Positive and negative control of multidrug resistance by the Sit4 protein phosphatase in Kluyveromyces lactis

The nuclear gene encoding the Sit4 protein phosphatase was identified in the budding yeast Kluyveromyces lactis. K. lactis cells carrying a disrupted sit4 allele are resistant to oligomycin, antimycin, ketoconazole, and econazole but hypersensitive to paromomycin, sorbic acid, and 4-nitroquinoline-N-oxide (4-NQO). Overexpression of SIT4 leads to an elevation in resistance to paromomycin and to lesser extent tolerance to sorbic acid, but it has no detectable effect on resistance to 4-NQO. These observations suggest that the Sit4 protein phosphatase has a broad role in modulating multidrug resistance in K. lactis. Expression or activity of a membrane transporter specific for paromomycin and the ABC pumps responsible for 4-NQO and sorbic acid would be positively regulated by Sit4p. In contrast, the function of a Pdr5-type transporter responsible for ketoconazole and econazole extrusion, and probably also for efflux of oligomycin and antimycin, is likely to be negatively regulated by the phosphatase. Drug resistance of sit4 mutants was shown to be mediated by ABC transporters as efflux of the anionic fluorescent dye rhodamine 6G, a substrate for the Pdr5-type pump, is markedly increased in sit4 mutants in an energy-dependent and FK506-sensitive manner.