STE6, the Yeast A-factor Transporter

In eukaryotic cells, most extracellular proteins exit the cell via the classical secretory pathway (ER-->Golgi-->secretory vesicles). A notable exception to this pattern is the Saccharomyces cerevisiae mating pheromone alpha-factor, an isoprenylated, methylated, oligopeptide signaling molecule which uses a distinctly non-classical mechanism for secretion. Export of alpha-factor from the yeast cell is mediated by STE6, a member of the ABC protein superfamily. STE6 is one of the few eukaryotic ABC proteins for which a true physiological substrate is known. The ability to carry out molecular manipulations with ease in yeast, together with the possibility of probing substrate-transporter interactions via genetic analysis, affords an excellent opportunity to rigorously dissect the workings of this ABC family member.