Investigations into mitogen-activated protein (MAP) kinase signal transduction pathways in Saccharomyces cerevisiae
Mitogen-activated protein (MAP) kinases comprise a highly conserved family of eukaryotic signal transduction enzymes. Stimuli that activate MAP kinase pathways include growth factor/receptor interactions, environmental stresses, and cell type-specific differentiation signals during development. MAP kinase substrates include transcription factors and cell cycle regulatory proteins that effect cellular responses appropriate to the initiating signals. These pathways are found in organisms as evolutionarily diverse as the budding yeast and humans, and their control over vital cellular processes has produced an ongoing effort to understand the complexities of MAP kinase regulation.
Multiple MAP kinases can exist within a single cell type, and in Saccharomyces cerevisiae there are at least four MAP kinase pathways. The mating pheromone response pathway is activated during the conjugation of haploid cells, and the osmoregulatory (HOG) pathway is activated by an increase in the extracellular osmolarity. These two pathways utilize the MAP kinases Fus3p and Hog1p, respectively. The sporulation-specific SMK1 pathway is required for the proper construction of spore walls and the completion of this developmental program.
To uncover amino acid sequences in Fus3p and Hog1p that restrict their functions to their cognate pathways, chimeric proteins were expressed and analyzed in vivo. These studies indicate that the amino-terminal portion of Hog1p up to and including subdomain IX of the catalytic core is sufficient for osmoregulatory pathway specificity. Genetic and biochemical approaches have shown that Fus3p can be activated in response to osmotic stress, and that the osmoregulatory pathway represses mating pathway activity. A FUS3 allele that encodes a kinase which is resistant to the HOG-mediated repression has been isolated. These observations show that one MAP kinase pathway can negatively regulate a parallel MAP kinase pathway, and they suggest a general mechanism that insulates MAP kinase pathways from adventitious activation.
The SWM2 gene was isolated as a multicopy suppressor of the spore wall defects associated with a smk1 MAP kinase mutant. SWM2 encodes an evolutionarily conserved protein that contains seven zinc finger domains of the "CCHC" type. This data suggests that Swm2p homologues are fundamental components of signal transduction pathways in diverse organisms.