Exploring an antifungal target in the plasma membrane H⁺-ATPase of fungi

The plasma membrane H⁺-ATPase is a promising new antifungal target that is readily probed with the sulfhydryl-reactive reagent omeprazole. Inhibition of the H⁺-ATPase by omeprazole is closely linked to cell killing, and it has been suggested that enzyme inhibition may result from a covalent interaction within the first two transmembrane segments (M1 and M2) (Monk et al. (1995) Biochim. Biophys. Acta 1239, 81-90). In this study, the molecular nature of this interaction was examined by screening a series of 26 well-characterized pmal mutations residing in the first two transmembrane segments of the H⁺-ATPase from Saccharomyces cerevisiae. Only two pmal mutants, A135G and G158D, G156C, were found to significantly decrease the sensitivity of cells for omeprazole. In contrast, enhanced sensitivity was observed at a number of positions, with D140C(A) and M128C producing the most significant increases in sensitivity. The introduction of cysteine at various locations within this region only marginally affected omeprazole sensitivity, suggesting that this region was not a direct site of covalent modification. Rather, its conformation influences omeprazole binding at some other locus. In order to determine the sidedness of the omeprazole interaction, a novel in vitro assay system was exploited that utilized liposomes co-reconstituted with the H⁺-ATPase and the light-driven proton pump bacteriorhodopsin. Omeprazole was found to completely inhibit proton transport by the H⁺-ATPase at 50 μM in this system. An asymmetrically-distributed chemical trap system involving glutathione was used to demonstrate that this inhibition appears localized to the extracellular portion of the enzyme. This work indicates that omeprazole can inhibit the H⁺-ATPase from its extracellular face, and this inhibition is influenced by changes in the M1, M2 region of the protein.