CELL-FUSION DURING YEAST MATING REQUIRES HIGH-LEVELS OF A-FACTOR MATING PHEROMONE

During conjugation, two yeast cells fuse to form a single zygote. Cell fusion requires extensive remodeling of the cell wall, both to form a seal between the two cells and to remove the intervening material. Th e two plasma membranes then fuse to produce a continuous cytoplasm. We report the characterization of two cell fusion defective (Fus(-)) mut ants, fus5 and fus8, isolated previously in our laboratory. Fluorescen ce and electron microscopy demonstrated that the fus5 and fus8 mutant zygotes were defective for cell wall remodeling/removal but not plasma membrane fusion. Strikingly, fus5 and fus8 were a specific; both muta tions caused the mutant phenotype when present in the MA Ta parent but not in the MA Tot parent. Consistent with an a-specific defect, the f us5 and fus8 mutants produced less a-factor than the isogenic wild-typ e strain. FUS5 and FUS8 were determined to be allelic to AXL1 and RAM1 , respectively, two genes known to be required for biogenesis of a-fac tor. Several experiments demonstrated that the partial defect in a-fac tor production resulted in the Fus(-) phenotype. First, overexpression of a-factor in the fus mutants suppressed the Fus(-) defect, Second, matings to an MAT alpha partner supersensitive to mating pheromone (ss t2 Delta) suppressed the Fus(-) defect in trans. Finally, the gene enc oding a-factor, MFA1, was placed under the control of a repressible pr omoter; reduced levels of wild-type a-factor caused an identical cell fusion defect during mating. We conclude that high levels of pheromone are required as one component of the signal for prezygotes to initiat e cell fusion.