A conserved leucine occupies the empty substrate site of LeuT in the Na(+)-free return state.

Neurotransmitter:sodium symporters (NSS) perform Na+-dependent amino-acid uptake, but a full structural description of their function is lacking. Here the authors present a structure of the bacterial NSS LeuT in the outward-oriented empty state and show that a Leu residue occupies the empty substrate site and is essential for function.