Selectivity in overlapping MAP kinase cascades

Some protein kinases operate in more than one mitogen-activated protein-kinase (MAPK) cascade. We here address the question whether specificity of the cascades necessitates physical sequestration of these "promiscuous" kinases (e.g. by binding to scaffolds). A model is constructed, in which two MAPK cascades depend on a single MAP-kinase kinase that is not sequestered in two subpopulations. We show that in this model selective signal transduction is possible provided that there is an additional interaction at the MAP-kinase level, there is no simultaneous activation of more than one response by either signal. We discuss a number of additional interactions that can generate the selectivity, as well as some kinetic features by which this mechanism may be recognized experimentally.     

MAP kinase cascades in elicitor signal transduction

 Protein kinases play important roles in elicitor signal transduction. In this article, I describe the current view of the role of mitogen-activated protein kinase (MAPK) cascades in elicitor signal transduction of plant cells based on our own research and recent developments in this field. In the past several years, it has become apparent that MAPK cascades play important roles in elicitor signal transduction in plants. Our early studies demonstrated the identification of p47 MAPK in tobacco as an elicitor-responsive protein kinase and possible involvement of p47 MAPK in elicitor signal transduction to induce defense responses, including defense gene expression and hypersensitive cell death. However, the molecular identity of p47 MAPK is still unclear. Recent important studies suggest that tobacco MAPK cascades that include SIPK, and/or WIPK, and NtMEK2, an upstream kinase for both SIPK and WIPK, have a crucial function in induction of defense responses and hypersensitive cell death. The orthologs of these protein kinases in Arabidopsis and alfalfa are also suggested to have similar functions. Furthermore, the identification of loss-of-function mutation in Arabidopsis reveals a negative regulatory role for putative MAPK cascades in plant defense mechanisms. Received: February 7, 2002 / Accepted: February 25, 2002     

Scaffold proteins in mammalian MAP kinase cascades

The mitogen-activated protein kinase (MAPK) signaling pathway, which is conserved from yeast to humans, is activated in response to a variety of extra- and intracellular stimuli, and plays key roles in multiple cellular processes, including proliferation, differentiation, and apoptosis. The MAPK pathway transmits its signal through the sequential phosphorylation of MAPK kinase kinase to MAPK kinase to MAPK. Specific and efficient activation of the MAPK cascades is crucial for proper cellular responses to stimuli. As shown in yeast, the mammalian MAPK signaling system may also employ scaffold proteins, in part, to organize the MAPK signaling components into functional MAPK modules, thereby enabling the efficient activation of specific MAPK pathways. This review article describes recent advances in the study of potential mammalian scaffold proteins that may help us understand the complex regulation, including the spatial and temporal control, of the mammalian MAPK signaling pathways.     

Tunable signal processing in synthetic MAP kinase cascades

The flexibility of MAPK cascade responses enables regulation of a vast array of cell fate decisions, but elucidating the mechanisms underlying this plasticity is difficult in endogenous signaling networks. We constructed insulated mammalian MAPK cascades in yeast to explore how intrinsic and extrinsic perturbations affect the flexibility of these synthetic signaling modules. Contrary to biphasic dependence on scaffold concentration, we observe monotonic decreases in signal strength as scaffold concentration increases. We find that augmenting the concentration of sequential kinases can enhance ultrasensitivity and lower the activation threshold. Further, integrating negative regulation and concentration variation can decouple ultrasensitivity and threshold from the strength of the response. Computational analyses show that cascading can generate ultrasensitivity and that natural cascades with different kinase concentrations are innately biased toward their distinct activation profiles. This work demonstrates that tunable signal processing is inherent to minimal MAPK modules and elucidates principles for rational design of synthetic signaling systems.     

MAP kinase cascades: scaffolding signal specificity

Scaffold proteins organize many MAP kinase pathways by interacting with several components of these cascades. Recent studies suggest that scaffold proteins provide local activation platforms that contribute to signal specificity by insulating different MAP kinase pathways.     

Roles of MAP kinase cascades in Caenorhabditis elegans

Mitogen-activated protein kinases (MAPKs) are serine/threonine protein kinases that are activated by diverse stimuli such as growth factors, cytokines, neurotransmitters and various cellular stresses. MAPK cascades are generally present as three-component modules, consisting of MAPKKK, MAPKK and MAPK. The precise molecular mechanisms by which these MAPK cascades transmit signals is an area of intense research, and our evolving understanding of these signal cascades has been facilitated in great part by genetic analyses in model organisms. One organism that has been commonly used for genetic manipulation and physiological characterization is the nematode Caenorhabditis elegans. Genes sequenced in the C. elegans genome project have furthered the identification of components involved in several MAPK pathways. Genetic and biochemical studies on these components have shed light on the physiological roles of MAPK cascades in the control of cell fate decision, neuronal function and immunity in C. elegans.     

Signal transduction by MAP kinase cascades in budding yeast

Budding yeast contain at least four distinct MAPK (mitogen activated protein kinase) cascades that transduce a variety of intracellular signals: mating-pheromone response, pseudohyphal/invasive growth, cell wall integrity, and high osmolarity adaptation. Although each MAPK cascade contains a conserved set of three protein kinases, the upstream activation mechanisms for these cascades are diverse, including a trimeric G protein, monomeric small G proteins, and a prokaryotic-like two-component system. Recently, it became apparent that there is extensive sharing of signaling elements among the MAPK pathways; however, little undesirable cross-talk occurs between various cascades. The formation of multi-protein signaling complexes is probably centrally important for this insulation of individual MAPK cascades.     

The potential for signal integration and processing in interacting MAP kinase cascades

The cellular response to environmental stimuli requires biochemical information processing through which sensory inputs and cellular status are integrated and translated into appropriate responses by way of interacting networks of enzymes. One such network, the mitogen-activated protein (MAP) kinase cascade is a highly conserved signal transduction module that propagates signals from cell surface receptors to various cytosolic and nuclear targets by way of a phosphorylation cascade. We have investigated the potential for signal processing within a network of interacting feed-forward kinase cascades typified by the MAP kinase cascade. A genetic algorithm was used to search for sets of kinetic parameters demonstrating representative key input-output patterns of interest. We discuss two of the networks identified in our study, one implementing the exclusive-or function (XOR) and another implementing what we refer to as an in-band detector (IBD) or two-sided threshold. These examples confirm the potential for logic and amplitude-dependent signal processing in interacting MAP kinase cascades demonstrating limited cross-talk. Specifically, the XOR function allows the network to respond to either one, but not both signals simultaneously, while the IBD permits the network to respond exclusively to signals within a given range of strength, and to suppress signals below as well as above this range. The solution to the XOR problem is interesting in that it requires only two interacting pathways, crosstalk at only one layer, and no feedback or explicit inhibition. These types of responses are not only biologically relevant but constitute signal processing modules that can be combined to create other logical functions and that, in contrast to amplification, cannot be achieved with a single cascade or with two non-interacting cascades. Our computational results revealed surprising similarities between experimental data describing the JNK/MKK4/MKK7 pathway and the solution for the IBD that evolved from the genetic algorithm. The evolved IBD not only exhibited the required non-monotonic signal strength-response, but also demonstrated transient and sustained responses that properly reflected the input signal strength, dependence on both of the MAPKKs for signaling, phosphorylation site preferences by each of the MAPKKs, and both activation and inhibition resulting from the overexpression of one of the MAPKKs.     

The Ste20 group kinases as regulators of MAP kinase cascades

Ste20p (sterile 20 protein) is a putative yeast mitogen-activated protein kinase kinase kinase kinase (MAP4K) involved in the mating pathway. Its homologs in mammals, Drosophila, Caenorhabditis elegans and other organisms make up a large emerging group of protein kinases including 28 members in human. The Ste20 group kinases are further divided into the p21-activated kinase (PAK) and germinal center kinase (GCK) families. They are characterized by the presence of a conserved kinase domain and a noncatalytic region of great structural diversity that enables the kinases to interact with various signaling molecules and regulatory proteins of the cytoskeleton. This review describes the phylogenetic relationships of the Ste20 group kinases based on discussions with many researchers in this field. With the newly established phylogenetic relationships, crucial arguments can be advanced regarding the functions of these kinases as upstream activators of the MAPK pathways and possible activity as MAP4Ks. Their involvement in apoptosis, morphogenesis and cytoskeletal rearrangements is also discussed.     

Molecular basis of MAP kinase regulation

Mitogen‐activated protein kinases (MAPKs; ERK1/2, p38, JNK, and ERK5) have evolved to transduce environmental and developmental signals (growth factors, stress) into adaptive and programmed responses (differentiation, inflammation, apoptosis). Almost 20 years ago, it was discovered that MAPKs contain a docking site in the C‐terminal lobe that binds a conserved 13‐16 amino acid sequence known as the D‐ or KIM‐motif (kinase interaction motif). Recent crystal structures of MAPK:KIM‐peptide complexes are leading to a precise understanding of how KIM sequences contribute to MAPK selectivity. In addition, new crystal and especially NMR studies are revealing how residues outside the canonical KIM motif interact with specific MAPKs and contribute further to MAPK selectivity and signaling pathway fidelity. In this review, we focus on these recent studies, with an emphasis on the use of NMR spectroscopy, isothermal titration calorimetry and small angle X‐ray scattering to investigate these processes.     

Antagonistic interactions between two MAP kinase cascades in plant development and immune signaling

Mitogen‐activated protein kinase (MAPK) signaling plays important roles in diverse biological processes. In Arabidopsis, MPK3/MPK6, MKK4/MKK5, and the MAPKKK YODA (YDA) form a MAPK pathway that negatively regulates stomatal development. Brassinosteroid (BR) stimulates this pathway to inhibit stomata production. In addition, MPK3/MPK6 and MKK4/MKK5 also serve as critical signaling components in plant immunity. Here, we report that MAPKKK3/MAPKKK5 form a kinase cascade with MKK4/MKK5 and MPK3/MPK6 to transduce defense signals downstream of multiple plant receptor kinases. Loss of MAPKKK3/MAPKKK5 leads to reduced activation of MPK3/MPK6 in response to different pathogen‐associated molecular patterns (PAMPs) and increased susceptibility to pathogens. Surprisingly, developmental defects caused by silencing of YDA are suppressed in the mapkkk3 mapkkk5 double mutant. On the other hand, loss of YDA or blocking BR signaling leads to increased PAMP‐induced activation of MPK3/MPK6. These results reveal antagonistic interactions between a developmental MAPK pathway and an immune signaling MAPK pathway.     

Signaling function of phagocytic NADPH oxidase: Activation of MAP kinase cascades in phagocytosis

Until recently, the production of reactive oxygen species by NADPH oxidase has been considered only in the context of the oxidative damage to pathogens inside the phagosome. However, homologues of phagocytic NADPH oxidase have been found in almost all cell types, where they produce hydrogen peroxide and thereby regulate the initial intracellular stages of MAP kinase cascades. In the present work, the activation of two MAP kinase cascades, p38 and Erk1/2, during phagocytosis has been studied. It was found that phagocytosis activates both cascades. The activation of Erk1/2 is dependent, and the activation of p38 is not dependent, on the activity of NADPH oxidase. Therefore, the activation of MAP kinases in phagocytes during phagocytosis occurs by a mechanism similar to that operating in nonphagocytic cells, indicating the universality of the function of NADPH oxidases in different cell types.     

Mitogen and stress response pathways: MAP kinase cascades and phosphatase regulation in mammals and yeast

Evolutionarily conserved from yeast to man, mitogen-activated protein kinase (MAPK) pathways respond to a variety of disparate signals which induce differentiation, proliferation, or changes in intracellular enzyme regulation. Recent advances have identified two new mammalian MAPK relatives, JNK1 and p38, and the pathways which are responsible for their activation.     

A novel functional link between MAP kinase cascades and the Ras/cAMP pathway that regulates survival

In mammalian cells, Ras regulates multiple effectors, including activators of mitogen-activated protein kinase (MAPK) cascades, phosphatidylinositol-3-kinase, and guanine nucleotide exchange factors (GEFs) for RalGTPases. In S. cerevisiae, Ras regulates the Kss1 MAPK cascade that promotes filamentous growth and cell integrity, but its major function is to activate adenylyl cyclase and control proliferation and survival ([; see Figure S1 in the Supplemental Data available with this article online). Previous work hints that the mating Fus3/Kss1 MAPK cascade cross-regulates the Ras/cAMP pathway during growth and mating, but direct evidence is lacking. Here, we report that Kss1 and Fus3 act upstream of the Ras/cAMP pathway to regulate survival. Loss of Fus3 increases cAMP and causes poor long-term survival and resistance to stress. These effects are dependent on Kss1 and Ras2. Activation of Kss1 by a hyperactive Ste11 MAPKKK also increases cAMP, but mating receptor/scaffold activation has little effect and may therefore insulate the MAPKs from cross-regulation. Catalytically inactive Fus3 represses cAMP by blocking accumulation of active Kss1 and by another function also shared by Kss1. The conserved RasGEF Cdc25 is a likely control point, because Kss1 and Fus3 complexes associate with and phosphorylate Cdc25. Cross-regulation of Cdc25 may be a general way that MAPKs control Ras signaling networks.