknown and is one of the research topics.

Glt_ph is the only member of the family for which a crystal structure is available. The protein is trimeric (as are the mammalian and bacterial homologues), and the protomers have an unusual membrane topology including helical hairpins, and broken helices. Each of the three subunits contains an independent substrate-binding site located in a compact C-terminal domain. The structural information on Glt_Ph provides a framework to study the mechanism of transport.

Groeneveld M, Weme RG, Duurkens RH, Slotboom DJ. Biochemical characterization of the C4-dicarboxylate transporter DctA from Bacillus subtilis. J Bacteriol. 2010;192(11):2900-7.
Groeneveld M, Slotboom DJ. Na(+):aspartate coupling stoichiometry in the glutamate transporter homologue Glt(Ph). Biochemistry. 2010;49(17):3511-3.
Groeneveld M, Slotboom DJ. Rigidity of the subunit interfaces of the trimeric glutamate transporter GltT during translocation. J Mol Biol. 2007;372(3):565-70.