Ste5

Abstract: A cascade of three protein kinases known as a mitogen-activated protein kinase (MAPK) cascade is commonly found as part of the signaling pathways in eukaryotic cells. Almost two decades of genetic and biochemical experimentation plus the recently completed DNA sequence of the Saccharomyces cerevisiae genome have revealed just five functionally distinct MAPK cascades in this yeast. Sexual conjugation, cell growth, and adaptation to stress, for example, all require MAPK-mediated cellular responses. A primary function of these cascades appears to be the regulation of gene expression in response to extracellular signals or as part of specific developmental processes. In addition, the MAPK cascades often appear to regulate the cell cycle and vice versa. Despite the success of the gene hunter era in revealing these pathways, there are still many significant gaps in our knowledge of the molecular mechanisms for activation of these cascades and how the cascades regulate cell function. For example, comparison of different yeast signaling pathways reveals a surprising variety of different types of upstream signaling proteins that function to activate a MAPK cascade, yet how the upstream proteins actually activate the cascade remains unclear. We also know that the yeast MAPK pathways regulate each other and interact with other signaling pathways to produce a coordinated pattern of gene expression, but the molecular mechanisms of this cross talk are poorly understood. This review is therefore an attempt to present the current knowledge of MAPK pathways in yeast and some directions for future research in this area.

Abstract: MAP kinases (MAPKs) and their upstream regulatory ki- nases comprise a functional unit that couples upstream input signals to a variety of outputs (reviewed by Blenis, 1993; Blumer and Johnson, 1994; Marshall, 1994). The defining characteristic in this module is the MAPK itself (also called ERK). It has a regulatory kinase, MAPK kinase or MEK (for MAPK/ERK kinase), necessary for its activa- tion. This enzyme is, in turn, regulated by another kinase-- raf, mos, or a group of structurally related kinases termed MEKK (for MEK kinase). Raf is found widely in metazoans and is the upstream member of the raf-MEK-MAPK mod- ule (Figure 1). Budding yeast lacks raf but has two identi- fied MEK kinases, STE11 and BCK1, which are structur- ally related to a mammalian MEKK (Lange-Carter et al., 1993). I shall refer to this structurally related group of ki- nases as the MEKK family. Members of this family have also been identified in tobacco (NPK1; Banno et al., 1993) and in fission yeast (byr2; Wang et al., 1991). These en- zymes are upstream components of the MAPK module MEKK-MEK-MAPK (Figure 1). A recent flurry of activity has revealed the manner in which the raf-MEK-MAPK module is regulated by coupling to membrane receptors 'in a process involving ras and its regulator SOS (reviewed by Blenis, 1993). Little is known in metazoan systems about how MEKK is regulated and about how receptors other than those involving tyrosine kinases feed into MAPK modules. The budding yeast Saccharomyces cerevisiae has at least six identified pathways that contain MAPKs or their presumed upstream regulators. Studies of these different pathways are providing a wealth of information on different possible inputs to MEKK-MEK-MAPK modules (from ser- pentine receptors and perhaps from two-component regu- latory systems) and on new components that play im- portant roles in the MEKK-MEK-MAPK module and are likely to be found in metazoans (Figure 1). In particular, recent work has revealed the existence of an intriguing novel component, STE5, which appears to be a scaffold for association of the three protein kinases in the MEKK- MEK-MAPK module of the yeast pheromone response pathway. This pathway also contains yet another protein kinase, STE20, which likely functions upstream of the MEKK as a link to the G protein. The existence of mamma- lian homologs of STE20 (p65 PAK) raises the possibility that STE20 relatives might also regulate mammalian MEK ki- nases. The MEKK-MEK-MAPK modules of budding yeast are involved in a wide variety of biological processes: they govern transitions in its life cycle--mating and invasive- ness in haploid strains and pseudohyphal development and spore formation in diploid strains--as well as mainte- nance cell wall integrity and response of cells to high osmolarity. A well-studied MAPK module of the fission yeast Schizosaccharomyces pombe is involved in mating and other steps in its life cycle, and an element of a MAPK module has been found in the fungal plant pathogen Usti- lago maydis, where it may play a role in responding to signals from its host plant. One of the purposes this review is to describe the components of these MAPK path- ways and to place them in biological context. Studies with these organisms present an opportunity to use facile ge- netic analysis to identify components of these pathways and to determine whether they play a role in more than one pathway. These studies reveal a case in which a com- ponent functions in more than one pathway within a given cell type (Roberts and Fink, 1994) two examples in which components are functionally redundant (Elion et al., 1991; Irie et al., 1993). These observations and the fact that a unicellular organism has at least six distinct MAPK pathways raise several questions about the specificity with which the MAPKs are activated. Additional goals of this review are to understand the bases for specificity in different pathways and to highlight some of the new molec- ular insights into these pathways that may be relevant to similar systems in metazoans. The diversity and function of MAPKs in yeast have been the subject of several excel- lent recent reviews (Neiman, 1993; Errede and Levin, 1993; Ammerer, 1994).

Abstract: Exposure of the yeast Saccharomyces cerevisiae to high extracellular osmolarity induces the Sln1p-Ypd1p-Ssk1p two-component osmosensor to activate a mitogen-activated protein (MAP) kinase cascade composed of the Ssk2p and Ssk22p MAP kinase kinase kinases (MAPKKKs), the Pbs2p MAPKK, and the Hog1p MAPK. A second osmosensor, Sho1p, also activated Pbs2p and Hog1p, but did so through the Ste11p MAPKKK. Although Ste11p also participates in the mating pheromone–responsive MAPK cascade, there was no detectable cross talk between these two pathways. The MAPKK Pbs2p bound to the Sho1p osmosensor, the MAPKKK Ste11p, and the MAPK Hog1p. Thus, Pbs2p may serve as a scaffold protein.

Abstract: In addition to preventing crosstalk among related signaling pathways, scaffold proteins might facilitate signal transduction by preforming multimolecular complexes that can be rapidly activated by incoming signal. In many cases, such as mitogen-activated protein kinase (MAPK) cascades, scaffold proteins are necessary for full activation of a signaling pathway. To date, however, no detailed biochemical model of scaffold action has been suggested. Here we describe a quantitative computer model of MAPK cascade with a generic scaffold protein. Analysis of this model reveals that formation of scaffold-kinase complexes can be used effectively to regulate the specificity, efficiency, and amplitude of signal propagation. In particular, for any generic scaffold there exists a concentration value optimal for signal amplitude. The location of the optimum is determined by the concentrations of the kinases rather than their binding constants and in this way is scaffold independent. This effect and the alteration of threshold properties of the signal propagation at high scaffold concentrations might alter local signaling properties at different subcellular compartments. Different scaffold levels and types might then confer specialized properties to tune evolutionarily conserved signaling modules to specific cellular contexts.