Publisher Summary The chapter introduces the mitogen-activated protein (MAP) kinase (MAPK) module. The identification of MAP kinase pathways exemplifies the power of combining biochemical and genetic approaches to molecular problems. The chapter discusses the mammalian MAPK pathways—ERKl/2 and MKKl/2 pathways—and stress-activated protein kinase pathways. The regulation of MAPK pathways by protein phosphatases is discussed in the chapter describing in detail about dual specificity phosphatases, serinenhreonine phosphatases, and protein tyrosine phosphatases. The chapter explores the cellular substrates of MAP kinases, wherein it discusses about protein kinase substrates for MAPKS, nuclear transcription factors, signaling components, and cytoskeletal proteins. Responses to MAPK pathways, regulation of cell growth and transformation, and regulation of cell differentiation and development have also been summarized in the chapter. The chapter describes the yeast MAPK pathways of saccharomyces cerevisiae (Budding Yeast) and Schizosaccharomyces pombe (Fission Yeast). The chapter provides the description of the intracellular targeting and spatial regulation of MAPK pathway components, signaling complexes, and the nuclear translocation of MAPK and MKK. Eukaryotic MAPK cascades provide excellent examples of signal transduction mechanisms that embody key principles common to many, if not all, signaling pathways. Many fundamental questions remain for future studies to investigate the mechanisms by which these pathways are regulated as well as the cellular responses that they control.

Signal transduction through MAP kinase cascades.

The ability to adapt to altered availability of free water is a fundamental property of living cells. The principles underlying osmoadaptation are well conserved. The yeast Saccharomyces cerevisiae is an excellent model system with which to study the molecular biology and physiology of osmoadaptation. Upon a shift to high osmolarity, yeast cells rapidly stimulate a mitogen-activated protein (MAP) kinase cascade, the high-osmolarity glycerol (HOG) pathway, which orchestrates part of the transcriptional response. The dynamic operation of the HOG pathway has been well studied, and similar osmosensing pathways exist in other eukaryotes. Protein kinase A, which seems to mediate a response to diverse stress conditions, is also involved in the transcriptional response program. Expression changes after a shift to high osmolarity aim at adjusting metabolism and the production of cellular protectants. Accumulation of the osmolyte glycerol, which is also controlled by altering transmembrane glycerol transport, is of central importance. Upon a shift from high to low osmolarity, yeast cells stimulate a different MAP kinase cascade, the cell integrity pathway. The transcriptional program upon hypo-osmotic shock seems to aim at adjusting cell surface properties. Rapid export of glycerol is an important event in adaptation to low osmolarity. Osmoadaptation, adjustment of cell surface properties, and the control of cell morphogenesis, growth, and proliferation are highly coordinated processes. The Skn7p response regulator may be involved in coordinating these events. An integrated understanding of osmoadaptation requires not only knowledge of the function of many uncharacterized genes but also further insight into the time line of events, their interdependence, their dynamics, and their spatial organization as well as the importance of subtle effects.

Osmotic Stress Signaling and Osmoadaptation in Yeasts

Regulation of transcription by MAP kinase cascades.

The MSN2 and MSN4 genes encode homologous and functionally redundant Cys2His2 zinc finger proteins. A disruption of both MSN2 and MSN4 genes results in a higher sensitivity to different stresses, including carbon source starvation, heat shock and severe osmotic and oxidative stresses. We show that MSN2 and MSN4 are required for activation of several yeast genes such as CTT1, DDR2 and HSP12, whose induction is mediated through stress-response elements (STREs). Msn2p and Msn4p are important factors for the stress-induced activation of STRE dependent promoters and bind specifically to STRE-containing oligonucleotides. Our results suggest that MSN2 and MSN4 encode a DNA-binding component of the stress responsive system and it is likely that they act as positive transcription factors.

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The saccharomyces cerevisiae zinc finger proteins msn2p and msn4p are required for transcriptional induction through the stress-response element (stre )

Whole-genome mRNA quantitation can be used to identify the genes that are most responsive to environmental or genotypic change. By searching for mutually similar DNA elements among the upstream non-coding DNA sequences of these genes, we can identify candidate regulatory motifs and corresponding candidate sets of coregulated genes. We have tested this strategy by applying it to three extensively studied regulatory systems in the yeast Saccharomyces cerevisiae: galactose response, heat shock, and mating type. Galactose-response data yielded the known binding site of Gal4, and six of nine genes known to be induced by galactose. Heat shock data yielded the cell-cycle activation motif, which is known to mediate cell-cycle dependent activation, and a set of genes coding for all four nucleosomal proteins. Mating type α and a data yielded all of the four relevant DNA motifs and most of the known a- and α-specific genes.

Finding DNA regulatory motifs within unaligned noncoding sequences clustered by whole-genome mRNA quantitation

The HOG signal pathway of the yeast Saccharomyces cerevisiae is defined by the PBS2 and HOG1 genes encoding members of the MAP kinase kinase and of the MAP kinase family, respectively. Mutations in this pathway (deletions of PBS2 or HOG1, or point mutations in HOG1) almost completely abolish the induction of transcription by osmotic stress that is mediated by stress response elements (STREs). We have demonstrated previously that STREs also mediate induction of transcription by heat shock, nitrogen starvation and oxidative stress. This study shows that they are also activated by low external pH, sorbate, benzoate or ethanol stress. Induction by these other stress signals appears to be HOG pathway independent. HOG1-dependent osmotic induction of transcription of the CTT1 gene encoding the cytosolic catalase T occurs in the presence of a protein synthesis inhibitor and can be detected rapidly after an increase of tyrosine phosphorylation of Hog1p triggered by high osmolarity. Consistent with a role of STREs in the induction of stress resistance, a number of other stress protein genes (e.g. HSP104) are regulated like CTT1. Furthermore, catalase T was shown to be important for viability under severe osmotic stress, and heat shock was demonstrated to provide cross-protection against osmotic stress.

M.R. Alexander

Papers

The HOG pathway controls osmotic regulation of transcription via the stress response element (STRE) of the Saccharomyces cerevisiae CTT1 gene.