Abstract
ABCB4 is an ATP-binding cassette transporter that extrudes phosphatidylcholine into the bile canaliculi of the liver. Its dysfunction or inhibition by drugs can cause severe, chronic liver disease or drug-induced liver injury. We determined the cryo-EM structure of nanodisc-reconstituted human ABCB4 trapped in an ATP-bound state at a resolution of 3.2 Å. The nucleotide binding domains form a closed conformation containing two bound ATP molecules, but only one of the ATPase sites contains bound Mg2+. The transmembrane domains adopt a collapsed conformation at the level of the lipid bilayer, but we observed a large, hydrophilic and fully occluded cavity at the level of the cytoplasmic membrane boundary, with no ligand bound. This indicates a state following substrate release but prior to ATP hydrolysis. Our results rationalize disease-causing mutations in human ABCB4 and suggest an 'alternating access' mechanism of lipid extrusion, distinct from the 'credit card swipe' model of other lipid transporters.
🔬 Techniques
✨ Fluorophores
🧪 Sample Preparation
🔬 Cell Lines
💻 Software Details
💾 Data Repositories
🏛️ Research Organizations (ROR)
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📊 Figures
Figure 1
Schematic model of ABCB4. ABCB4 consists of twelve TMHs spanning the plasma membrane and two cytosolic NBFs containing the Walker A, Walker B, and signature motifs. N and C indicate the N- and C-termi...
Figure 2
Amino acid sequence alignment of human ABCB1 and ABCB4. Identical amino acids are red . Functional domains are shown above the sequence alignment. TMHs are predicted from the crystal structure of mous...
Figure 3
Formation of mixed micelles of bile salts, phospholipids, and cholesterol in bile canaliculus. ABCB11 mediates the efflux of bile salts into bile. The bile salt monomers are essential for the phosphol...
Figure 4
Phospholipid substrates for ABCB4. (a) C 6 -NBD-PC, (b) PC, (c) PE, and (d) SM.
Figure 5
Inhibitors of ABCB4: (a) cyclosporine A, (b) valspodar, (c) verapamil, (d) vinblastine, (e) paclitaxel, and (f) itraconazole.
Figure 6
Model of molecular mechanism of ABCB4-mediated transport. (I) A substrate, mainly PC, enters the binding pocket of ABCB4 from the inner leaflet through the gaps between TMHs. (II) Binding and/or hydro...
Figure images are served from the NIH/NLM PubMed Central Open Access Subset or Europe PMC; copyright remains with the publishers and authors.
💬 Discussion
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