#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 AT

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

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

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

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 anthracycl

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 P

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

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