Structure, function, and inhibition of catalytically asymmetric ABC transporters: Lessons from the PDR subfamily

ATP-binding cassette (ABC) superfamily comprises a large group of ubiquitous transmembrane proteins that play a crucial role in transporting a diverse spectrum of substrates across cellular membranes. They participate in a wide array of physiological and pathological processes including nutrient uptake, antigen presentation, toxin elimination, and drug resistance in cancer and microbial cells. ABC transporters couple ATP binding and hydrolysis to undergo conformational changes allowing substrate translocation. Within this superfamily, a set of ABC transporters has lost the capacity to hydrolyze ATP at one of their nucleotide-binding sites (NBS), called the non-catalytic NBS, whose importance became evident with extensive biochemistry carried out on yeast pleiotropic drug resistance (PDR) transporters. Recent single-particle cryogenic electron microscopy (cryo-EM) advances have further catapulted our understanding of the architecture of these pumps. We provide here a comprehensive overview of the structural and functional aspects of catalytically asymmetric ABC pumps with an emphasis on the PDR subfamily. Furthermore, given the increasing evidence of efflux-mediated antifungal resistance in clinical settings, we also discuss potential grounds to explore PDR transporters as therapeutic targets.