| Neurotransmitter:sodium symporter (NSS) proteins remove neurotransmitters released into the synapse through a transport process driven by the physiological sodium ion (Na+) gradient. NSSs for dopamine, noradrenaline, and serotonin are targeted by the psychostimulants cocaine and amphetamines, as well as by antidepressants. The crystal structure of LeuT, a prokaryotic NSS homologue, revealed the NSS molecular architecture and has been the basis for extensive structural, biochemical, and computational investigations of the mechanism of transporter proteins with a LeuT-like fold. In this dissertation, the conformational states sampled by LeuT are explored using single-molecule fluorescence resonance energy transfer imaging methods, with special focus on the motions of transmembrane helix 1a that lead to inward release of substrate. We also explored how dynamics are modulated by substrate, Na+, and protons to produce efficient transport. These advances represent a first of a kind study of the dynamics of an integral membrane protein at a truly single-molecule scale. Advances in instrumentation, analysis tools, and organic fluorophores were all required to achieve these goals, and such advances are also described. While these experiments were performed with detergent-solubilized protein, preliminary work suggests that imaging of LeuT in proteoliposomes is feasible and a fluorescence sensor assay could be used to simultaneously detect conformational dynamics and transport function. |
