ABC Transporters: Exporteur

ATP-binding cassette or ABC transporters form the largest family of membrane transporters.

These transporters contain two transmembrane domains that span the lipid bilayer and two cytoplasmic nucleotide-binding or ATP-binding domains.

ABC transporters can act as importers as well as exporters of solutes at the cell membrane.

While the exporters are present in both prokaryotes and eukaryotes, importers are only found in prokaryotes and some plants.

Consider an ABC exporter present on a eukaryotic cell membrane.

Initially, the transporter is present in the inward-facing conformation, with the solute-binding site exposed towards the cytosol. When a cytosolic solute binds to the transporter, it increases the affinity of ATP-binding domains towards ATP.

Binding of the ATP molecules to the transporter triggers a change in the protein conformation from inward-facing to outward-facing. This conformational change releases the solute into the extracellular space.

The attached ATP is then hydrolyzed into ADP and inorganic phosphate, prompting the two ATP-binding domains to separate. This reverts the transporter to its initial inward-facing conformation.