MRP subfamily ABC transporters from plants and yeast

This review addresses recent advances in the understanding of the function and organization of one subfamily of ATP-binding cassette (ABC) transporters, the multidrug resistance-associated proteins (MRPs), from plants and yeast. So named because of the phenotypes conferred by their animal prototypes, many MRPs are glutathione-conjugate or multi-specific organic anion Mg2+-ATPases active in the transport of glutathionated compounds and other bulky amphipathic anions. Acting downstream of the first two phases of toxin detoxification, typified by, but not limited to the reactions catalysed by the cytochromes P450 and glutathione S-transferases, respectively, processes that likely converge and depend on the MRPs from plants and yeast include the detoxification of herbicides and other organic xenobiotics, the alleviation of oxidative damage, the storage of endotoxins, heavy metal sequestration, and the vacuolation of natural pigments. Distinguished by a unique domain organization and the facility to mediate MgATP-energized transport, the MRPs are of profound significance in two respects. First, the fact that the MRPs, like most ABC transporters, are energized directly by MgATP rather than by, for example, a preformed H+ gradient, offers a new, non-chemiosmotic perspective on the molecular basis of energy-dependent solute transport in plants. Second, given the existence of 29 ABC protein genes in the yeast genome alone, characterization of the MRPs—the first class of ABC transporters from a plant source to have been defined at both the molecular and biochemical levels—indicates that many more ABC transporters and ABC transporter-mediated processes remain to be discovered in plants.