Pheromone induction promotes Ste11 degradation through a MAPK feedback and ubiquitin-dependent mechanism.

Ste11 is the mitogen-activated protein kinase (MAPK) kinase kinase in the MAPK cascades that mediate mating, high osmolarity glycerol, and filamentous growth responses in Saccharomyces cerevisiae. We show stimulation of the mating pathway by pheromone promotes an accelerated turnover of Ste11 through a MAPK feedback and ubiquitin-dependent mechanism. This degradation is pathway specific, because Ste11 is stable during activation of the high osmolarity glycerol pathway. Because the steady-state amount of Ste11 does not change significantly during pheromone induction, we infer that maintenance of MAPK activation involves repeated cycles in which naive Ste11 is activated and then targeted for degradation. This model predicts that elimination of active Ste11 would rapidly curtail MAPK activation upon attenuation of the upstream signal. This prediction is confirmed by the finding that blocking ubiquitin-dependent Ste11 degradation during pheromone induction abolishes the characteristic attenuation profile for MAPK activation.