The Saccharomyces cerevisiae weak-acid-inducible ABC transporter pdr12 transports fluorescein and preservative anions from the cytosol by an energy-dependent mechanism

Growth of Saccharomyces cerevisiae in the presence of the weak-acid preserv ative sorbic acid results in the induction of the ATP-binding cassette (ABC ) transporter Pdr12 in the plasma membrane (P, Piper, Y, Mahe, S, Thompson, R. Pandjaitan, C. Holyoak, R, Egner, M. Muhlbauer, P. Coote, and K. Kuchle r, EMBO J, 17:4257-4265, 1998). Pdr12 appears to mediate resistance to wate r-soluble, monocarboxylic acids with chain lengths of from C-1 to C-7. Expo sure to acids with aliphatic chain lengths greater than C-7 resulted in no observable sensitivity of Delta pdr12 mutant cells compared to the parent. Parent and Delta pdr12 mutant cells were grown in the presence of sorbic ac id and subsequently loaded with fluorescein. Upon addition of an energy sou rce in the form of glucose, parent cells immediately effluxed fluorescein f rom the cytosol into the surrounding medium. In contrast, under the same co nditions, cells of the Delta pdr12 mutant were unable to efflux any of the dye. When both parent and Delta pdr12 mutant cells were grown without sorbi c acid and subsequently loaded with fluorescein, upon the addition of gluco se no efflux of fluorescein was detected from either strain. Thus, we have shown that Pdr12 catalyzes the energy-dependent extrusion of fluorescein fr om the cytosol. Lineweaver-Burk analysis revealed that sorbic and benzoic a cids competitively inhibited ATP-dependent fluorescein efflux, Thus, these data provide strong evidence that sorbate and benzoate anions compete with fluorescein for a putative monocarboxylate binding site on the Pdr12 transp orter.