InterPro domain: IPR044726

ABC transporters belong to the ATP-Binding Cassette (ABC) superfamily, which uses the hydrolysis of ATP to energise diverse biological systems. ABC transporters minimally consist of two conserved regions: a highly conserved ATP binding cassette (ABC) and a less conserved transmembrane domain (TMD). These can be found on the same protein or on two different ones. Most ABC transporters function as a dimer and therefore are constituted of four domains, two ABC modules and two TMDs.

ABC transporters are involved in the export or import of a wide variety of substrates ranging from small ions to macromolecules. The major function of ABC import systems is to provide essential nutrients to bacteria. They are found only in prokaryotes and their four constitutive domains are usually encoded by independent polypeptides (two ABC proteins and two TMD proteins). Prokaryotic importers require additional extracytoplasmic binding proteins (one or more per systems) for function. In contrast, export systems are involved in the extrusion of noxious substances, the export of extracellular toxins and the targeting of membrane components. They are found in all living organisms and in general the TMD is fused to the ABC module in a variety of combinations. Some eukaryotic exporters encode the four domains on the same polypeptide chain [ 1 ].

The ATP-Binding Cassette (ABC) superfamily forms one of the largest of all protein families with a diversity of physiological functions [ 2 ]. Several studies have shown that there is a correlation between the functional characterisation and the phylogenetic classification of the ABC cassette [ 2 , 2 ]. More than 50 subfamilies have been described based on a phylogenetic and functional classification [ 3 , 3 , 4 ]; (for further information see http://www.tcdb.org/tcdb/index.php?tc=3.A.1).

This entry represents the six-transmembrane domain 2 (TMD2) of the ABC transporters that belong to the ABCC subfamily [ 4 ]. This TM subunit exhibits the type 3 ATP-binding cassette (ABC) exporter fold, which is characterised by 6 TM helices per subunit (domain), or a total of 12 TM helices for the complete transporter. The type 3 ABC exporters are found in both prokaryotes and eukaryotes, where they mediate the cellular secretion of toxic compounds, a various type of lipids and polypeptides [ 5 ].

1. Getting in or out: early segregation between importers and exporters in the evolution of ATP-binding cassette (ABC) transporters. J. Mol. Evol. 48, 22-41
2. The ABC of ABCS: a phylogenetic and functional classification of ABC systems in living organisms. Res. Microbiol. 152, 211-29
3. ABC transporters: physiology, structure and mechanism--an overview. Res. Microbiol. 152, 205-10
4. ABCC subfamily vacuolar transporters are involved in Pb (lead) detoxification in Saccharomyces cerevisiae. Appl Biochem Biotechnol 175, 65-74
5. ABC transporters: the power to change. Nat Rev Mol Cell Biol 10, 218-27