Interacting JNK-docking sites in MKK7 promote binding and activation of JNK mitogen-activated protein kinases

Unique MAP Kinase binding sites

Map kinases are drug targets for autoimmune disease, cancer, and apoptosis-related diseases. Drug discovery efforts have developed MAP kinase inhibitors directed toward the ATP binding site and neighboring "DFG-out" site, both of which are targets for inhibitors of other protein kinases. On the other hand, MAP kinases have unique substrate and small molecule binding sites that could serve as inhibition sites. The substrate and processing enzyme D-motif binding site is present in all MAP kinases, and has many features of a good small molecule binding site. Further, the MAP kinase p38alpha has a binding site near its C-terminus discovered in crystallographic studies. Finally, the MAP kinases ERK2 and p38alpha have a second substrate binding site, the FXFP binding site that is exposed in active ERK2 and the D-motif peptide induced conformation of MAP kinases. Crystallographic evidence of these latter two binding sites is presented.

p38 pathway kinases as anti-inflammatory drug targets

Mitogen-activated protein kinases (MAPK) are intracellular signaling molecules involved in cytokine synthesis. Several classes of mammalian MAPK have been identified, including extracellular signal-regulated kinase, c-jun N-terminal kinase, and p38 MAP kinase. p38alpha is a key MAPK involved in tumor necrosis factor alpha and other cytokine production, as well as enzyme induction (cyclooxygenase-2, inducible nitric oxide synthase, and matrix metalloproteinases) and adhesion molecule expression. An understanding of the broad biologic and pathophysiological roles of p38 MAPK family members has grown significantly over the past decade, as has the complexity of the signaling network leading to their activation. Downstream substrates of MAPK include other kinases (e.g., mitogen-activated protein-kinase-activated protein kinase 2) and factors that regulate transcription, nuclear export, and mRNA stability and translation. The high-resolution crystal structure of p38alpha has led to the design of selective inhibitors that have good pharmacological activity. Despite the strong rationale for MAPK inhibitors in human disease, direct proof of concept in the clinic has yet to be demonstrated, with most compounds demonstrating dose-limiting adverse effects. The role of MAPK in inflammation makes them attractive targets for new therapies, and efforts are continuing to identify newer, more selective inhibitors for inflammatory diseases.

MAP kinases and the control of nuclear events

The mitogen-activated protein kinases (MAPKs) are a family of serine/threonine kinases that play an essential role in signal transduction by modulating gene transcription in the nucleus in response to changes in the cellular environment. They include the extracellular signal-regulated protein kinases (ERK1 and ERK2); c-Jun N-terminal kinases (JNK1, JNK2, JNK3); p38s (p38alpha, p38beta, p38gamma, p38delta) and ERK5. The molecular events in which MAPKs function can be separated in discrete and yet interrelated steps: activation of the MAPK by their upstream kinases, changes in the subcellular localization of MAPKs, and recognition, binding and phosphorylation of MAPK downstream targets. The resulting pattern of gene expression will ultimately depend on the integration of the combinatorial signals provided by the temporal activation of each group of MAPKs. This review will focus on how the specificity of signal transmission by MAPKs is achieved by scaffolding molecules and by the presence of structural motifs in MAPKs that are dynamically regulated by phosphorylation and protein-protein interactions. We discuss also how MAPKs recognize and phosphorylate their target nuclear proteins, including transcription factors, co-activators and repressors and chromatin-remodeling molecules, thereby affecting an intricate balance of nuclear regulatory molecules that ultimately control gene expression in response to environmental cues.

Manipulation of host signalling pathways by anthrax toxins

Infectious microbes face an unwelcoming environment in their mammalian hosts, which have evolved elaborate multicelluar systems for recognition and elimination of invading pathogens. A common strategy used by pathogenic bacteria to establish infection is to secrete protein factors that block intracellular signalling pathways essential for host defence. Some of these proteins also act as toxins, directly causing pathology associated with disease. Bacillus anthracis, the bacterium that causes anthrax, secretes two plasmid-encoded enzymes, LF (lethal factor) and EF (oedema factor), that are delivered into host cells by a third bacterial protein, PA (protective antigen). The two toxins act on a variety of cell types, disabling the immune system and inevitably killing the host. LF is an extraordinarily selective metalloproteinase that site-specifically cleaves MKKs (mitogen-activated protein kinase kinases). Cleavage of MKKs by LF prevents them from activating their downstream MAPK (mitogen-activated protein kinase) substrates by disrupting a critical docking interaction. Blockade of MAPK signalling functionally impairs cells of both the innate and adaptive immune systems and induces cell death in macrophages. EF is an adenylate cyclase that is activated by calmodulin through a non-canonical mechanism. EF causes sustained and potent activation of host cAMP-dependent signalling pathways, which disables phagocytes. Here I review recent progress in elucidating the mechanisms by which LF and EF influence host signalling and thereby contribute to disease.

Mechanisms of MAPK signalling specificity

MAPK (mitogen-activated protein kinase) signalling pathways contribute to the regulation of diverse responses, including normal and pathological aspects of cell growth, division, differentiation and death. Their ubiquity and versatility raise the issue of how they achieve specific coupling of signal with cellular response. How do the kinases in the cascade distinguish their correct substrates from the vast excess of incorrect substrates? Furthermore, how do different signals elicit distinct responses when they are transmitted by the same components? This short review highlights several mechanisms that can promote specificity in MAPK signalling, including tethering interactions between MAPKs and their substrates and regulators mediated by docking sites, feedback loops and cross-pathway regulatory circuits, and the selective activation of scaffold proteins.

The c-jun kinase/stress-activated pathway: regulation, function and role in human disease

c-Jun N-terminal kinases (JNKs), also referred to as stress-activated kinases (SAPKs), were initially characterized by their activation in response to cell stress such as UV irradiation. JNK/SAPKs have since been characterized to be involved in proliferation, apoptosis, motility, metabolism and DNA repair. Dysregulated JNK signaling is now believed to contribute to many diseases involving neurodegeneration, chronic inflammation, birth defects, cancer and ischemia/reperfusion injury. In this review, we present our current understanding of JNK regulation and their involvement in homeostasis and dysregulation in human disease.

MAP kinase pathways: the first twenty years

The MAP kinases, discovered approximately 20 years ago, together with their immediate upstream regulators, are among the most highly studied signal transduction molecules. This body of work has shaped many aspects of our present views of signal transduction by protein kinases. The effort expended in this area reflects the extensive participation of these regulatory modules in the control of cell fate decisions, i.e., proliferation, differentiation and death, across all eukaryotic phylla and in all tissues of metazoans. The discovery of these kinases is reviewed, followed by a discussion of some of the features of this signaling module that account for its broad impact on cell function and its enormous interest to many investigators.

Physiological roles of MKK4 and MKK7: insights from animal models

c-Jun NH2-terminal protein kinase (JNK) is a mitogen-activated protein kinase (MAPK) involved in the regulation of numerous physiological processes during development and in response to stress. Its activity is increased upon phosphorylation by the MAPK kinases, MKK4 and MKK7. Similar to the early embryonic death of mice caused by the targeted deletion of the jnk genes, mice lacking mkk4 or mkk7 die before birth. The inability of MKK4 and MKK7 to compensate for each other's functions in vivo is consistent with their synergistic effect in mediating JNK activation. However, the phenotypic analysis of the mutant mouse embryos indicates that MKK4 and MKK7 have specific roles that may be due to their selective regulation by extracellular stimuli and their distinct tissue distribution. MKK4 and MKK7 also have different biochemical properties. For example, whereas MKK4 can activate p38 MAPK, MKK7 functions as a specific activator of JNK. Here we summarize the studies that have shed light on the mechanism of activation of MKK4 and MKK7 and on their physiological functions.