P-glycoprotein

P-glycoprotein 1 (permeability glycoprotein, abbreviated as P-gp or Pgp) also known as multidrug resistance protein 1 (MDR1) or ATP-binding cassette sub-family B member 1 (ABCB1) or cluster of differentiation 243 (CD243) is an important protein of the is_associated_with::cell membrane that pumps many foreign substances out of cells. More formally, it is an ATP-dependent efflux pump with broad substrate specificity. It exists in animals, fungi and bacteria and likely evolved as a defense mechanism against harmful substances.

P-gp is extensively distributed and expressed in the is_associated_with::intestinal epithelium where it pumps is_associated_with::xenobiotics (such as toxins or drugs) back into the intestinal lumen, in liver cells where it pumps them into is_associated_with::bile ducts, in the cells of the proximal tubule of the kidney where it pumps them into urine-conducting ducts, and in the is_associated_with::capillary endothelial cells composing the is_associated_with::blood–brain barrier and is_associated_with::blood-testis barrier, where it pumps them back into the capillaries. Some is_associated_with::cancer cells also express large amounts of P-gp, which renders these cancers multi-drug resistant.

P-gp is a is_associated_with::glycoprotein that in humans is encoded by the ABCB1 is_associated_with::gene. P-gp is a well-characterized is_associated_with::ABC-transporter (which transports a wide variety of substrates across extra- and intracellular membranes) of the MDR/TAP subfamily.

P-gp was discovered in 1971 by is_associated_with::Victor Ling.

Function
The protein belongs to the superfamily of ATP-binding cassette (ABC) transporters. ABC proteins transport various molecules across extra- and intra-cellular membranes. ABC genes are divided into seven distinct subfamilies (ABC1, MDR/TAP, MRP, ALD, OABP, GCN20, White). This protein is a member of the MDR/TAP subfamily. Members of the MDR/TAP subfamily are involved in is_associated_with::multidrug resistance. P-gp is an ATP-dependent drug efflux pump for is_associated_with::xenobiotic compounds with broad substrate specificity. It is responsible for decreased drug accumulation in multidrug-resistant cells and often mediates the development of resistance to anticancer drugs. This protein also functions as a transporter in the is_associated_with::blood–brain barrier.

P-gp transports various substrates across the cell membrane including:
 * Drugs such as is_associated_with::colchicine, is_associated_with::tacrolimus and is_associated_with::quinidine
 * Chemotherapeutic agents such as is_associated_with::etoposide, is_associated_with::doxorubicin, and is_associated_with::vinblastine
 * is_associated_with::Lipids
 * is_associated_with::Steroids
 * Xenobiotics
 * is_associated_with::Peptides
 * is_associated_with::Bilirubin
 * is_associated_with::Cardiac glycosides like is_associated_with::digoxin
 * is_associated_with::Immunosuppressive agents
 * is_associated_with::Glucocorticoids like is_associated_with::dexamethasone
 * HIV-type 1 antiretroviral therapy agents like is_associated_with::protease inhibitors and nonnucleoside reverse transcriptase inhibitors.

Its ability to transport the above substrates accounts for the many roles of P-gp including: It is inhibited by many drugs, such as:
 * Regulating the distribution and bioavailability of drugs
 * Increased intestinal expression of P-glycoprotein can reduce the absorption of drugs that are substrates for P-glycoprotein. Thus, there is a reduced bioavailability, and therapeutic plasma concentrations are not attained. On the other hand, supratherapeutic plasma concentrations and drug toxicity may result because of decreased P-glycoprotein expression
 * Active cellular transport of is_associated_with::antineoplastics resulting in is_associated_with::multidrug resistance to these drugs
 * The removal of toxic metabolites and xenobiotics from cells into is_associated_with::urine, bile, and the intestinal lumen
 * The transport of compounds out of the is_associated_with::brain across the is_associated_with::blood–brain barrier
 * Digoxin uptake
 * Prevention of is_associated_with::ivermectin and is_associated_with::loperamide entry into the is_associated_with::central nervous system
 * The migration of is_associated_with::dendritic cells
 * Protection of hematopoietic is_associated_with::stem cells from toxins.
 * is_associated_with::Amiodarone
 * is_associated_with::Azithromycin
 * is_associated_with::Captopril
 * is_associated_with::Clarithromycin
 * is_associated_with::Cyclosporine
 * is_associated_with::Piperine
 * is_associated_with::Quercetin
 * is_associated_with::Quinidine
 * is_associated_with::Quinine
 * is_associated_with::Reserpine
 * is_associated_with::Ritonavir
 * is_associated_with::Tariquidar
 * is_associated_with::Verapamil

Structure
P-gp is a 170 kDa transmembrane is_associated_with::glycoprotein, which includes 10-15 kDa of N-terminal glycosylation. The N-terminal half of the molecule contains 6 transmembrane domains, followed by a large cytoplasmic domain with an ATP-binding site, and then a second section with 6 transmembrane domains and an ATP-binding site that shows over 65% of amino acid similarity with the first half of the polypeptide. In 2009, the first structure of a mammalian P-glycoprotein was solved (3G5U). The structure was derived from the mouse MDR3 gene product heterologously expressed in Pichia pastoris yeast. The structure of mouse P-gp is similar to structures of the bacterial ABC transporter MsbA (3B5W and 3B5X) that adopt an inward facing conformation that is believed to be important for binding substrate along the inner leaflet of the membrane. Additional structures (3G60 and 3G61) of P-gp were also solved revealing the binding site(s) of two different cyclic peptide substrate/inhibitors. The promiscuous binding pocket of P-gp is lined with aromatic amino acid side chains. However, the murine P-gp structure is incomplete, missing an intermediate linker sequence proved to be essential for substrate recognition and ATP hydrolysis. Through Molecular Dynamic (MD) simulations, this sequence was proved to have a direct impact in the transporter's structural stability (in the nucleotide-binding domains) and defining a lower boundary for the internal drug-binding pocket.

Mechanism of action
Substrate enters P-gp either from an opening within the inner leaflet of the membrane or from an opening at the cytoplasmic side of the protein. ATP binds at the cytoplasmic side of the protein. Following binding of each, ATP hydrolysis shifts the substrate into a position to be excreted from the cell. Release of the phosphate (from the original ATP molecule) occurs concurrently with substrate excretion. ADP is released, and a new molecule of ATP binds to the secondary ATP-binding site. Hydrolysis and release of ADP and a phosphate molecule resets the protein, so that the process can start again.

Tissue distribution
P-gp is expressed primarily in certain cell types in the is_associated_with::liver, is_associated_with::pancreas, is_associated_with::kidney, colon, and is_associated_with::jejunum. P-gp is also found in is_associated_with::brain is_associated_with::capillary is_associated_with::endothelial cells.

Detecting the activity of the transporter
The activity of the transporter can be determined by both membrane ATPase and cellular is_associated_with::calcein assays.

Radioactive is_associated_with::verapamil can be used for measuring P-gp function with is_associated_with::positron emission tomography.

P-gp is also used to differentiate transitional B-cells from naive B-cells. Dyes such as Rhodamine123 and MitoTracker Dyes from Invitrogen can be used to make this differentiation.

History
P-gp was first cloned and characterized in 1976. It was shown to be responsible for conferring multidrug resistance upon mutant cultured cancer cells that had developed resistance to cytotoxic drugs.

The structure of P-gp was resolved by is_associated_with::x-ray crystallography in 2009.