UCSF

Being able to accurately respond to the environment is a trait crucial to cellular survival. However, signaling pathways that mediate the internal response to an external signal frequently share components. To ensure that specific pathways remain insulted from one another such that signaling fidelity is maintained, regulatory mechanisms must be in place to ensure signaling specificity. In Saccharomyces cerevisiae, a pair of mitogen-activated protein kinase (MAPK) pathways involved in the distinct developmental programs of mating pheromone response and filamentous growth shares many components but avoid pathway cross-talk. We have identified one of the mechanisms that inhibits erroneous activation of the filamentous growth pathway during activation of the mating response pathway. To ensure signaling specificity, the mating pheromone response pathway specific MAPK Fus3 targets the destruction of the filamentous growth-pathway specific transcription factor Tec1. We have further found that this proteolysis of Tec1 is dependent on the dual phosphorylation of Tec1 by Fus3 and the subsequent recognition of phospho-Tec1 by the SCFCdc4 ubiquitin ligase, the complex responsible for the ubiquitylation that signals for proteolysis. Specifically, Cdc4, the F-box protein component of SCFCdc4, binds directly to Tec1 via phosphorylations on both threonine 273 and threonine 276 in a mode of substrate recognition that differs significantly from the previously characterized Cdc4 substrate binding motif (Cdc4 phospho-degron) and requires residues on Cdc4 outside the core substrate binding pocket. This mechanism of Tec1 regulation by destruction explains the conundrum of the absence of filamentation gene expression despite the activation of a population of Kss1, the filamentation pathway specific MAPK, during pheromone signaling. It also presents an example of a novel mode of Cdc4 substrate recognition that, taken together with recently published data on Sic1, another Cdc4 substrate, could represent a more general means of high affinity Cdc4 substrate binding.