In this report we describe the functional expression of EmrE, a 110- amino-acid multidrug transporter from Escherichia coli, in the yeast Saccharomyces cerevisiae. To allow for phenotypic complementation, a mutant strain sensitive to a series of cationic lipophilic drugs was first identified. A hemagglutinin epitope-tagged version of EmrE (HA-EmrE) conferring resistance to a wide variety of drugs, including acriflavine, ethidium, methyl viologen, and the neurotoxin 1-methyl-4-phenylpyridinium (MPP+), was functionally expressed in this strain. HA-EmrE is expressed in yeast at relatively high levels (0.5 mg/liter), is soluble in a mixture of organic solvents, and can be functionally reconstituted in proteoliposomes. In bacterial cells, EmrE removes toxic compounds by active transport through the plasma membrane, lowering their cytosolic concentration. However, yeast cells expressing HA-EmrE take up 14C-methyl viologen as well as control cells do. Thus, we investigated the basis of the enhanced resistance to the above compounds. Using Cu2+ ions or methylamine, we could selectively permeabilize the plasma membrane or deplete the proton electrochemical gradients across the vacuolar membrane, respectively. Incubation of yeast cells with copper ions caused an increase in 14C-methyl viologen uptake. In contrast, treatment with methylamine markedly diminished the extent of uptake. Conversely, the effect of Cu2+ and methylamine on a plasma membrane uptake system, proline, was essentially the opposite: while inhibited by the addition of Cu2+, it remained unaffected when cells were treated with methylamine. To examine the intracellular distribution of HA-EmrE, a functional chimera between HA-EmrE and the green fluorescent protein (HA- EmrE-GFP) was prepared. The pattern of HA-EmrE-GFP fluorescence distribution was virtually identical to that of the vacuolar marker FM 4-64, indicating that the transporter is found mainly in this organelle. Therefore, HA-EmrE protects yeast cells by lowering the cytoplasmic concentrations through removal of the toxin to the vacuole. This novel way of detoxification has been previously suggested to function in organisms in which a large vacuolar compartment exists. This report represents the first molecular description of such a mechanism.