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#ABCB4

2 public questions tagged with this topic.

Membrane Transportation

ABCB4 (MDR2) primarily functions as a:

ABCB4 designated MDR2/MDR3 in rodents and humans localizes to canalicular domain of hepatocytes and functions as phosphatidylcholine floppase, not as glucose transporter. Glucose symport is mediated by SLC5 family SGLT1 and SGLT2 which cotransport Na+ and glucose using sodium gradient rather than direct ATP hydrolysis, distinct from ABC primary pumps. ABCB4 mechanism follows canonical ABC cycle: ATP binding dimerizes nucleotide-binding domains, shifts transmembrane domains outward, flips phosphatidylcholine from cytosolic leaflet to lumenal leaflet where bile acids extract it into mixed micelles that buffer bile salt detergent activity and solubilize cholesterol. Loss-of-function produces toxic, phospholipid-depleted bile injuring biliary epithelium, causing progressive familial intrahepatic cholestasis type 3, gallstones, and pregnancy cholestasis. Accurate discrimination between secondary active SLC symporters for nutrients and primary ABC floppases for lipids is essential for understanding liver pathophysiology and correctly interpreting transporter classification in examinations. 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: Davit-Spraul et al., J Hepatol 2010, ABCB4 deficiency; Alberts, 7th ed., Chapter 11 flippases vs SLC.

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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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