Energy-coupling factor (ECF) transporters mediate uptake of micronutrients in
prokaryotes. The transporters consist of an S-component that binds the
transported substrate and an ECF module (EcfAA'T) that binds and hydrolyses ATP.
The mechanism of transport is poorly understood but presumably involves an
unusual step in which the membrane-embedded S-component topples over to carry
the substrate across the membrane. In many ECF transporters, the S-component
dissociates from the ECF module after transport. Subsequently, substrate-bound
S-components out-compete the empty proteins for re-binding to the ECF module in
a new round of transport. Here we present crystal structures of the
folate-specific transporter ECF-FolT from Lactobacillus delbrueckii. Interaction
of the ECF module with FolT stabilizes the toppled state, and simultaneously
destroys the high-affinity folate-binding site, allowing substrate release into
the cytosol. We hypothesize that differences in the kinetics of toppling can
explain how substrate-loaded FolT out-competes apo-FolT for association with the
ECF module.
