THE OSMOREGULATORY PATHWAY REPRESSES MATING PATHWAY ACTIVITY IN SACCHAROMYCES-CEREVISIAE - ISOLATION OF A FUS3 MUTANT THAT IS INSENSITIVE TO THE REPRESSION MECHANISM

Mitogen-activated protein (MAP) kinase cascades are conserved signal t ransduction pathways that are required for eukaryotic cells to respond to a variety of stimuli, Multiple MAP kinase pathways can function wi thin a single cell type; therefore, mechanisms that insulate one MAP k inase pathway from adventitious activations by parallel pathways may e xist, We have studied interactions between the mating pheromone respon se and the osmoregulatory (high-osmolarity glycerol response [HOG]) pa thways in Saccharomyces cerevisiae which utilize the MAP kinases Fus3p and Hog1p, respectively, Inactivating mutations in HOG pathway kinase s cause an increase in the phosphotyrosine content of Fus3p, greater e xpression of pheromone-responsive genes, and increased sensitivity to growth arrest by pheromone. Therefore, the HOG pathway represses matin g pathway activity, In a HOG1(+) strain, Fus3p phosphotyrosine increas es modestly and transiently following an increase in the extracellular osmolarity; however, it increases to a greater extent and for a susta ined duration in a hog1-Delta strain. Thus, the HOG-mediated repressio n of mating pathway activity may insulate the mating pathway from acti vation by osmotic stress. A FUS3 allele whose gene product is resistan t to the HOG-mediated repression of its phosphotyrosine content has be en isolated. This mutant encodes an amino acid substitution in the hig hly conserved DPXDEP motif in subdomain XI, Other investigators have s hown that the corresponding amino acid is also mutated in a gain-of-fu nction allele of the MAP kinase encoded by the rolled locus in Drosoph ila melanogaster, These data suggest that the DPXDEP motif plays a rol e in the negative regulation of MAP kinases.