title: Ion-controlled conformational dynamics in the outward-open transition from an occluded state of LeuT.
abstract: Neurotransmitter:sodium symporter (NSS) proteins are secondary Na(+)-driven active transporters that terminate neurotransmission by substrate uptake. Despite the availability of high-resolution crystal structures of a bacterial homolog of NSSs-Leucine Transporter (LeuT)-and extensive computational and experimental structure-function studies, unanswered questions remain regarding the transport mechanisms. We used microsecond atomistic molecular-dynamics (MD) simulations and free-energy computations to reveal ion-controlled conformational dynamics of LeuT in relation to binding affinity and selectivity of the more extracellularly positioned Na(+) binding site (Na1 site). In the course of MD simulations starting from the occluded state with bound Na(+), but in the absence of substrate, we find a spontaneous transition of the extracellular vestibule of LeuT into an outward-open conformation. The outward opening is enhanced by the absence of Na1 and modulated by the protonation state of the Na1-associated Glu-290. Consistently, the Na(+) affinity for the Na1 site is inversely correlated with the extent of outward-open character and is lower than in the occluded state with bound substrate; however, the Na1 site retains its selectivity for Na(+) over K(+) in such conformational transitions. To the best of our knowledge, our findings shed new light on the Na(+)-driven transport cycle and on the symmetry in structural rearrangements for outward- and inward-open transitions.
authors.affiliation: Institute for Biocomplexity and Informatics, University of Calgary, Calgary, Alberta, Canada.
authors.forename: Chunfeng
authors.initials: C
authors.lastname: Zhao
authors.forename: Sebastian
authors.initials: S
authors.lastname: Stolzenberg
authors.forename: Luis
authors.initials: L
authors.lastname: Gracia
authors.forename: Harel
authors.initials: H
authors.lastname: Weinstein
authors.forename: Sergei
authors.initials: S
authors.lastname: Noskov
authors.forename: Lei
authors.initials: L
authors.lastname: Shi
mesh_terms: D005110:Extracellular Space / Q000378:metabolism; D056004:Molecular Dynamics Simulation; D050480:Plasma Membrane Neurotransmitter Transport Proteins / Q000737:chemistry* / Q000378:metabolism*; D011487:Protein Conformation / Q000187:drug effects; D012964:Sodium / Q000378:metabolism / Q000494:pharmacology*; D013816:Thermodynamics
chemical_list: D050480:Plasma Membrane Neurotransmitter Transport Proteins; D012964:Sodium