Crystal structure of a substrate-free aspartate transporter.

Archaeal glutamate transporter homologs catalyze the coupled uptake of aspartate and three sodium ions. After the delivery of the substrate and sodium ions to the cytoplasm, the empty binding site must reorient to the outward-facing conformation to reset the transporter. Here, we report a crystal structure of the substrate-free transporter GltTk from Thermococcus kodakarensis, which provides insight into the mechanism of this essential step in the translocation cycle.