Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:3.6.3.1 (Mg2+-ATPase)
 1,484 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The small apical (canalicular) domains of hepatocytes form a luminal meshwork of tubules between adjacent hepatocytes and are the sites of primary bile formation. Organic compounds are transported across this membrane domain against high concentration gradients. It has been recognized in recent years that the hepatocyte is harnessed with a set of canalicular ATP-dependent transport proteins, specialized in this uphill transport. Bile salts, organic anions, cations, and neutral amphipaths are all pumped into the bile via such primary active transporters. Functionally, these transporters resemble ABC transporters overexpressed in cells with the multidrug resistance phenotype. Indeed, those transporters that have been characterized at the molecular level turn out to be new, or already recognized, members of this family. Phospholipid secretion across the canalicular membrane of the mouse is also mediated by a member of this family, mdr2 P-glycoprotein. This was demonstrated by the absence of phospholipid secretion into bile of mice with a disrupted mdr2 gene and by subsequent demonstration of phospholipid translocation in cells that overexpress this protein. The recognition of mdr2 P-glycoprotein as a phospholipid flippase sheds new light on the function of P-glycoproteins and is an important step in understanding the mechanism of biliary lipid secretion.
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PMID:Hepatic canalicular membrane 1: The role of mdr2 P-glycoprotein in hepatobiliary lipid transport. 903 62

The ABC superfamily of transporters includes the mammalian P-glycoprotein family (Class I and Class II P-gps), the multidrug resistance-associated protein (MRP), the Pgh-1 product of Plasmodium falciparum gene pfmdr1, all of which are associated with cellular pleiotropic drug resistance phenomena. STE6, the yeast transporter for the farnesylated peptide pheromone a, is also a member of this family. Structural similarities in this family translate into functional homology as expression of mouse Mdr3S (P-gp), P. falciparum Pgh-1, and human MRP partially restore mating in a sterile yeast mutant lacking a functional STE6 gene. The demonstration that Class II P-gps function as phosphatidylcholine (PC) translocators raise the possibility that other ABC transporters may also interact with physiological lipids. We report the identification of the synthetic lipid and PC analog ET-18-OCH3 (edelfosine) as a substrate for not only Class II P-gp but also for Class I P-gps and surprisingly for the other ABC transporters MRP, Pgh-1, and STE6. Expression of these proteins in the yeast Saccharomyces cerevisiae JPY201 was found to confer cellular resistance to cytotoxic concentrations of this lipid by a factor of 4-20-fold in a growth inhibition assay. The noted activity of ABC transporters toward this synthetic lipid was specific as a mutant variant of Mdr3 (Mdr3F) with reduced activity could not convey cellular resistance to ET-18-OCH3. ET-18-OCH3 was also found capable of blocking a-peptide pheromone transport and STE6 complementation by these ABC proteins. The inhibitory effect of ET-18-OCH3 on cell growth and a-factor transport could be abrogated by incubation with the lipid acceptor protein BSA or by enzymatic cleavage by microsomal alkylglycerol mono-oxygenase (MAMO). MAMO and BSA reversal of the ether lipid effect was only seen in the presence of a functional transporter. These results suggest that the group of cytotoxic synthetic PC analogs studied reveal possible structural and functional aspects common to the ABC transporters tested. Furthermore, the studies with BSA and MAMO suggest that the mechanism of transport of ET-18-OCH3 by these ABC transporters may be related to the flippase mechanism of PC transport by Mdr2.
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PMID:Functional interactions between synthetic alkyl phospholipids and the ABC transporters P-glycoprotein, Ste-6, MRP, and Pgh 1. 1009 17

Multidrug resistance (MDR) is a serious medical problem and presents a major challenge to the treatment of disease and the development of novel therapeutics. ABC transporters that are associated with multidrug resistance (MDR-ABC transporters) translocate hydrophobic drugs and lipids from the inner to the outer leaflet of the cell membrane. To better elucidate the structural basis for the "flip-flop" mechanism of substrate movement across the lipid bilayer, we have determined the structure of the lipid flippase MsbA from Escherichia coli by x-ray crystallography to a resolution of 4.5 angstroms. MsbA is organized as a homodimer with each subunit containing six transmembrane alpha-helices and a nucleotide-binding domain. The asymmetric distribution of charged residues lining a central chamber suggests a general mechanism for the translocation of substrate by MsbA and other MDR-ABC transporters. The structure of MsbA can serve as a model for the MDR-ABC transporters that confer multidrug resistance to cancer cells and infectious microorganisms.
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PMID:Structure of MsbA from E. coli: a homolog of the multidrug resistance ATP binding cassette (ABC) transporters. 1718 84

The ABC transporters are ubiquitous membrane proteins that couple adenosine triphosphate (ATP) hydrolysis to the translocation of diverse substrates across cell membranes. Clinically relevant examples are associated with cystic fibrosis and with multidrug resistance of pathogenic bacteria and cancer cells. Here, we report the crystal structure at 3.2 angstrom resolution of the Escherichia coli BtuCD protein, an ABC transporter mediating vitamin B12 uptake. The two ATP-binding cassettes (BtuD) are in close contact with each other, as are the two membrane-spanning subunits (BtuC); this arrangement is distinct from that observed for the E. coli lipid flippase MsbA. The BtuC subunits provide 20 transmembrane helices grouped around a translocation pathway that is closed to the cytoplasm by a gate region whereas the dimer arrangement of the BtuD subunits resembles the ATP-bound form of the Rad50 DNA repair enzyme. A prominent cytoplasmic loop of BtuC forms the contact region with the ATP-binding cassette and appears to represent a conserved motif among the ABC transporters.
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PMID:The E. coli BtuCD structure: a framework for ABC transporter architecture and mechanism. 1200 8

Lipids in biological membranes are asymmetrically distributed across the bilayer; the amine-containing phospholipids are enriched on the cytoplasmic surface of the plasma membrane, while the choline-containing and sphingolipids are enriched on the outer surface. The maintenance of transbilayer lipid asymmetry is essential for normal membrane function, and disruption of this asymmetry is associated with cell activation or pathologic conditions. Lipid asymmetry is generated primarily by selective synthesis of lipids on one side of the membrane. Because passive lipid transbilayer diffusion is slow, a number of proteins have evolved to either dissipate or maintain this lipid gradient. These proteins fall into three classes: 1) cytofacially-directed, ATP-dependent transporters ("flippases"); 2) exofacially-directed, ATP-dependent transporters ("floppases"); and 3) bidirectional, ATP-independent transporters ("scramblases"). The flippase is highly selective for phosphatidylserine and functions to keep this lipid sequestered from the cell surface. Floppase activity has been associated with the ABC class of transmembrane transporters. Although they are primarily nonspecific, at least two members of this class display selectivity for their substrate lipid. Scramblases are inherently nonspecific and function to randomize the distribution of newly synthesized lipids in the endoplasmic reticulum or plasma membrane lipids in activated cells. It is the combined action of these proteins and the physical properties of the membrane bilayer that generate and maintain transbilayer lipid asymmetry.
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PMID:Regulation of transbilayer plasma membrane phospholipid asymmetry. 1257 5

The spread of multidrug resistance (MDR) is a world health crisis that presents a significant challenge to the treatment of cancer and infection. MDR can be caused by a group of ABC (MDR-ABC) transporters that move hydrophobic drug molecules and lipids across the cell membrane. To gain insight into the conformational changes these transporters undergo when flipping hydrophobic substrates across the lipid bilayer, we have determined the structure of the lipid flippase MsbA from Vibrio cholera (VC-MsbA) to 3.8A. Structural comparison of VC-MsbA to MsbA from Escherichia coli reveals that the transporters share a structurally conserved core of transmembrane alpha-helices, but differ in the relative orientations of their nucleotide-binding domains (NBD). The transmembrane domain of VC-MsbA is captured in a closed conformation and the structure supports a "power stroke" model of transporter dynamics where opposing NBDs associate upon ATP binding. The separation of the alpha and beta domains of the NBD suggests the possibility that their association could make them competent to bind ATP and gives further insight into the structural basis for catalytic regulation.
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PMID:Structure of MsbA from Vibrio cholera: a multidrug resistance ABC transporter homolog in a closed conformation. 1758 Mar 80

Translocation of lipid-linked oligosaccharide (LLO) intermediates across membranes is an essential but poorly understood process in eukaryotic and bacterial glycosylation pathways. Membrane proteins defined as translocases or flippases are implicated to mediate the translocation reaction. The membrane protein Wzx has been proposed to mediate the translocation across the plasma membrane of lipopolysaccharide (LPS) O antigen subunits, which are assembled on an undecaprenyl pyrophosphate lipid carrier. Similarly, PglK (formerly WlaB) is a Campylobacter jejuni-encoded ABC-type transporter proposed to mediate the translocation of the undecaprenylpyrophosphate-linked heptasaccharide intermediate involved in the recently identified bacterial N-linked protein glycosylation pathway. A combination of genetic and carbohydrate structural analyses defined and characterized flippase activities in the C. jejuni N-linked protein glycosylation and the Escherichia coli LPS O antigen biosynthesis. PglK displayed relaxed substrate specificity with respect to the oligosaccharide structure of the LLO intermediate and complemented a wzx deficiency in E. coli O-antigen biosynthesis. Our experiments provide strong genetic evidence that LLO translocation across membranes can be catalyzed by two distinct proteins that do not share any sequence similarity.
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PMID:Two distinct but interchangeable mechanisms for flipping of lipid-linked oligosaccharides. 1649

In juvenile rats born from mothers with obstructive cholestasis during pregnancy (OCP), transient latent cholestasis together with alterations in the secretion of biliary lipids have been reported. Here we investigated whether the expression of genes involved in this function is already modified at birth and examined the effect of treating pregnant rats with ursodeoxycholic acid (UDCA; i.g., 60 microg/100 g b.w./day). Cholanemia was markedly higher in mothers with OCP, and was further increased by UDCA. In the Control pups, cholanemia increased after birth, whereas in OCP and OCP+UDCA pups, hypercholanemia decreased after birth. Steady-state mRNA levels in neonatal liver were measured by real-time quantitative RT-PCR. The expression of basolateral bile acid transporters was not affected by OCP and was unchanged (Oatp1/1a1 and Oatp4/1b2) or moderately increased (Ntcp and Oatp2/1a4) by UDCA. In both groups, the expression of ABC proteins was either not modified (Bsep, Bcrp and Mrp2) or enhanced (Mrp1 and Mrp3), that of phospholipid flippase Mdr2 was not changed, whereas that of cholesterol transporter Abcg5/Abcg8 was impaired. The expression of the nuclear receptor FXR was not affected by OCP or UDCA, whereas that of SHP and key enzymes in bile acid synthesis (Cyp7a1, Cyp8b1 and Cyp27) was increased in both groups. In conclusion, OCP affects the expression in the neonatal liver of genes involved in hepatobiliary function, which cannot be prevented, at this stage, by treating pregnant rats with UDCA, even though this treatment has been found to partially restore normal lipid secretion later during post-natal development.
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PMID:Effect of maternal cholestasis and treatment with ursodeoxycholic acid on the expression of genes involved in the secretion of biliary lipids by the neonatal rat liver. 1676 92

Multidrug transporters of the ABC family facilitate the export of diverse cytotoxic drugs across cell membranes. This is clinically relevant, as tumour cells may become resistant to agents used in chemotherapy. To understand the molecular basis of this process, we have determined the 3.0 A crystal structure of a bacterial ABC transporter (Sav1866) from Staphylococcus aureus. The homodimeric protein consists of 12 transmembrane helices in an arrangement that is consistent with cross-linking studies and electron microscopic imaging of the human multidrug resistance protein MDR1, but critically different from that reported for the bacterial lipid flippase MsbA. The observed, outward-facing conformation reflects the ATP-bound state, with the two nucleotide-binding domains in close contact and the two transmembrane domains forming a central cavity--presumably the drug translocation pathway--that is shielded from the inner leaflet of the lipid bilayer and from the cytoplasm, but exposed to the outer leaflet and the extracellular space.
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PMID:Structure of a bacterial multidrug ABC transporter. 1697 36

Clinical relevance is implicated between the genetic polymorphisms of the ABC (ATP-binding cassette) transporter ABCG2 (ABC subfamily G, member 2) and the individual differences in drug response. We expressed a total of seven non-synonymous SNP (single nucleotide polymorphism) variants in Flp-In-293 cells by using the Flp (flippase) recombinase system. Of these, ABCG2 F208S and S441N variants were found to be expressed at markedly low levels, whereas their mRNA levels were equal to those of the other SNP variants and ABCG2 WT (wild-type). Interestingly, protein expression levels of the ABCG2 F208S and S441N variants increased 6- to 12-fold when Flp-In-293 cells were treated with MG132, a proteasome inhibitor. Immunoprecipitation followed by immunoblot analysis showed that the ABCG2 F208S and S441N variant proteins were endogenously ubiquitinated in Flp-In-293 cells, and treatment with MG132 significantly enhanced the level of these ubiquitinated variants. Immunofluorescence microscopy demonstrated that MG132 greatly affected the ABCG2 F208S and S441N variants in terms of both protein levels and intracellular distribution. Immunoblot analysis revealed that those variants were N-glycosylated; however, their oligosaccharides were immature compared with those present on ABCG2 WT. The ABCG2 F208S and S441N variant proteins do not appear to be processed in the Golgi apparatus, but undergo ubiquitin-mediated protein degradation in proteasomes, whereas ABCG2 WT is sorted to the plasma membrane and then degraded via the lysosomal pathway. The present study provides the first evidence that certain genetic polymorphisms can affect the protein stability of ABCG2. Control of proteasomal degradation of ABCG2 would provide a novel approach in cancer chemotherapy to circumvent multidrug resistance of human cancers.
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PMID:Ubiquitin-mediated proteasomal degradation of non-synonymous SNP variants of human ABC transporter ABCG2. 1823 72


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