Structure and mechanism of ABC transporters

Abstract

All living organisms depend on primary and secondary membrane transport for the supply of external nutrients and removal or sequestration of unwanted (toxic) compounds. Due to the chemical diversity of cellular molecules, it comes as no surprise that a significant part of the proteome is dedicated to the active transport of cargo across the plasma membrane or the membranes of subcellular organelles. Transport against a chemical gradient can be driven by, for example, the free energy change associated with ATP hydrolysis (primary transport), or facilitated by the potential energy of the chemical gradient of another molecule (secondary transport). Primary transporters include the rotary motor ATPases (F-, A-, and V-ATPases), P-type ATPases and a large family of integral membrane proteins referred to as "ABC" (ATP binding cassette) transporters. ABC transporters are widespread in all forms of life and are characterized by two nucleotide-binding domains (NBD) and two transmembrane domains (TMDs). ATP hydrolysis on the NBD drives conformational changes in the TMD, resulting in alternating access from inside and outside of the cell for unidirectional transport across the lipid bilayer. Common to all ABC transporters is a signature sequence or motif, LSGGQ, that is involved in nucleotide binding. Both importing and exporting ABC transporters are found in bacteria, whereas the majority of eukaryotic family members function in the direction of export. Recent progress with the X-ray crystal structure determination of a variety of bacterial and eukaryotic ABC transporters has helped to advance our understanding of the ATP hydrolysis-driven transport mechanism but has also illustrated the large structural and functional diversity within the family.

Figure 1.. Structural features of ABC transporters

(A) Outward-facing maltose transporter with ADP•VO₄ in catalytic sites and maltose bound to the transmembrane domain ([TMD] 3puv.pdb) [23]. (B) Homodimeric exporter Sav1866 from Staphylococcus aureus in the outward-facing conformation with ADP in catalytic sites (2hyd.pdb) [37]. (C) P-glycoprotein in the inward-facing conformation with an inhibitor molecule bound at the TMDs (4m2t.pdb) [27]. (D) The nucleotide-binding domain (NBD) sandwich dimer of the maltose transporter (MalK) as seen from the cytoplasmic side. (E) The cavity formed by the TMDs of outward-facing Sav1866. Note that the cavity does not provide access to the outer leaflet of the lipid bilayer. (F) Cross-section through the TMDs of glycoprotein showing the two inhibitor molecules. ABC, ATP-binding cassette; MRP, multidrug resistance associated protein; NBD, nucleotide-binding domain; TMD, transmembrane domain.

Figure 2.. Schematic of the mechanism of ABC exporters and importers

(A) The inward-facing exporter binds substrate “D” (drug) from the cytoplasm or the inner leaflet of the bilayer. After binding two molecules of MgATP, the nucleotide-binding domains (NBDs) dimerize and switch the transmembrane domain (TMDs) from the inward- to the outward-facing conformation, followed by the release of the drug to the extracellular milieu. ATP hydrolysis, ADP/Pi release and NBD dissociation resets the transporter to the inward-facing conformation. Note that there are likely intermediate conformations, some asymmetric, that have not yet been resolved by crystallography. (B) The inward-facing type I transporter (e.g., MalFGK₂) binds to the substrate containing periplasmic binding protein and two molecules of MgATP. NBDs dimerize and result in the outward-facing conformation. Substrate leaves the binding protein and binds to the TMDs mid-membrane. ATP is hydrolyzed and product release, together with NBD dissociation, resets the transporter to the inward-facing conformation. (C) The outward facing type II importer (e.g., BtuCD) binds to substrate binding protein and two molecules of MgATP. Dimerization of the NBDs results in the occluded conformation with substrate confined to a sealed cavity mid-membrane. Subsequent ATP hydrolysis and NBD dissociation allows substrate to escape into the cytoplasm. A fourth, asymmetric, conformation as seen for BtuCDF is not shown. Abbreviations: ABC, ATP-binding cassette; NBD, nucleotide binding domain; TMD, transmembrane domain.