Alternating Access Mechanism of Glutamate Transporters

Abstract

In the central nervous system, glutamate transporters are responsible for the glutamate clearance following rounds of neurotransmission. They are molecular pumps, which utilizes the energy of pre-existing electrochemical gradients of ions to drive substrate uptake against steep concentration gradients. Sodium coupled aspartate transporter from Pyrococcus horikoshii, GltPh, is a homologue of the mammalian transporters and has served as a model system, within which to understand the molecular details of transport. The previously determined crystal structures of GltPh revealed the substrate and sodium binding sites located near the extracellular solution leaving the question of how they reach the cytoplasm unanswered. Recently, we have determined the crystal structure of a double cysteine mutant of GltPh, captured by cross-linking in a novel conformational state. In this state the substrate-binding sites are near the cytoplasmic surface of the protein. These findings suggest a novel and unexpected mechanism, by which GltPh and, by analogy mammalian glutamate transporters catalyze trans-membrane transport of their substrates.