ABC (ATP-binding cassette) transporters are primary active membrane proteins that translocate solutes (allocrites) across lipid bilayers. The prototypical ABC transporter consists of four domains: two cytoplasmic NBDs (nucleotide-binding domains) and two TMDs (transmembrane domains). The NBDs, whose primary sequence is highly conserved throughout the superfamily, bind and hydrolyse ATP to power the transport cycle. The TMDs, whose primary sequence and protein fold can be quite disparate, form the translocation pathway across the membrane and generally (but not always) determine allocrite specificity. Structure determination of ABC proteins initially took advantage of the relative ease of expression and crystallization of the hydrophilic bacterial NBDs in isolation from the transporter complex, and revealed detailed information on the structural fold of these domains, the amino acids involved in the binding and hydrolysis of nucleotide, and the head-to-tail arrangement of the NBD–NBD dimer interface. More recently, several intact transporters have been crystallized and three types have, so far, been characterized: type I and II ABC importers, and ABC exporters. All three are present in prokaryotes, but only the ABC exporters appear to be present in eukaryotes. Their structural determination has provided insight into the mechanisms of energy and signal transduction between the NBDs and TMDs (i.e. between the ATP- and allocrite-binding sites) and, for some, the nature of the allocrite-binding site(s) within the TMDs. In this chapter, we focus primarily on the ABC exporters and describe the structural, biochemical and biophysical evidence for and against the controversial bellows-like mechanism proposed for allocrite efflux.

For a primary active pump, such as the human ATP‐binding‐cassette (ABC) transporter ABCB1, coupling of drug‐binding by the two transmembrane domains (TMDs) to the ATP catalytic cycle of the two nucleotide‐binding domains (NBDs) is fundamental to the transport mechanism, but is poorly understood at the biochemical level. Structure data suggest that signals are transduced through intracellular loops of the TMDs that slot into grooves on the NBDs. At the base of these grooves is the Q loop. We therefore mutated the eponymous glutamine in one or both NBD Q loops and measured the effect on conformation and function by using a conformation‐sensitive antibody (UIC2) and a fluorescent drug (Bodipy‐verapamil), respectively. We showed that the double mutant is trapped in the inward‐open state, which binds the drug, but cannot couple to the ATPase cycle. Our data also describe marked redundancy within the transport mechanism, because single‐Q‐loop mutants are functional for Bodipy‐verapamil transport. This result allowed us to elucidate transduction pathways from twin drug‐binding cavities to the Q loops using point mutations to favor one cavity over the other. Together, the data show that the Q loop is the central flexion point where the aspect of the drug‐binding cavities is coupled to the ATP catalytic cycle.—Zolnerciks, J. K., Akkaya, B. G., Snippe, M., Chiba, P., Seelig, A., Linton, K. J., The Q loops of the human multidrug resistance transporter ABCB1 are necessary to couple drug binding to the ATP catalytic cycle. FASEB J. 28, 4335–4346 (2014). www.fasebj.org

ABCB4 flops phosphatidylcholine into the bile canaliculus to protect the biliary tree from the detergent activity of bile salts. Homozygous-null ABCB4 mutations cause the childhood liver disease, progressive familial intrahepatic cholestasis, but cause and effect is less clear, with many missense mutations linked to less severe cholestatic diseases. ABCB4S320F, in particular, is described in 13 patients, including in heterozygosity with ABCB4A286V, ABCB4A953D, and null mutants, whose symptoms cover the spectrum of cholestatic disease. We sought to define the impact of these mutations on the floppase, explain the link with multiple conditions at the molecular level, and investigate the potential for reversal. ABCB4S320F, ABCB4A286V, and ABCB4A953D expression was engineered in naïve cultured cells. Floppase expression, localization, and activity were measured by western blot, confocal microscopy, and lipid transport assays, respectively. ABCB4S320F was fully active for floppase activity but expression at the plasma membrane was reduced to 50%. ABCB4A286V expressed and trafficked efficiently but could not flop lipid, and ABCB4A953D expressed poorly and was impaired in floppase activity. Proteasome inhibition stabilized nascent ABCB4S320F and ABCB4A953D but did not improve plasma membrane localization. Cyclosporin-A improved plasma membrane localization of both ABCB4S320F and ABCB4A953D, but inhibited floppase activity. Conclusion : The level of ABCB4 functionality correlates with, and is the primary determinant of, cholestatic disease severity in these patients. ABCB4S320F homozygosity, with half the normal level of ABCB4, is the tipping point between more benign and potentially fatal cholestasis and makes these patients more acutely sensitive to environmental effects. Cyclosporin-A increased expression of ABCB4S320F and ABCB4A953D, suggesting that chemical chaperones could be exploited for therapeutic benefit to usher in a new era of personalized medicine for patients with ABCB4-dependent cholestatic disease. (Hepatology 2014;59:1921–1931)

Multidrug resistance-associated protein 2 (MRP2/ABCC2) is a polyspecific efflux transporter of organic anions expressed in hepatocyte canalicular membranes. MRP2 dysfunction, in Dubin-Johnson syndrome or by off-target inhibition, for example by the uricosuric drug probenecid, elevates circulating bilirubin glucuronide and is a cause of jaundice. Here, we determine the cryo-EM structure of rat Mrp2 (rMrp2) in an autoinhibited state and in complex with probenecid. The autoinhibited state exhibits an unusual conformation for this class of transporter in which the regulatory domain is folded within the transmembrane domain cavity. In vitro phosphorylation, mass spectrometry and transport assays show that phosphorylation of the regulatory domain relieves this autoinhibition and enhances rMrp2 transport activity. The in vitro data is confirmed in human hepatocyte-like cells, in which inhibition of endogenous kinases also reduces human MRP2 transport activity. The drug-bound state reveals two probenecid binding sites that suggest a dynamic interplay with autoinhibition. Mapping of the Dubin-Johnson mutations onto the rodent structure indicates that many may interfere with the transition between conformational states.

Bile is synthesized in the liver and is essential for the emulsification of dietary lipids and lipid-soluble vitamins. It is a complex mixture of amphiphilic bile acids (BAs; which act as detergent molecules), the membrane phospholipid phosphatidylcholine (PC), cholesterol and a variety of endogenous metabolites and waste products. Over the last 20 years, the combined effort of clinicians, geneticists, physiologists and biochemists has shown that each of these bile components is transported across the canalicular membrane of the hepatocyte by its own specific ATP-binding cassette (ABC) transporter. The bile salt export pump (BSEP) ABCB11 transports the BAs and drives bile flow from the liver, but it is now clear that two lipid transporters, ABCB4 (which flops PC into the bile) and the P-type ATPase ATP8B1/CDC50 (which flips a different phospholipid in the opposite direction) play equally critical roles that protect the biliary tree from the detergent activity of the bile acids. Understanding the interdependency of these lipid floppases and flippases has allowed the development of an assay to measure ABCB4 function. ABCB4 harbours numerous mis-sense mutations which probably reflects the spectrum of liver disease rooted in ABCB4 aetiology. Characterization of the effect of these mutations at the protein level opens the possibility for the development of personalized prognosis and treatment.