MUTATIONS WITHIN THE FIRST LSGGQ MOTIF OF STE6P CAUSE DEFECTS IN A-FACTOR TRANSPORT AND MATING IN SACCHAROMYCES-CEREVISIAE

Mating between the two haploid cell types (a and alpha) of the yeast S accharomyces cerevisiae depends upon the efficient secretion and deliv ery of the a- and alpha-factor pheromones to their respective target c ells. However, a quantitative correlation between the level of transpo rted a-factor and mating efficiency has never been determined, a-Facto r is transported hy Ste6p, a member of the ATP-binding cassette (ABC) family of transporter proteins. In this study, several missense mutati ons were introduced in or near the conserved LSGGQ motif within the fi rst nucleotide-binding domain of Ste6p, Quantitation of extracellular a-factor levels indicated that these mutations caused a broad range of a-factor transport defects, and those directly within the LSGGQ motif caused the most severe defects. Overall, we observed a strong correla tion between the level of transported a-factor and the mating efficien cy of these strains, consistent with the role of Ste6p as the a-factor transporter. The LSGGQ mutations did not cause either a significant a lteration in the steady-state level of Ste6p or a detectable change in its subcellular localization. Thus, it appears that these mutations i nterfere with the ability of Ste6p to transport a-factor out of the MA Ta cell. The possible involvement of the LSGGQ motif in transporter fu nction is consistent with the strong conservation of this sequence mot if throughout the ABC transporter superfamily.