COORDINATION OF THE MATING AND CELL INTEGRITY MITOGEN-ACTIVATED PROTEIN-KINASE PATHWAYS IN SACCHAROMYCES-CEREVISIAE

Mating pheromone stimulates a mitogen-activated protein (MAP) kinase a ctivation pathway in Saccharomyces cerevisiae that induces cells to di fferentiate and form projections oriented toward the gradient of phero mone secreted by a mating partner, The polarized growth of mating proj ections involves new cell wall synthesis, a process that relies on act ivation of the cell integrity MAP kinase, Mpk1. In this report, we sho w that Mpk1 activation during pheromone induction requires the transcr iptional output of the mating pathway and protein synthesis, Consequen tly, Mpk1 activation occurs subsequent to the activation of the mating pathway MAP kinase cascade, Additionally, Spa2 and Bni1, a formin fam ily member, are two coil-coil-related proteins that are involved in th e timing and other aspects of mating projection formation, Both protei ns also affect the timing and extent of Mpk1 activation, This correlat ion suggests that projection formation comprises part of the pheromone -induced signal that coordinates Mpk1 activation with mating different iation, Stimulation of Mpk1 activity occurs through the cell integrity phosphorylation cascade and depends on Pkc1 and the redundant MAP/Erk kinases (MEKs), Mkk1 and Mkk2. Surprisingly, Mpk1 activation by phero mone was only partially impaired in cells lacking the MEK kinase Bck1. This Bck1-independent mechanism reveals the existence of an alternati ve activator of Mkk1/Mkk2 in some strain backgrounds that at least fun ctions under pheromone-induced conditions.