#ABC transporter

Membrane Transportation

Which ABC transporter plays a role in cholesterol and phospholipid transport in hepatocytes?

Row labeled Q30 appears truncated but query addresses which ABC transporter handles cholesterol and phospholipid transport in hepatocytes. In canalicular membrane ABCB4 MDR3 is established phosphatidylcholine floppase essential for biliary phospholipid secretion, pairing with bile salt export via ABCB11 and cholesterol export via ABCG5/G8. Its ATP-driven flipping of phosphatidylcholine to outer leaflet allows bile acids to extract lipid into mixed micelles that solubilize cholesterol and buffer bile acid detergent toxicity. At sinusoidal membrane, ABCA1 effluxes cholesterol and phospholipid to apolipoprotein A1 forming nascent HDL, contributing to reverse cholesterol transport. ABCB1 MDR1 exports amphipathic drugs xenobiotics rather than bulk biliary lipids, while SGLT1 and SGLT2 are sodium-glucose cotransporters secondary active members of SLC5 family irrelevant to lipid secretion. Clinically ABCB4 defects produce low-phospholipid bile, cholestasis, cholelithiasis and progressive liver injury, highlighting distinct partitioning of lipid transport functions among hepatic ABC proteins for bile formation and systemic lipoprotein metabolism. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Borst et al., Annu Rev Biochem 2000, ABC transporters in lipid transport; Alberts, Chapter 11.

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Membrane Transportation

Which ABC transporter plays a role in cholesterol and phospholipid transport in hepatocytes?

Cholesterol and phospholipid homeostasis in hepatocytes involves coordinated action of several ABC transporters at canalicular membrane. ABCB4 MDR3 flips phosphatidylcholine from inner to outer leaflet making it available for bile salt extraction forming mixed micelles that protect biliary epithelium and solubilize cholesterol exported by ABCG5/G8 heterodimer. ABCB1 MDR1 primarily exports hydrophobic xenobiotics and drugs, not bulk phospholipid for bile formation, though it can translocate short-chain lipids in vitro. ABCA1, not listed among options but relevant, mediates phospholipid and cholesterol efflux to apolipoprotein A1 for nascent HDL biogenesis at basolateral side. SGLT family members SLC5A1 and SLC5A2 are sodium-glucose symporters secondary active, unrelated to lipid transport. Mutations in ABCB4 cause progressive familial intrahepatic cholestasis type 3 with low biliary phospholipid, cholangitis and cholelithiasis, while ABCG5/G8 defects cause sitosterolemia, demonstrating specific partitioning of cholesterol and phosphatidylcholine pathways among distinct ABC transporters in liver physiology and disease. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Oude Elferink & Paulusma 2007, Hepatic lipid transporters; Nicolaou et al., J Hepatol 2012.

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Membrane Transportation

The ABC transporter CFTR is unique because:

CFTR ABCC7 stands as atypical member of ABC superfamily because evolution transformed an active exporter scaffold into ATP-gated ion channel. Typical ABC exporters use ATP binding and hydrolysis to drive large TMD movements translocating substrate uphill. In CFTR, transmembrane domains create chloride-selective pore allowing passive flow down electrochemical gradient when open, while nucleotide-binding domains still bind ATP and regulate gating rather than transport solute stoichiometrically. Two ATP sites: NBD1 non-canonical hydrolyzes slowly, NBD2 drives channel closure upon hydrolysis. Additionally R domain phosphorylation by protein kinase A is required to permit NBD dimerization and opening. Thus transport is not solute-coupled uphill pumping but gated diffusion of Cl- and HCO3-, regulating airway surface liquid volume. This unique adaptation explains why potentiators like ivacaftor increase open probability and correctors assist folding, strategies distinct from inhibitors targeting classical ABC efflux pumps that expel hydrophobic drugs. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Gadsby et al., Nature 2006, Unique CFTR channel mechanism; Hwang & Sheppard, 2009.

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Membrane Transportation

Which ABC transporter is associated with multidrug resistance in cancer?

Cancer multidrug efflux is dominated not by CFTR or P-type ATPases but by ABC transporters that expel amphipathic chemotherapeutics from cytoplasm. ABCB1 P-glycoprotein MDR1 is best studied, overexpressed in colonic, renal, adrenocortical carcinomas and after chemotherapy induction in leukemia, lymphoma and breast cancer. Its polyspecific hydrophobic binding chamber accommodates doxorubicin, daunorubicin, vinblastine, vincristine, paclitaxel and etoposide causing cross-resistance. ABCC1 MRP1 exports glutathione conjugates of drugs, ABCG2 BCRP expels mitoxantrone and topotecan. Expression correlates with poor response, reduced disease-free interval, and altered pharmacokinetics influencing oral bioavailability and blood-brain barrier penetration. Mechanistically transporter uses ATP hydrolysis at two NBDs to switch from inward-high affinity to outward-low affinity releasing drug. Inhibitors verapamil, cyclosporine, tariquidar attempted clinically to reverse resistance, but toxicity and redundant export pathways limit success, prompting development of nanoparticle formulations to bypass efflux and selective modulators. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Gottesman et al., Annu Rev Med 2002, MDR in cancer; Szakacs et al., Nat Rev Drug Discov 2006.

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Membrane Transportation

The ABC transporter in plants responsible for detoxification of xenobiotics is found in:

Plants lack animal Na+/K+ gradient but generate proton motive force via plasma membrane P-type H+-ATPase and vacuolar V-type H+-ATPase and pyrophosphatase, driving uptake and compartmentalization of metabolites. Detoxification of xenobiotics, herbicides, and endogenous secondary metabolites employs ATP-binding cassette transporters localized predominantly in vacuolar membrane tonoplast and plasma membrane. Vacuolar ABCC members often called MRP-like transport glutathione S-conjugates, phytochelatin heavy metal complexes, and glucuronide conjugates into vacuole for sequestration, reducing cytosolic toxicity. Plasma membrane ABCG members extrude antimicrobial terpenoids and cuticular lipids. In Arabidopsis, AtABCC1 and AtABCC2 are classic vacuolar transporters conferring tolerance to arsenic and cadmium phytochelatin complexes. Tonoplast localization enables long-term storage away from sensitive metabolic processes, paralleling hepatic canalicular ABC exporters. Exclusive peroxisomal or endoplasmic reticulum residence is not typical for xenobiotic detoxification ABC pumps; tonoplast and plasma membrane are major detox sites contributing to environmental adaptation. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Martinoia et al., Planta 2002, Vacuolar transporters; Kang et al., PNAS 2011, ABC in detox.

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Membrane Transportation

Which ABC transporter is found in the liver and is responsible for phosphatidylcholine transport?

Hepatic bile secretion relies on coordinated action of several canalicular ABC transporters in hepatocytes. ABCB11 BSEP exports bile acids, ABCG5/ABCG8 heterodimer exports cholesterol, and ABCB4 previously called MDR2 in mice and MDR3 in humans is dedicated to phosphatidylcholine. ABCB4 acts as ATP-dependent floppase moving phosphatidylcholine from inner cytoplasmic leaflet to outer exoplasmic leaflet of canalicular membrane where bile salt micelles extract it into biliary space forming mixed micelles. This phospholipid shield protects cholangiocyte membranes from detergent attack by hydrophobic bile salts and solubilizes cholesterol preventing crystal formation. Mice lacking Abcb4 develop spontaneous cholangitis, bile duct proliferation, and biliary fibrosis due to regurgitation of low-phospholipid toxic bile. In humans, biallelic loss causes progressive familial intrahepatic cholestasis type 3 presenting in childhood, while heterozygous variants predispose to low-phospholipid-associated cholelithiasis, intrahepatic cholestasis of pregnancy and drug-induced cholestasis, demonstrating essential protective role in hepatic physiology. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.

Ref: Oude Elferink & Paulusma, Pflugers Arch 2007, ABCB4 bile phospholipid secretion.

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Membrane Transportation

The ABC transporter superfamily is involved in:

ABC transporter superfamily represents large gene family with 48 members in humans associated with inherited disorders and drug resistance, present also in bacteria where many function as importers. Core architecture comprises two transmembrane domains typically six helices each forming substrate translocation chamber with diverse selectivity filters, and two nucleotide binding domains located cytoplasmically containing Walker A phosphate binding P-loop, Walker B magnesium coordination hhhhhD, signature C motif LSGGQ characteristic of ABC plus H-loop and Q-loop coordinating ATP. Transport cycle involves ATP binding inducing tight dimerization of nucleotide binding domains sandwiching nucleotides, converting transmembrane domains from inward to outward facing releasing substrate, ATP hydrolysis and ADP plus phosphate release resetting to inward. This mechanism promiscuous for chemically diverse substrates: anionic chloride via CFTR, cationic lipids cholesterol via ABCA1 contributing to HDL formation, phosphatidylcholine via ABCB4, bile salts via ABCB11, peptides via TAP for MHC class I presentation, and myriad hydrophobic drugs including vinca alkaloids, anthracyclines exported by P-glycoprotein MDR1 conferring multidrug resistance in cancer and bacterial antibiotic resistance. Therefore transport of lipophilic molecules and multidrug resistance exemplifies family function.

Ref: Higgins, ABC Transporters Annual Review Cell Biol, Lipophilic Drug Export and MDR Phenotype.

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Membrane Transportation

What is the function of the ABC transporter superfamily?

ATP-binding cassette superfamily constitutes one of largest and most functionally diverse transporter groups present in all domains of life, with 48 members in human genome implicated in cystic fibrosis, Tangier disease and multidrug resistance. Each functional unit minimal consists of two transmembrane domains typically six helices each forming translocation pathway determining substrate specificity and two cytosolic nucleotide binding domains containing highly conserved Walker A P-loop GXXGXGKS binding phosphate, Walker B hhhhDE binding magnesium, signature motif LSGGQ unique to ABC, and Q-loop. Upon ATP binding NBDs dimerize sandwiching two ATP, inducing conformational switch from inward facing to outward facing releasing substrate, then hydrolysis resets. Transported substrates range widely: ions chloride via CFTR channel, inorganic molecules, amino acids, sugars, lipids including cholesterol and phospholipids, bile acids, peptides for antigen presentation, siderophores, and structurally unrelated chemotherapeutic drugs effluxed by P-glycoprotein ABCB1 conferring multidrug resistance in cancer. Some members regulate channels. Thus function extends far beyond ion conduction to broad organic export and import.

Ref: Dean et al., Genome Research, ABC Transporter Superfamily Substrates and Multidrug Resistance.

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