Combining mutations in the incoming and outgoing pheromone signal pathwayscauses a synergistic mating defect in Saccharomyces cerevisiae

Mating pheromones stimulate Saccharomyces cerevisiae yeast cells to form a pointed projection that becomes the site of cell fusion during conjugation. To investigate the role of mating projections, we screened for mutations t hat enhanced the weak mating defect of MATa ste2-T326 cells that are defect ive in forming pointed projections. These cells are also 10-fold more sensi tive to alpha-factor pheromone because ste2-T326 encodes truncated alpha-fa ctor receptors that are not regulated properly. Mutations in AXL1, STE6 and FUS3 were identified in the screen. AXL1 was studied further because it is required for efficient a-factor pheromone production and for selecting the site for bud morphogenesis. Mutation of AXL1 did not enhance the morphogen esis or pheromone sensitivity defects of ste2-T326. Instead, the synergisti c mating defect was apparently due to decreased a-factor production because the axl1 Delta ste2-T326 cells mated well with a sst2 alpha mating partner that is supersensitive to a-factor. When combined with a wild-type mating partner, the ste2-T326 axl1 Delta cells failed to mate because they did not lock cell walls, one of the earliest steps in conjugation. Analysis of axl 1 Delta in combination with other mutations that cause defects in morphogen esis or pheromone sensitivity (e.g. bar1, sst2, afr1) indicated that both p henotypes of ste2-T326 cells, supersensitivity to a-factor and the defect i n forming pointed projections, contributed to the synergistic mating defect . We suggest a model that the synergistic mating defect is caused by the co mbined effects of ste2-T326 and axl1 Delta on the presentation of alpha-fac tor to partner cells. Altogether, these results demonstrate an important li nkage between the incoming and outgoing pheromone signals during the interc ellular communication that promotes yeast mating. Copyright (C) 1999 John W iley & Sons, Ltd.