External and intrinsic mechanical stress restricts pheromone signaling by direct phosphorylation of the Ste5 scaffold by Pkc1

Ste5 and Far1 function as prototypical scaffold proteins activating a MAP-kinase cascade and polarizing yeast cells in response to pheromones. Here we show that Pkc1-dependent phosphorylation of conserved serine residues in their RING-H2 domains down-regulates pheromone induced signaling upon mechanical stress. Phosphorylation of Ste5 on serine 185 by Pkc1 interferes with its membrane-association in vivo by preventing binding to the receptor linked Gbg protein as determined by in vitro assays. Quantitative readouts of pheromone signaling show that Pkc1 induces rapid dispersal of Ste5 from mating projections upon mechanically-induced cell wall stress as well as during cell-cell fusion, leading to inhibition of MAPK activity and polarized growth. Cells expressing non-phosphorylatable Ste5 (Ste5S185A) undergo increased lysis during mechanical stress and cell-cell fusion compared to wild-type controls, and this effect is exacerbated by simultaneous expression of non-phosphorylatable Far1 (Far13A). Taken together, these results uncover a cross-talk mechanism explaining how mating cells inhibit pheromone signaling upon extrinsic or intrinsic mechanical stress to orchestrate polarized growth and cell-cell fusion.