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J Gen Physiol. 2019 Jul 3. pii: jgp.201912318. doi: 10.1085/jgp.201912318. [Epub ahead of print]

A comparison of the transport kinetics of glycine transporter 1 and glycine transporter 2.

Author information

1
Institute of Pharmacology and the Gaston H. Glock Research Laboratories for Exploratory Drug Development, Center of Physiology and Pharmacology, Medical University of Vienna, Vienna, Austria.
2
Department of Pharmaceutical Chemistry, Faculty of Life Sciences, University of Vienna, Vienna, Austria.
3
Institute of Science and Technology Austria, Klosterneuburg, Austria.
4
Neuroproteomics, Paracelsus Private Medical University, Salzburg, Austria.
5
Institute of Pharmacology and the Gaston H. Glock Research Laboratories for Exploratory Drug Development, Center of Physiology and Pharmacology, Medical University of Vienna, Vienna, Austria walter.sandtner@meduniwien.ac.at.

Abstract

Transporters of the solute carrier 6 (SLC6) family translocate their cognate substrate together with Na+ and Cl- Detailed kinetic models exist for the transporters of GABA (GAT1/SLC6A1) and the monoamines dopamine (DAT/SLC6A3) and serotonin (SERT/SLC6A4). Here, we posited that the transport cycle of individual SLC6 transporters reflects the physiological requirements they operate under. We tested this hypothesis by analyzing the transport cycle of glycine transporter 1 (GlyT1/SLC6A9) and glycine transporter 2 (GlyT2/SLC6A5). GlyT2 is the only SLC6 family member known to translocate glycine, Na+, and Cl- in a 1:3:1 stoichiometry. We analyzed partial reactions in real time by electrophysiological recordings. Contrary to monoamine transporters, both GlyTs were found to have a high transport capacity driven by rapid return of the empty transporter after release of Cl- on the intracellular side. Rapid cycling of both GlyTs was further supported by highly cooperative binding of cosubstrate ions and substrate such that their forward transport mode was maintained even under conditions of elevated intracellular Na+ or Cl- The most important differences in the transport cycle of GlyT1 and GlyT2 arose from the kinetics of charge movement and the resulting voltage-dependent rate-limiting reactions: the kinetics of GlyT1 were governed by transition of the substrate-bound transporter from outward- to inward-facing conformations, whereas the kinetics of GlyT2 were governed by Na+ binding (or a related conformational change). Kinetic modeling showed that the kinetics of GlyT1 are ideally suited for supplying the extracellular glycine levels required for NMDA receptor activation.

PMID:
31270129
DOI:
10.1085/jgp.201912318

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