Fig. 5.

Proposed role of the outward-occluded state in the mechanism of ABC exporters. Ligand binding to the inward-open (apo) conformation (state i) facilitates transition of the transporter into the inward-closed (apo) conformation (state ii). ATP binding is associated with a transition into a hypothetical occluded conformation (state iii), and subsequent formation of a nucleotide-bound outward-open conformation (state iv). Upon release of the ligand, the ABC exporter adopts an outward-occluded conformation (state v) (current McjD structure). ATP hydrolysis resets the transporter back to the inward-facing conformation (state i). The TMs 1 and 2 from each monomer are highlighted in dark blue and red. The substrate is shown as a yellow diamond.