STE7

Ste7 is a mitogen-activated protein kinase kinase (MAPKK) in Saccharomyces cerevisiae that functions as a central component of two related MAPK signaling cascades: the pheromone response pathway used for mating and the filamentous (invasive) growth pathway triggered by nutrient limitation. As a MAPKK, Ste7 phosphorylates and activates the downstream MAP kinases Fus3 and Kss1.

Activation flows from the MAPKKK Ste11 to the MAPKK Ste7, which then activates the MAPKs Fus3 and Kss1 by phosphorylation on their activation loops. Ste7 is recruited to the signaling complex by the scaffold Ste5 at the plasma membrane, which helps organize Ste11, Ste7, and the MAPKs in response to pheromone. Phosphorylation of Ste7 by Ste11 enables it to phosphorylate Fus3 and Kss1, enabling downstream signaling.

In the pheromone response, Fus3 mediates mating-specific outcomes such as transcriptional changes, G1 cell cycle arrest, and the formation of mating projections called shmoos. In nutrient-poor conditions, Kss1 drives invasive or pseudohyphal growth, promoting altered cell morphology and behavior. The two branches of signaling yield distinct transcriptional programs depending on cellular context.

Genetic studies show that STE7 is essential for a proper pheromone response and mating; mutants lacking Ste7 fail to activate Fus3-dependent signaling and mating processes. Ste7 activity is tightly regulated by upstream signals and modulated by phosphatases and other regulatory inputs, ensuring appropriate cell behavior in response to environmental cues.