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ATP-binding cassette transporter
ATP-binding cassette transporters (ABC transporters) are members of a protein superfamily that is one of the largest and oldest families with representatives in all extant phyla from prokaryotes to humans. ABC transporters are transmembrane proteins that utilize the energy of adenosine triphosphate (ATP) binding and 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. They transport a wide variety of substrates across extra- and intracellular membranes, including metabolic products, lipids and sterols, and drugs. ABC transporters are classified as proteins based on the sequence and organization of their ATP-binding cassette (ABC) domain(s). ABC transporters are involved in tumor resistance, cystic fibrosis and a range of other inherited human diseases along with both prokaryotic and eukaryotic (including human) development of resistance to multiple drugs.
More than 100 ABC transporters are distributed from prokaryotes to humans. ABC genes are essential for many processes in the cell, and mutations in these genes cause or contribute to several human genetic diseases. 48 ABC genes have been reported in humans. Among these, 16 genes have been determined and 14 of these are related with diseases present in humans such as cystic fibrosis, adrenoleukodystrophy, Stargadt’s disease, drug-resistant tumors, Dubin-Johnson syndrome, Byler’s disease, progressive familiar intrahepatic cholestasis, X-linked sideroblastic anemia, ataxia, and persistent and hyperinsulimenic hypoglycemia in children.〔 ABC transporters are also involved in multiple drug resistance, and this is how some of them were first identified. When the ABC transporter proteins are overexpressed in cancer cells they can export anticancer drugs and render tumors resistant.
== Function ==
ABC transporters utilize the energy of ATP binding and hydrolysis to transport various substrates across cellular membranes. They 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'' or ''effluxers'', 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.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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