Studies with novel Pdr5p substrates demonstrate a strong size dependence for xenobiotic efflux

The yeast (Saccharomyces cerevisiae) multidrug transporter Pdr5p effluxes a broad range of substrates that are variable in structure and mode of action. Previous work suggested that molecular size and ionization could be important parameters. In this study, we compared the relative sensitivity of isogenic PDR5 and pdr5 strains toward putative substrates that are similar in chemical structure. Three series were used: imidazole-containing compounds, trialkyltin chlorides, and tetraalkyltin compounds. We demonstrate that the Pdr5p transporter is capable of mediating transport of substrates that neither ionize nor have electron pair donors and that are much simpler in structure than those transported by the human MDR1-encoded P-glycoprotein. Furthermore, the size of the substrate is critical and independent of any requirement for hydrophobicity. Substrates have surface volumes greater than 90 A(3) with an optimum response at approximately 200-225 A(3) as determined by molecular modeling. Assays measuring the efflux from cells of [(3)H]chloramphenicol and [(3)H]tritylimidazole were used. A concentration-dependent inhibition of chloramphenicol transport was observed with imidazole derivatives but not with either the organotin compounds or the antitumor agent doxorubicin. In contrast, several of the organotin compounds were potent inhibitors of tritylimidazole efflux, but the Pdr5p substrate tetrapropyltin was ineffective in both assays. This argues for the existence of at least three substrate-binding sites on Pdr5p that differ in behavior from those of the mammalian P-glycoprotein. Evidence also indicates that some substrates are capable of interacting at more than one site. The surprising observation that Pdr5p mediates resistance to tetraalkyltins suggests that one of the sites might use only hydrophobic interactions to bind substrates.