| Neurotransmitter:sodium symporter (NSS) proteins, the targets of antidepressants and psychostimulants, clear neurotransmitters from the synaptic cleft in a Na+-coupled transport mechanism. Transport is thought to occur via conformational rearrangements that alternately expose the substrate-binding site to each side of the membrane, but little is known about the mechanism by which ligand binding coordinates motions at the two faces. In this dissertation, single-molecule fluorescence resonance energy transfer (smFRET) techniques are used to image the dynamics of the prokaryotic NSS LeuT with sufficient resolution to describe the conformational states at both the intra- and extracellular faces for the first time. We found that the two sides do not move as a rigid body, contrary to popular models, and that previously undetected intermediate states are associated with transport activity. We also describe how ions and substrates influence conformational dynamics to create a productive transport cycle. |
