ABC Transporter

Cross references:    Adenosine         Adenosine Triphosphate    (ATP) 
cyclic Adenosine Monophosphate     (cAMP)    Guanosine Triphosphate (GTP)   
       


ATP-Binding Cassette Transporter (Wiki) 
http://en.wikipedia.org/wiki/ATP-binding_cassette_transporter 
"ATP-binding cassette transporters (ABC-transporter) are members of a protein superfamily that is one of the largest and most ancient families with representatives in all extant phyla from prokaryotes to humans." 

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In prokaryotes, importers mediate the uptake of nutrients into the cell. Eukaryotes do not possess any importers."

"ABC transporters are transmembrane proteins that utilize the energy of adenosine triphosphate (ATP) hydrolysis to carry out certain biological processes including translocation of various substrates across membranes and non-transport-related processes such as translation of RNA and DNA repair.

 
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The common feature of all ABC transporters is that they consist of two distinct domains, the transmembrane domain (TMD) and the nucleotide-binding domain (NBD). The TMD, also known as membrane-spanning domain (MSD) or integral membrane (IM) domain, consists of alpha helices, embedded in the membrane bilayer. ... importers have a high-affinity binding protein (BP) that specifically associates with the substrate in the periplasm for delivery to the appropriate ABC transporter. ... It recognizes a variety of substrates and undergoes conformational changes to transport the substrate across the membrane.

Structure of an ABC Importer



File:Btucd.jpg





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ABC transporters utilize the energy of ATP hydrolysis to transport various substrates across cellular membranes.
"  The substrate-loaded binding protein acts as the ligand. 

Mechanism of Transport for Importers

File:Abc importer.jpg - Wikipedia,

https://en.wikipedia.org/wiki/File:Abc_importer.jpg

https://upload.wikimedia.org/wikipedia/commons/d/d1/Abc_importer.jpg


"The resting state of importers is inward-facing, where the nucleotide binding domain (NBD) dimer interface is held open by the TMDs and facing outward but occluded from the cytoplasm. Upon docking of the closed, substrate-loaded binding protein towards the periplasmic side of the transmembrane domains, ATP binds and the NBD dimer closes. This switches the resting state of transporter into an outward-facing conformation, in which the TMDs have reoriented to receive substrate from the binding protein. After hydrolysis of ATP, the NBD dimer opens and substrate is released into the cytoplasm. " 


"Both ABC importers and exporters have a common mechanism in transporting substrates because of the similarities in their structures.




Structure of an ABC Exporter

File:Abc-sav.jpg

"In most exporters, the N-terminal transmembrane domain and the C-terminal ABC domains are fused as a single polypeptide chain, arranged as TMD-NBD-TMD-NBD."  " Exporters do not have the binding protein but have an intracellular domain (ICD) that joins the membrane-spanning helices and the ABC domain. The ICD is believed to be responsible for communication between the TMD and NBD.


Mechanism of Transport for Exporters

Proposed Mechanism of Transport for ABC Exporters. This model was based on structural and biochemical studies on MsbA.

Note:  Click on the picture to enlarge it so that you can read the labels.   




Structure, Function, and Evolution of Bacterial ATP-Binding Cassette Systems (PubMed) 
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2415747/?tool=pubmed 
Full length article.  "ABC systems are widespread among living organisms and have been detected in all genera of the three kingdoms of life, with remarkable conservation in the primary sequence of the cassette and in the organization of the constitutive domains or subunits



This is a discussion of the function of ABC-transporters in prokaryotes. 

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ATP-binding cassette transporters (ABC-transporter) are divided into three main functional categories.

In prokaryotes, importers mediate the uptake of nutrients into the cell. The substrates that can be transported include ions, amino acids, peptides, sugars, and other molecules that are mostly hydrophilic. The membrane-spanning region of the ABC transporter protects hydrophilic substrates from the lipids of the membrane bilayer thus providing a pathway across the cell membrane. Eukaryotes do not possess any importers.

Exporters, which are both present in prokaryotes and eukaryotes, function as pumps that extrude toxins and drugs out of the cell. In gram-negative bacteria, exporters transport lipids and some polysaccharides from the cytoplasm to the periplasm.

The third subgroup of ABC proteins do not function as transporters, but are rather involved in translation and DNA repair processes.










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