Ägg transportera
Fånga upp och transportera ägg till livmodern, befruktning av äggen sker här. The ABC transporters , ATP synthase ATP -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 often consist of multiple subunits, one or two of which are transmembrane proteins and one or two of which are membrane-associated AAA ATPases.
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Most of the uptake systems also have an extracytoplasmic receptor, a solute binding protein. 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 including human 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. 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.
When the ABC transport proteins are overexpressed in cancer cells, they can export anticancer drugs and render tumors resistant. 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. Bacterial ABC transporters are essential in cell viability, virulence , and pathogenicity. 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.
For instance, a potential lethal increase in osmotic strength is counterbalanced by activation of osmosensing ABC transporters that mediate uptake of solutes. In bacterial efflux systems, certain substances that need to be extruded from the cell include surface components of the bacterial cell e. Although most eukaryotic ABC transporters are effluxers, some are not directly involved in transporting substrates.
It is found to mediate the secretion of the steroid aldosterone by the adrenals, and its inhibition blocked the migration of dendritic immune cells, [ 19 ] possibly related to the outward transport of the lipid platelet activating factor PAF. 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. All ABC transport proteins share a structural organization consisting of four core 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. 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.
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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.
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. An example is the E. Most exporters, such as in the multidrug exporter Sav [ 26 ] 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 NBD.
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 BP 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 ICD that joins the membrane-spanning helices and the ABC domain.