Magnesium Adenosine 5[prime]-Triphosphate-Energized Transport of Glutathione-S-Conjugates by Plant Vacuolar Membrane Vesicles

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Abstract
By characterization of the uptake of glutathione-S-conjugates, principally dinitrophenyl-S-glutathione (DNP-GS), by vacuolar membrane vesicles, we demonstrate that a subset of energy-dependent transport processes in plants are not H+-coupled but instead are directly energized by MgATP. The most salient features of this transport pathway are: (a) its specific, obligate requirement for MgATP as energy source; (b) the necessity for hydrolysis of the [gamma]-phosphate of MgATP for uptake; (c) the insensitivity of uptake to uncouplers of the transtonoplast H+ gradient (carbonylcyanide 4-trifluoromethoxyphenylhydrazone, gramicidin-D, and NH4Cl); (d) its pronounced sensitivity to vanadate and partial inhibition by vinblastine and verapamil; (e) the lack of chemical modification of DNP-GS either during or after transport; (f) the capacity of S-conjugates of chloroacetanilide herbicides, such as metolachlor-GS, but not free herbicide, to inhibit uptake; and (g) the ability of vacuolar membrane vesicles purified from a broad range of plant species, including Arabidopsis, Beta, Vigna, and Zea, to mediate MgATP-dependent, H+-electrochemical potential difference-independent DNP-GS uptake. On the basis of these findings it is proposed that the transport of DNP-GS across the vacuolar membrane of plant cells is catalyzed by a glutathione-conjugate transporter that directly employs MgATP rather than the energy contained in the transtonoplast H+-electrochemical potential difference to drive uptake. The broad distribution of the vacuolar DNP-GS transporter and its inhibition by metolachlor-GS are consistent with the notion that it plays a general role in the vacuolar sequestration of glutathione-conjugable cytotoxic agents.