ABC transporter

The ABC transporters, ATP synthase -binding cassette transporters are a transport system superfamily that is one of the largest and possibly one of the oldest gene families. It is represented in all extant phyla, from prokaryotes to humans. ABC transporters belong to translocases.
ABC transporters often consist of multiple subunits, one or two of which are transmembrane proteins and one or two of which are membrane-associated AAA ATPases. The ATPase subunits utilize the energy of adenosine triphosphate binding and hydrolysis to provide the energy needed for the translocation of substrates across membranes, either for uptake or for export of the substrate.
Most of the uptake systems also have an extracytoplasmic receptor, a solute binding protein. Some homologous ATPases function in non-transport-related processes such as translation of RNA and DNA repair. ABC transporters are considered to be an ABC superfamily based on the similarities of the sequence and organization of their ATP-binding cassette domains, even though the integral membrane proteins appear to have evolved independently several times, and thus comprise different protein families. Like the ABC exporters, it is possible that the integral membrane proteins of ABC uptake systems also evolved at least three times independently, based on their high resolution three-dimensional structures. ABC uptake porters take up a large variety of nutrients, biosynthetic precursors, trace metals and vitamins, while exporters transport lipids, sterols, drugs, and a large variety of primary and secondary metabolites. Some of these exporters in humans are involved in tumor resistance, cystic fibrosis and a range of other inherited human diseases. High level expression of the genes encoding some of these exporters in both prokaryotic and eukaryotic organisms result in the development of resistance to multiple drugs such as antibiotics and anti-cancer agents.
Hundreds of ABC transporters have been characterized from both prokaryotes and eukaryotes. ABC genes are essential for many processes in the cell, and mutations in human genes cause or contribute to several human genetic diseases. Forty eight ABC genes have been reported in humans. Among these, many have been characterized and shown to be causally related to diseases present in humans such as cystic fibrosis, adrenoleukodystrophy, Stargardt disease, drug-resistant tumors, Dubin–Johnson syndrome, Byler's disease, progressive familiar intrahepatic cholestasis, X-linked sideroblastic anemia, ataxia, and persistent and hyperinsulimenic hypoglycemia. ABC transporters are also involved in multiple drug resistance, and this is how some of them were first identified. When the ABC transport 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 present both 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.

Prokaryotic

Bacterial ABC transporters are essential in cell viability, virulence, and pathogenicity. Iron ABC uptake systems, for example, are important effectors of virulence. Pathogens use siderophores, such as Enterobactin, to scavenge iron that is in complex with high-affinity iron-binding proteins or erythrocytes. These are high-affinity iron-chelating molecules that are secreted by bacteria and reabsorb iron into iron-siderophore complexes. The chvE-gguAB gene in Agrobacterium tumefaciens encodes glucose and galactose importers that are also associated with virulence. Transporters are extremely vital in cell survival such that they function as protein systems that counteract any undesirable change occurring in the cell. For instance, a potential lethal increase in osmotic strength is counterbalanced by activation of osmosensing ABC transporters that mediate uptake of solutes. Other than functioning in transport, some bacterial ABC proteins are also involved in the regulation of several physiological processes.
In bacterial efflux systems, certain substances that need to be extruded from the cell include surface components of the bacterial cell, proteins involved in bacterial pathogenesis, heme, hydrolytic enzymes, S-layer proteins, competence factors, toxins, antibiotics, bacteriocins, peptide antibiotics, drugs and siderophores. They also play important roles in biosynthetic pathways, including extracellular polysaccharide biosynthesis and cytochrome biogenesis.

Eukaryotic

Although most eukaryotic ABC transporters are effluxers, some are not directly involved in transporting substrates. In the cystic fibrosis transmembrane regulator and in the sulfonylurea receptor, ATP hydrolysis is associated with the regulation of opening and closing of ion channels carried by the ABC protein itself or other proteins.
Human ABC transporters are involved in several diseases that arise from polymorphisms in ABC genes and rarely due to complete loss of function of single ABC proteins. Such diseases include Mendelian diseases and complex genetic disorders such as cystic fibrosis, adrenoleukodystrophy, Stargardt disease, Tangier disease, immune deficiencies, progressive familial intrahepatic cholestasis, Dubin–Johnson syndrome, Pseudoxanthoma elasticum, persistent hyperinsulinemic hypoglycemia of infancy due to focal adenomatous hyperplasia, X-linked sideroblastosis and anemia, age-related macular degeneration, familial hypoapoproteinemia, Retinitis pigmentosum, cone rod dystrophy, and others. The human ABCB family is responsible for multiple drug resistance against a variety of structurally unrelated drugs. ABCB1 or MDR1 P-glycoprotein is also involved in other biological processes for which lipid transport is the main function. It is found to mediate the secretion of the steroid aldosterone by the adrenals, and its inhibition blocked the migration of dendritic immune cells, possibly related to the outward transport of the lipid platelet activating factor. It has also been reported that ABCB1 mediates transport of cortisol and dexamethasone, but not of progesterone in ABCB1 transfected cells. MDR1 can also transport cholesterol, short-chain and long-chain analogs of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin, and glucosylceramide. Multispecific transport of diverse endogenous lipids through the MDR1 transporter can possibly affect the transbilayer distribution of lipids, in particular of species normally predominant on the inner plasma membrane leaflet such as PS and PE.
More recently, ABC-transporters have been shown to exist within the placenta, indicating they could play a protective role for the developing fetus against xenobiotics. Evidence has shown that placental expression of the ABC-transporters P-glycoprotein and breast cancer resistance protein are increased in preterm compared to term placentae, with P-gp expression further increased in preterm pregnancies with chorioamnionitis. To a lesser extent, increasing maternal BMI also associated with increased placental ABC-transporter expression, but only at preterm.

Structure

All ABC transport proteins share a structural organization consisting of four core domains. These domains consist of two trans-membrane domains and two cytosolic domains. The two T domains alternate between an inward and outward facing orientation, and the alternation is powered by the hydrolysis of adenosine triphosphate or ATP. ATP binds to the A subunits and it is then hydrolyzed to power the alternation, but the exact process by which this happens is not known. The four domains can be present in four separate polypeptides, which occur mostly in bacteria, or present in one or two multi-domain polypeptides. When the polypeptides are one domain, they can be referred to as a full domain, and when they are two multi-domains they can be referred to as a half domain. The T domains are each built of typically 10 membrane spanning alpha helices, through which the transported substance can cross through the plasma membrane. Also, the structure of the T domains determines the specificity of each ABC protein. In the inward facing conformation, the binding site on the A domain is open directly to the surrounding aqueous solutions. This allows hydrophilic molecules to enter the binding site directly from the inner leaflet of the phospholipid bilayer. In addition, a gap in the protein is accessible directly from the hydrophobic core of the inner leaflet of the membrane bilayer. This allows hydrophobic molecules to enter the binding site directly from the inner leaflet of the phospholipid bilayer. After the ATP powered move to the outward facing conformation, molecules are released from the binding site and allowed to escape into the exoplasmic leaflet or directly into the extracellular medium.
The common feature of all ABC transporters is that they consist of two distinct domains, the transmembrane domain and the nucleotide-binding domain . The TMD, also known as membrane-spanning domain or integral membrane domain, consists of alpha helices, embedded in the membrane bilayer. It recognizes a variety of substrates and undergoes conformational changes to transport the substrate across the membrane. The sequence and architecture of TMDs is variable, reflecting the chemical diversity of substrates that can be translocated. The NBD or ATP-binding cassette domain, on the other hand, is located in the cytoplasm and has a highly conserved sequence. The NBD is the site for ATP binding. 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. An example is the E. coli hemolysin exporter HlyB. Importers have an inverted organization, that is, NBD-TMD-NBD-TMD, where the ABC domain is N-terminal whereas the TMD is C-terminal, such as in the E. coli MacB protein responsible for macrolide resistance.
The structural architecture of ABC transporters consists minimally of two TMDs and two NBDs. Four individual polypeptide chains including two TMD and two NBD subunits, may combine to form a full transporter such as in the E. coli BtuCD importer involved in the uptake of vitamin B₁₂. Most exporters, such as in the multidrug exporter Sav1866 from Staphylococcus aureus, are made up of a homodimer consisting of two half transporters or monomers of a TMD fused to a nucleotide-binding domain. A full transporter is often required to gain functionality. Some ABC transporters have additional elements that contribute to the regulatory function of this class of proteins. In particular, importers have a high-affinity binding protein that specifically associates with the substrate in the periplasm for delivery to the appropriate ABC transporter. Exporters do not have the binding protein but have an intracellular domain that joins the membrane-spanning helices and the ABC domain. The ICD is believed to be responsible for communication between the TMD and NBD.