Abstract

Na⁺/Cl^--dependent transporters terminate synaptic transmission by using electrochemical gradients to drive the uptake of neurotransmitters, including the biogenic amines, from the synapse to the cytoplasm of neurons and glia. These transporters are the targets of therapeutic and illicit compounds, and their dysfunction has been implicated in multiple diseases of the nervous system. Here we present the crystal structure of a bacterial homologue of these transporters from Aquifex aeolicus, in complex with its substrate, leucine, and two sodium ions. The protein core consists of the first ten of twelve transmembrane segments, with segments 1−5 related to 6−10 by a pseudo-two-fold axis in the membrane plane. Leucine and the sodium ions are bound within the protein core, halfway across the membrane bilayer, in an occluded site devoid of water. The leucine and ion binding sites are defined by partially unwound transmembrane helices, with main-chain atoms and helix dipoles having key roles in substrate and ion binding. The structure reveals the architecture of this important class of transporter, illuminates the determinants of substrate binding and ion selectivity, and defines the external and internal gates.

1. Department of Biochemistry and Molecular Biophysics and
2. Howard Hughes Medical Institute, Columbia University, 650 West 168th Street, New York, New York 10032, USA

Correspondence to: Eric Gouaux^1,2 Correspondence and requests for materials should be addressed to E.G. (Email: jeg52@columbia.edu). The coordinates for the structure have been deposited in the Protein Data Bank under the accession code 2A65.

Received 23 May 2005; Accepted 4 July 2005

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