Saccharomyces cerevisiae STE5 is an essential component of the pheromone-mediated-mitogen activated protein kinase (MAPK) pathway. The STE5 protein recruits MAPK module kinases (STE11, STE7, and FUS3) to give a specificity for the pheromone pathway. The STE5 protein contains a putative PEST motif for ubiquitin-dependent protein degradation, and its level may be important for regulation of pheromone signal transduction. In this article, we studied the roles of proteins associated with the STE5 protein for its stabilization by analyzing ste deletion mutants. Here, we found that the STE11 kinase performed the most important role in stabilization of the STE5 protein. The level of STE5 protein was significantly low in the absence of STE11 kinase, suggesting essential roles of STE11 in stabilization of the STE5 protein. Immunodetection and Northern blot analyses showed that the low level of the STE5 protein in the ste11 delta mutant is not due to the level of gene expression but to that of protein stability. Measurement of relative binding affinities showed that the STE11 protein tightly interacts with the STE5 protein for its stabilization.