THE ATP BINDING CASSETTE TRANSPORTERS PDR5 AND SNQ5 OF SACCHAROMYCES-CEREVISIAE CAN MEDIATE TRANSPORT OF STEROIDS IN-VIVO

Multiple or pleiotropic drug resistance in the yeast Saccharomyces cer evisiae can arise from overexpression of the Pdr5 and Snq2 ATP binding cassette multidrug transporters. Expression of Pdr5 and Snq2 is regul ated by the two transcription factors Pdr1 and Pdr3, as multidrug-resi stant pdr1 and pdr3 gain-of-function mutants overexpress both drug eff lux pumps. One such pdr1 mutant allele was previously cloned in a gene tic screen by its ability to suppress the squelching toxicity mediated by an estradiol-inducible chimeric VP16-human estrogen receptor (VEO) expressed in yeast (Gilbert, D. M., Heery, D. M., Lesson, R., Chambon , P., and Lemoine, Y. (1993) Mol. Cell. Biol. 13, 462-472). In this st udy, we demonstrate that relief of estradiol toxicity in yeast cells e xpressing VEO requires functional PDR5 and SNQ2 genes, since a Delta p dr5 Delta snq2 double deletion leads to an increased estradiol toxicit y. Furthermore, using URA3 as an estradiol-inducible reporter gene, we show that Pdr5 and Snq2, when overexpressed from high-copy plasmids, can reduce the intracellular concentration of estradiol. In contrast, a Delta pdr5 Delta snq2 double deletion mutant accumulates almost 30-f old more intracellular estradiol than the isogenic wild type, Indirect immunofluorescence showed that a pdr1-3 mutant massively overexpresse s Pdr5 at the plasma membrane, suggesting that estradiol efflux from t he cells occurs across the plasma membrane. Our data demonstrate that Pdr5 and Snq2 can transport steroid substrates in vivo and suggest tha t steroids and/or related membrane lipids could represent physiologica l substrates for certain yeast ABC transporters, which are otherwise i nvolved in the development of pleiotropic drug resistance.