Pdr5p is one of the major multidrug efflux PUMPS whose overexpression confers multidrug resistance (MDR) in Saccharomyces cerevisiae. By using our original assay system, a fungal strain producing inhibitors for Pdr5p was obtained and classified as Fitsarium sp. Y-53. The purified inhibitors were identified as ionophore antibiotics, enniatin B, B I, and D, respectively. A non-toxic concentration of each enniatin (5 mug/ml, similar to7.8 muM) strongly inhibited a Pdr5p-mediated efflux of cycloheximide or cerulenin in Pdr5p-overexpressing cells. The enniatins accumulated a fluorescent dye rhodamine 123, a substrate of Pdr5p, into yeast cells. The mode of Pdr5p inhibition of enniatin was competitive against FK506, and its inhibitory activity was more potent with less toxicity than that of FK506. The enniatins showed similar inhibitory profile as FK506 against S 1360 mutants (S1360A and S1360F) of Pdr5p. The enniatins did not inhibit the function of Snq2p, a homologue of Pdr5p. Thus, it was round that enniatins are potent and specific inhibitors for Pdr5p, with less toxicities than that of FK506. (C) 2005 Elsevier Inc. All rights reserved.