LeuT: a prokaryotic stepping stone on the way to a eukaryotic neurotransmitter transporter structure

Ion-coupled secondary transport is utilized by a broad range of integral membrane proteins to catalyze the energetically unfavorable movement of solute molecules across a lipid bilayer. Members of the solute carrier 6 (SLC6) family, present in both prokaryotes and eukaryotes, are sodium-coupled symporters that play crucial roles in processes as diverse as nutrient uptake and neurotransmitter clearance. The crystal structure of LeuT, a bacterial member of this family, provided the first atomic-level glimpse into overall architecture, pinpointed the substrate and sodium binding sites and implicated candidate helices and residues in the "gating" conformational changes that accompany ion binding and release. The structure is consistent with a wealth of elegant biochemical data on the eukaryotic counterparts and has for the first time permitted the construction of accurate homology models that can be directly tested experimentally. Sequence identity is especially high near the substrate and sodium binding sites and, thus, molecular insights within these regions have been substantial. However, there are several topics relevant to transport mechanism, inhibition and regulation that structure/function studies of LeuT cannot adequately address, suggesting the need for a eukaryotic transporter crystal structure.