Protein scaffolds in MAP kinase signalling

doi:10.1016/j.cellsig.2008.11.013

Cellular Signalling

Volume 21, Issue 4, April 2009, Pages 462-469

https://doi.org/10.1016/j.cellsig.2008.11.013 Get rights and content

Abstract

The mitogen-activated protein kinase (MAPK) pathway allows cells to interpret external signals and respond in an appropriate way. Diverse cellular functions, ranging from differentiation and proliferation to migration and inflammation, are regulated by MAPK signalling. Therefore, cells have developed mechanisms by which this single pathway modulates numerous cellular responses from a wide range of activating factors. This specificity is achieved by several mechanisms, including temporal and spatial control of MAPK signalling components. Key to this control are protein scaffolds, which are multidomain proteins that interact with components of the MAPK cascade in order to assemble signalling complexes. Studies conducted on different scaffolds, in different biological systems, have shown that scaffolds exert substantial control over MAPK signalling, influencing the signal intensity, time course and, importantly, the cellular responses. Protein scaffolds, therefore, are integral elements to the modulation of the MAPK network in fundamental physiological processes.

Introduction

The mitogen activated protein kinase (MAPK) pathway is an intracellular signalling cascade, activated by diverse external cues, that regulates many cellular functions including cell proliferation and differentiation [1]. Several growth factors, such as epidermal growth factor (EGF), vascular endothelial growth factor (VEGF), insulin, neurotrophins, and inflammatory cytokines, activate MAPKs. Cells are stimulated by these factors and respond to changes in their environment in part through manipulation of MAPK signalling. Five distinct groups of MAPKs have been identified in mammals. These are MAPK/ERK kinase/extracellular regulated kinase (MEK/ERK), c-Jun N-terminal kinase (JNK), p38, ERK5 and ERK3. The MAPKs can control events both in the nucleus, such as gene regulation, and extra-nuclear events, such as cytoskeletal reorganisation, through phosphorylation and activation of targets in the cytosol and nucleus. Several excellent publications have addressed general MAPK functions [1], [2], [3]. In this review, we focus on the role of protein scaffolds in the modulation of MAPK signalling.

The best characterised of the MAPK pathways is the MEK/ERK cascade. This pathway is activated by protein tyrosine kinase receptors, such as EGF receptor (EGFR) or VEGF receptor (VEGFR) [1]. Briefly, when growth factors bind to their cognate receptors, the receptors undergo dimerisation, inducing phosphorylation by intrinsic tyrosine kinases. Proteins that contain SH2 (Src homology 2) domains are recruited to the receptors and bind to specific phosphotyrosine residues. One of these SH2-containing proteins, Grb2, is constitutively bound to the Ras activator Sos, and normally localises to the cytosol. Relocation to the membrane activates Sos, which in turn activates Ras. Ras is a GTPase, which hydrolyses guanosine triphosphate (GTP) to guanosine diphosphate (GDP). Ras is active in the GTP-bound state. GTP-Ras activates downstream effectors (see below), which propagate signalling. Regulation of the GTP/GDP bound state of Ras provides tight control over its activity. This control is mediated by two different classes of proteins, GTPase activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). GAPs reduce the pool of GTP bound Ras by increasing the intrinsic GTPase activity of Ras (and therefore the rate of GTP hydrolysis). Consequently, Ras GAPs act as “off” switches and reduce Ras activity. In contrast to GAPs, GEFs facilitate the exchange of GDP for GTP, increasing the pool of GTP-bound Ras. Consequently, Ras GEFs increase Ras activity. There are three Ras isoforms in mammals, namely H-Ras, K-Ras and N-Ras. At their C-terminal ends, Ras proteins contain a CAAX motif, in which a cysteine (C) is followed by two aliphatic amino acids (A) and any other amino acid (X). This motif is farnesylated by farnesyl transferase, causing Ras to be localised to both the plasma membrane and internal membranes [4].

Ras-GTP recruits Raf kinase to the membrane. There are three known isoforms of Raf, namely A-Raf, B-Raf and C-Raf (also termed Raf-1), each having distinct functions [5]. When localised to the plasma membrane, the Raf kinases become active and catalyse the phosphorylation and activation of MEK1 and MEK2, which in turn activate ERK1 and ERK2. Once active, ERKs dimerise and either translocate to the nucleus, where they phosphorylate transcription factors, or remain in the cytosol where they phosphorylate substrates in multiple cellular compartments (reviewed in [3]). The predominant sequelae of MEK/ERK signalling are proliferation and differentiation.

The c-Jun N-terminal kinase (JNK) family contains three ubiquitously expressed members, JNK1, JNK2 and JNK3 [3]. The major activators of the JNK pathway are cytokines, selected G-protein coupled receptors (GPCR) and cell stress, such as inhibition of DNA or protein synthesis. JNK is phosphorylated by either MEK4 or MEK7, which are themselves phosphorylated by several kinases, including MEKK1-4, MLK2/3 and DLK. Following activation, JNK is translocated to the nucleus where it phosphorylates and activates several transcription factors, including c-Jun, ATF-2, STAT3 and HSF-1 [3]. JNKs control apoptosis and the development of multiple cell types in the immune system.

There are four members of the p38 kinase family, namely α, β, γ and δ. Cytokines, hormones, G-protein coupled receptors and cell stress, for example, heat or osmotic shock, all stimulate these enzymes [1]. p38 kinases, which are targets of both MEK3 and MEK6, have numerous substrates, including MAPK interacting kinases (Mnk) 1 and Mnk 2, and eukaryotic initiation factor 4e (eIF4e). p38 regulates angiogenesis, cell proliferation, inflammation and cytokine production.

Section snippets

Protein scaffolds

Cells have developed a class of proteins, termed scaffolds or adaptor proteins, which confer spatial and temporal regulation of the MAPK pathway. Protein scaffolds bind to multiple components of the MAPK cascade, bringing them into close proximity and thereby facilitating efficient propagation of the signal (Fig. 1). Consequently, scaffolds act as signal modules, providing an intricate level of control over MAPK signalling. Originally identified in yeast [6], several scaffolds that modulate

Protein scaffolds and MAPK specificity

Despite progress in our comprehension of MAPK signalling, one question that remains poorly understood is how a particular stimulus elicits the correct response. This topic, termed MAPK specificity, seems remarkable when one considers the diverse range of cellular responses induced by many different activators, all of which signal through the MEK/ERK pathway (Fig. 2). Spatial and temporal changes to MAPK signalling influence the cellular response to a specific stimulus and are of particular

MAPK scaffolds as therapeutic targets

The MAPK pathway has been strongly implicated in numerous different cancers (reviewed in [78]). Several MAPK components have been targeted for therapy, but this strategy has failed to yield effective pharmacotherapeutic agents. The lack of efficacy may be due, at least in part, to the lack of selectivity when inhibiting a key signalling kinase, such as MEK, Raf, or p38. Because these kinases regulate myriad cellular functions, inhibiting their activity is likely to affect multiple processes,

Perspectives

Remarkable progress has been made in the last decade in our comprehension of the molecular mechanisms by which MAPK signalling is regulated by protein scaffolds. One of the most intriguing conundrums in understanding MAPK function is how a specific stimulus can induce a specific response, considering the diverse cellular functions that are controlled through MAPKs. This specificity appears to be provided, at least in part, by protein scaffolds. These multidomain proteins control the kinetics of

Acknowledgements

Work in the authors' laboratory is funded by grants from the National Institutes of Health.

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ReviewProtein scaffolds in MAP kinase signalling

Abstract

Introduction

Section snippets

Protein scaffolds

Protein scaffolds and MAPK specificity

MAPK scaffolds as therapeutic targets

Perspectives

Acknowledgements

Endocr. Rev.

Trends Biochem. Sci.

Microbiol Mol. Biol. Rev.

Annu. Rev. Immunol.

Nat. Rev., Mol. Cell Biol.

J. Cell Sci.

Annu. Rev. Cell Dev. Biol.

Nat. Rev., Mol. Cell Biol.

Biochem. Soc. Trans.

Cell

J. Cell Sci.

Curr. Biol.

Mol. Cell. Biol.

Curr. Biol.

Mol. Cell. Biol.

J. Immunol.

Cancer Res.

Trends Cell Biol.

FEBS Lett.

Cell Motil. Cytoskelet.

J. Biol. Chem.

Mol. Cell. Biol.

Proc. Natl. Acad. Sci. U. S. A.

J. Biol. Chem.

Neuron

J. Biol. Chem.

Mol. Biol. Cell

J. Cell Sci.

EMBO Reports

Mol. Biol. Cell

J. Biol. Chem.

Science

Dev. Cell

Curr. Opin. Cell Biol.

Science

J. Cell Biol.

J. Biol. Chem.

Nat. Cell Biol.

J. Biol. Chem.

Dev. Cell

Biochem. Biophys. Res. Commun.

Cell. Signal.

Curr. Opin. Cell Biol.

J. Biol. Chem.

Review
Protein scaffolds in MAP kinase signalling