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ABC Transporters
Another type of ATP-driven membrane pump is the ATP-binding cassette transporter or simply ABC transporter. The discovery of these pumps came from studies with human cancer. When cultured cells are exposed to a specific type of drug, they eventually become resistant to that drug and many related drugs. This is known as multi-drug resistance or MDR. Multidrug resistance was shown to be a result of the expression of membrane proteins that became known as MDR proteins or P-glycoproteins. They use energy to pump drugs out of the cell before the drugs can elicit their response. The MDR protein consists of four domains - two are membrane-spanning domains that bind the substrate molecule (i.e. the drug) while the other two are ATP-binding domains called ATP-binding cassettes (ABCs). Both eukaryotic and prokaryotic cells have ABC transporters. However (1) eukaryotic cells usually have ABC transporters made of a single polypeptide subunit while prokaryotic cells typically express multi-subunit ABC proteins (2) eukaryotic cells generally use ABC transporters to export the molecules out of the cell while prokaryotic cells usually import them into the cell.
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