ATP-Binding Cassette Transporter
Introduction to ATP-Binding Cassette Transporter
ATP binding cassette (ABC) transporter(s) is an integral membrane protein or proteins that transport molecules actively across lipid membranes using energy derived from ATP hydrolysis to ADP. The process occurs against a concentration gradient. This group of transporters is virtually present in all living things and it makes up a larger part of a wide range of biological processes.
It is wise to note that ABC domain can be found in proteins that can couple ATP hydrolysis to function even further than just transport, for instance repair of DNA. However, while such proteins can play a crucial role in enhancing human understanding of catalytic processes, modern review is often focused on ABC transporter family only.
There are different ABC transporter types including
- ABCA subfamily
- ABCB subfamily
- ABCC subfamily
- ABCD subfamily of peroxisomal ABC transporters
- ABCG subfamily
They transport a wide range of substances including small molecules such as sugars, amino acids and ions to larger compounds such as drugs, antibiotics, oligopeptides and lipids. ABC transporters take part in uptake of various nutrients and toxin secretion of toxins in bacteria. It also confers multidrug resistance in cases of cancer or bacterial cells. The cells achieve this by pumping a wide range of anti-cancer drugs and anti-biotics into various extracellular spaces.
ABC transporters are also relevant in medical field because some of their mutations have been implicated in cystic fibrosis among other related genetic disorders. The proteins constitute an ancient family of transporters. They are believed to have diversified before eukaryotes, archea and bacteria and diverged on other evolutionary paths. Therefore, human ABC transporters have bacterial homologues in different living organisms such Escherichia coli.
General structure ATP-Binding Cassette Transporter
All ABC transporters have a common structure. This is regardless of their functions as exporters or importers or even the substance being transported. They consist of two transmembrane domains and are the integral membrane proteins. They also include two NBDS or nucleotide binding domains. Furthermore, they are composed of water soluble proteins and closely associated with TMDs located on one side of the transporter membrane.
The TMD parts of the ATP transporter form a channel for transport and they consist of various membrane spanning alpha helices that present a structural variability among the transporters. Even so, there are 8 to 20 transmembrane helices for importers and 12 for the exporters.
NBD parts in contrast are highly conserved in the family and the present characteristics such as Walker A and B motifs found in all ATP binding proteins. There is also a signature motif that is related to the ABC transporter family. Additionally, the NBDs are ABC transporter engines because they are bound together while the ATP powers transport.
ATP binding induces different conformational changes in NBDs and force them to a closer contact transmitted to TMDs. This causes a conformational change that opens a conduit between TMDs to the outside or inside of the cell.
The exact mechanism of NBDs and TMDs coupling is unknown. It was however suggested that different TMDs could easily develop strong mechanisms for coupling between the two domains.
Bacterial ABC transporters
All bacteria have two ABC transporters including eukaryotic and prokaryotic type. The former have genes for EK type ABC transporters only. They are highly functional and feature gene organizational differences between EK type ABC and PK type ABC transporters.
PK type ABC transporters are importers which need additional extracellular proteins known as substrate binding proteins. They are also specifically for gram negative bacteria periplasmic binding proteins and they recruit substrates from extracellular space besides delivering them to transporters.
There are also ABC transporters in E.coli and it includes characterization of different ABC transporter genes fully encoded. However, the number of functional ABC transporters in E.coli is not clear. 48 ABC transporter gene systems in humans have been identified so far. Researchers have also found out that the more substances a living organism needs to export or import, the more the number of ABC transporter systems it has in its genome.
Therefore, in eukaryotes, motile organisms such as animals usually need few ABC transporters compared to non-motile organisms for example plants.
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