The ND5 component of the respiratory complex I is a large, hydrophobic subunit encoded by the mitochondrial genome. Its bacterial homologue, the NDH-1 subunit NuoL, acts as a cation transporter in the absence of other NDH-1 subunits. Mutations in human ND5 are frequently observed in neurodegenerative diseases. Wild type and mutant variants of ND5 fused to GFP or a FLAG peptide were targeted to the endoplasmatic reticulum (ER) or the inner mitochondrial membrane of Saccharomyces cerevisiae, which lacks an endogenous complex I. The localization of ND5 fusion proteins was confirmed by microscopic analyses of S. cerevisiae cells, followed by cellular fractionation and immunostaining. The impact of the expression of ND5 fusion proteins on the growth of S. cerevisiae in the presence and absence of added salts was studied. ER-resident ND5 conferred Li(+) sensitivity to S. cerevisiae, which was lost when the E145V variant of ND5 was expressed. All variants of ND5 tested led to increased resistance of S. cerevisiae at high external concentrations of Na(+) or K(+). The data seem to indicate that ND5 influences the salt homeostasis of S. cerevisiae independent of other complex I subunits, and paves the way for functional studies of mutations found in mitochondrially encoded complex I genes.