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.
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