#ABC transporters

Membrane Transportation

The Walker B sequence in ABC transporters is responsible for:

Walker B motif in ABC nucleotide-binding domains follows consensus hhhhDE where h denotes hydrophobic residue. This segment forms beta-strand adjacent to Walker A P-loop. The aspartate carboxylate coordinates Mg2+ ion essential for ATP binding and neutralizing negative charge of phosphate chain, while downstream glutamate acts as catalytic base activating water for nucleophilic attack on gamma phosphate during hydrolysis. Structural studies of MalK, Sav1866 and P-glycoprotein show glutamate positioned near gamma phosphate; mutation to glutamine yields hydrolysis-deficient protein that binds ATP tightly and traps NBD dimer in pre-hydrolytic state, useful for crystallizing outward-facing conformation. Walker A provides lysine for phosphate contact and is traditionally described as ATP binding motif, whereas Walker B contributes chemistry of hydrolysis and Mg2+ coordination, though textbook questions sometimes swap emphasis. Disruption of Walker B abolishes ATPase activity, prevents resetting to inward-facing state, halts drug transport cycle, and in bacterial importers prevents substrate delivery to transmembrane domains.

Ref: Oldham & Chen, J Mol Biol 2008, ABC NBD catalytic mechanism; Jones & George, 2004, Walker B role.

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Membrane Transportation

Which of the following is not a function of ABC transporters?

ABC transporters are defined as primary active systems utilizing ATP binding and hydrolysis at conserved nucleotide-binding domains to drive uphill movement of substrate. Their tasks include export of lipophilic drugs and xenobiotics by ABCB1, ABCC1, ABCG2 conferring multidrug resistance, phosphatidylcholine flipping by ABCB4 for biliary micelle formation, bile salt export by ABCB11, cholesterol and phospholipid efflux by ABCA1 to apolipoprotein A1 in HDL biogenesis, and peptide delivery by TAP1/TAP2 into endoplasmic reticulum for antigen presentation. In prokaryotes they import sugars, amino acids, micronutrients with high affinity binding proteins. Because transport absolutely requires ATP hydrolysis and NBD dimerization cycle, ATP-independent diffusion down concentration gradient through simple lipid solubility or passive channels cannot be considered ABC-mediated function. Passive diffusion lacks saturability, energy coupling and conserved Walker motifs, distinguishing facilitated or simple diffusion from ATP-switch alternating-access mechanism central to ABC superfamily and its clinical relevance. 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: Locher, Nature 2016, ABC mechanisms; Dean, Genome Res 2001, ABC functions.

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Membrane Transportation

Which of the following describes the Walker A sequence in ABC transporters?

Walker A motif, discovered by John Walker through alignment of ATP synthase beta subunit, kinases and ABC proteins, is phosphate-binding P-loop with consensus GXXGXGKT/S where X denotes any residue. In ABC nucleotide-binding domains this loop lies at N-terminus of alpha-helix following central beta-sheet, with invariant lysine side chain forming ion pair with beta and gamma phosphates of ATP and conserved threonine or serine coordinating Mg2+ ion essential for hydrolysis. Together with Walker B aspartate that chelates Mg2+ and signature LSGGQ from opposite NBD completing active site, Walker A cradles nucleotide in bipartite sandwich dimer. Mutagenesis studies show lysine to methionine or arginine substitutions in P-glycoprotein Walker A abolish ATP binding, prevent NBD closure, eliminate drug-stimulated ATPase activity and trap transporter inward-facing. While Walker B glutamate acts as catalytic base polarizing water for nucleophilic attack on gamma phosphate, initial recognition and positioning of ATP depends predominantly on Walker A P-loop, explaining conservation across ATPases and kinases.

Ref: Walker et al., EMBO J 1982, P-loop motifs; Jones & George, Cell Mol Life Sci 2004, NBD mechanism.

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Membrane Transportation

What is the function of ABC transporters?

ATP-binding cassette superfamily includes hundreds of members across all domains of life encoded by 49 human genes divided into ABCA to ABCG subfamilies. Core architecture comprises two transmembrane domains forming substrate path and specificity filter, and two cytosolic nucleotide-binding domains containing Walker A P-loop GXXGXGKS/T for phosphate binding, Walker B hhhhDE coordinating Mg2+ and catalytic glutamate, and signature LSGGQ hallmark. ATP binding drives NBD dimerization sandwiching two ATPs, converting TMDs to outward-facing conformation; hydrolysis and release of ADP and Pi resets inward-facing state enabling vectorial translocation against gradient. In bacteria they primarily serve as high-affinity importers for sugars, amino acids, phosphate, vitamins and siderophores using periplasmic binding proteins. In eukaryotes they act as exporters moving phospholipids, sterols, bile acids, retinoids, heme, xenobiotics, and immunologically TAP1/TAP2 transport peptides for MHC I presentation. CFTR ABCC7 even evolved into chloride channel gated by ATP binding, illustrating diversification of same ATP-switch engine for varied functions.

Ref: Dean et al., Genome Res 2001, Human ABC family; Rees et al., Nat Rev Mol Cell Biol 2009.

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Membrane Transportation

Which transporter is involved in multidrug resistance (MDR)?

Multidrug resistance in oncology frequently originates from increased efflux reducing intracellular drug accumulation below cytotoxic threshold. ATP-binding cassette transporters, particularly ABCB1 P-glycoprotein, ABCC1 MRP1 and ABCG2 BCRP, are major contributors. They are primary active pumps using two ATP hydrolysis events per cycle to switch transmembrane domains from high-affinity inward-facing to low-affinity outward-facing state, literally vacuuming hydrophobic substrates from inner membrane leaflet. Their drug-binding pocket is large, flexible and polyspecific, accommodating anthracyclines, vinca alkaloids, taxanes, epipodophyllotoxins, tyrosine kinase inhibitors and immunosuppressants despite distinct structures. Physiologically these transporters localize at intestine, liver canaliculus, kidney tubule, blood-brain barrier and placenta providing xenobiotic protection. Overexpression after chemotherapy through gene amplification or transcriptional activation via pregnane X receptor leads to cross-resistance to many drugs, alters pharmacokinetics of orally administered compounds, and motivates development of third-generation inhibitors like tariquidar, though clinical success remains limited by toxicity and compensatory mechanisms. 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., Nat Rev Cancer 2002, Multidrug resistance; Ambudkar et al., Annu Rev Pharmacol 1999.

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Membrane Transportation

The CFTR protein, which is defective in cystic fibrosis, is part of which transporter family?

Cystic fibrosis most common lethal autosomal recessive disorder in Caucasian populations results from mutations in CFTR gene on chromosome 7q31.2 encoding cystic fibrosis transmembrane conductance regulator. Biochemical classification places CFTR within C subfamily of ATP binding cassette transporters ABCC7 despite functional divergence. Topology includes two membrane spanning domains each six helices forming anion selective pore, two nucleotide binding domains NBD1 and NBD2 containing Walker motifs and LSGGQ signature dimerizing upon ATP binding, and unique regulatory R domain with multiple PKA phosphorylation sites controlling gating. Unlike typical ABC exporter that alternates access to pump substrates, CFTR functions as low conductance chloride channel allowing passive efflux of Cl- and HCO3- down electrochemical gradient when phosphorylated R domain permits NBD dimerization and ATP binding opens pore. Most common mutation deletion Phe508 in NBD1 impairs folding trafficking degraded by ER quality control. Potentiators ivacaftor increase open probability, correctors lumacaftor aid folding. Hence defective protein belongs to ABC transporter family.

Ref: Riordan et al., Science 1989, CFTR Gene Identification - ABC Transporter Family Classification.

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