FROM VACUOLAR GS-X PUMPS TO MULTISPECIFIC ABC TRANSPORTERS

While the concept of H+-coupling has dominated studies of energy-depen dent organic solute transport in plants for over two decades, recent s tudies have demonstrated the existence of a group of organic solute tr ansporters, belonging to the ATP-binding cassette (ABC) superfamily, t hat are directly energized by MgATP rather than by a transmembrane H+- electrochemical potential difference. Originally identified in microbi al and animal cells, the ABC superfamily is one of the largest and mos t widespread protein families known. Competent in the transport of a b road range of substances including sugars, peptides, alkaloids, inorga nic anions, and lipids, all ABC transporters are constituted of one or two copies each of an integral membrane sector and cytosolically orie nted ATP-binding domain. To date, two major subclasses, the multidrug resistance-associated proteins (MRPs) and multidrug resistance protein s (MDRs) (so named because of the phenotypes conferred by their animal prototypes), have been identified molecularly in plants. However, onl y the MRPs have been defined functionally. This review therefore focus es on the functional capabilities, energetics, organization, and regul ation of the plant MRPs. Otherwise known as GS-X pumps, or glutathione -conjugate or multispecific organic anion Mg2+-ATPases, the MRPs are c onsidered to participate in the transport of exogenous and endogenous amphipathic anions and glutathionated compounds from the cytosol into the vacuole. Encoded by a multi-gene family and possessing a unique do main organization, the types of processes that likely converge and dep end on plant MRPs include herbicide detoxification, cell pigmentation, the alleviation of oxidative damage, and the storage of antimicrobial compounds. Additional functional capabilities might include channel r egulation or activity, and/or the transport of heavy metal chelates. T he identification of the MRPs, in particular, and the demonstration of a central role for ABC transporters, in general, in plant function no t only provide fresh insights into the molecular basis of energy-depen dent solute transport but also offer the prospect for manipulating and investigating many fundamental processes that have hitherto evaded an alysis at the transport level.