Structure and mechanism of the bacterial transporter LeuT

Ion‑coupled secondary transport enables concentrative uptake of diverse molecules across membranes and is exemplified by LeuT, a bacterial homolog of the Neurotransmitter:Sodium Symporter (NSS) family. To deepen understanding of LeuT structure and mechanism, we combined transport assays, binding studies, and X‑ray crystallography using different substrates and mutations. We show that LeuT transports multiple amino acids and that transportable substrates stabilize an occluded intermediate with a single high‑affinity binding site. Structural and functional data indicate that a second, low‑affinity site appears only in an open‑to‑out state. These findings support a detailed alternating‑access mechanism applicable to related NSS transporters.