MAPK specificity in the yeast pheromone response independent of transcriptional activation

The mechanisms whereby different external cues stimulate the same mitogen-a ctivated protein kinase (MAPK) cascade, yet trigger an appropriately distin ct biological response, epitomize the conundrum of specificity in cell sign aling. In yeast, shared upstream components of the mating pheromone and fil amentous growth pathways activate two related MAPKs, Fus3 and Kss1, which i n turn regulate programs of gene expression via the transcription factor St e12 [1]. As fus3, but not kss1, strains are impaired for mating, Fus3 exhib its specificity for the pheromone response. To account for this specificity , it has been suggested that Fus3 physically occludes Kss1 from pheromone-a ctivated signaling complexes, which are formed on the scaffold protein Ste5 [2]. However, we find that genome-wide expression profiles of pheromone-tr eated wild-type, fus3, and kss1 deletion strains are highly correlated for all induced genes and, further, that two catalytically inactive versions of Fus3 fail to abrogate the pheromone-induced transcriptional response. Cons istently, Fus3 and Kss1 kinase activity is induced to an equivalent extent in pheromone-treated cells. In contrast, both in vivo and in an in vitro-re constituted MAPK system, Fus3, but not Kss1, exhibits strong substrate sele ctivity toward Farl, a bifunctional protein required for polarization and G (1) arrest [3, 4]. This effect accounts for the failure to repress G(1)-S s pecific transcription in fus3 strains and, in part, explains the mating def ect of such strains. MAPK specificity in the pheromone response evidently o ccurs primarily at the substrate level, as opposed to specific kinase activ ation by dedicated signaling complexes.