The yeast STE6 gene encodes a homologue of the mammalian multidrug resistance P-glycoprotein

MAMMALIAN tumours displaying multidrug resistance overexpress a plasma membrane protein (P-glycoprotein), which is encoded by the MDR1 gene 1 and apparently functions as an energy-dependent drug efflux pump. Tissue-specific expression of MDR1 and other members of the MDR gene family has been observed in normal cells 2 , suggesting a role for P-glycoproteins in secretion. We have isolated a gene from the yeast Saccharomyces cerevisiae that encodes a protein very similar to mammalian P-glycoproteins. Deletion of this gene resulted in sterility of MAT a, but not of MAT α cells. Subsequent analysis revealed that the yeast P-glycoprotein is the product of the STE6 gene, a locus previously shown to be required in MAT a cells for production of a-factor pheromone 3 . Our findings suggest that the STE6 protein functions to export the hydrophobic a-factor lipopeptide in a manner analogous to the efflux of hydrophobic cytotoxic drugs catalysed by the related mammalian P-glycoprotein. Thus, the evolutionarily conserved family of MDR -like genes, including the hlyB gene 4 of Escherichia coli and the STE6 gene of S. cerevisiae , encodes components of secretory pathways distinct from the classical, signal sequence-dependent protein translocation system.