Learning the ABCs one at a time: structure and mechanism of ABC transporters.

ATP-binding cassette (ABC) transporters are essential proteins that are found across all kingdoms of life. ABC transporters harness the energy of ATP hydrolysis to drive the import of nutrients inside bacterial cells or the export of toxic compounds or essential lipids across bacteria and eukaryotic membranes. Typically, ABC transporters consist of transmembrane domains (TMDs) and nucleotide-binding domains (NBDs) to bind their substrate and ATP, respectively. The TMDs dictate what ligands can be recognised, whereas the NBDs are the power engine of the ABC transporter, carrying out ATP binding and hydrolysis. It has been proposed that they utilise the alternating access mechanism, inward- to outward-facing conformation, to transport their substrates. Here, we will review the recent progress on the structure determination of eukaryotic and bacterial ABC transporters as well as the novel mechanisms that have also been proposed, that fall out of the alternating access mechanism model.