ATP-binding cassette (ABC) transporters: structures and roles in bacterial pathogenesis.

Adenosine triphosphate (ATP)-binding cassette (ABC) transporter systems are divided into importers and exporters that facilitate the movement of diverse substrate molecules across the lipid bilayer, against the concentration gradient. These transporters comprise two highly conserved nucleotide-binding domains (NBDs) and two transmembrane domains (TMDs). Unlike ABC exporters, prokaryotic ABC importers require an additional substrate-binding protein (SBP) as a recognition site for specific substrate translocation.