ATP-dependent transport of organic anions in secretory vesicles of Saccharomyces cerevisiae.

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Secretory mutants (sec1, sec6) of Saccharomyces cerevisiae accumulate large pools of secretory vesicles at the restrictive temperature (37 degrees C) because of a block in the delivery of vesicles to the cell surface. We report that secretory vesicles isolated from sec mutants exhibit ATP-dependent uptake of two classes of organic anions that are substrates for the canalicular carriers of mammalian liver. Transport of the bile acid taurocholate (TC) and the glutathione conjugate of 1-chloro-2,4-dinitrobenzene (GS-DNP) into vesicles was temperature dependent and saturable and required ATP and Mg2+. Estimates of Km and Vmax were 177 microM and 1.2 nmol.min-1.mg-1 and 262 microM and 0.53 nmol.min-1.mg-1 for TC and GS-DNP, respectively. TC and GS-DNP did not complete for transport. TC transport was sensitive to vanadate and 4,4'-diisothiocyanostilbene-2,2'-disulfonate, inhibited by glycocholate, and retained partial activity when UTP and GTP, but not nonhydrolyzable ATP analogues, replaced ATP. Dissipation of the electrochemical potential with a nitrate buffer and ionophores partially decreased (30-40%) the transport of both anions. Direct testing of the influence of membrane potential was performed in sec6-4 mutants, in which the expression of electrogenic [H+]ATPase activity is reduced by > 85% in glucose-containing medium. Vesicles from sec6-4 retained full activity for ATP-dependent TC and GS-DNP transport. These results indicate that the transporters operate independently of the membrane potential and that ATP is required. These findings reveal that yeast possess separate ATP-dependent transport mechanisms for elimination of bile acids and glutathione conjugates. The mechanisms are functionally similar to those present in mammalian systems.

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