Molecular cloning and expression of the Saccharomyces cerevisiae STS1 gene product. A yeast ABC transporter conferring mycotoxin resistance.

We have cloned a yeast gene that confers a multidrug resistance phenotype on Saccharomyces cerevisiae when present in multiple copies. The STS1 (for Sporidesmin Toxicity Suppressor) gene encodes a 1511-residue protein whose predicted structural organization is characterized by 12 alpha-helical membrane segments and two domains containing consensus sites for ATP binding, indicating that STS1 is a new yeast ATP-binding cassette (ABC) transporter. A chromosomal deletion of STS1 leads to viable delta sts1 cells of both mating types, suggesting that STS1 is not essential for cell growth. However, delta sts1 cells exhibit supersensitivity to sporidesmin and to other structurally unrelated drugs such as cycloheximide. Conversely, overexpression of STS1 leads to increased resistance to the same drugs. Although Northern analysis showed that STS1 mRNA is present in all yeast cell types, its drastically reduced level in alpha-factor-arrested cells indicates that expression of STS1 is regulated by mating pheromones. Subcellular fractionation and immunoblotting using monoclonal antibodies, which recognize a fully functional epitope-tagged Sts1 protein, showed that Sts1 is a 175-kDa membrane protein localized mainly to intracellular membranes.