Review articleThe ups and downs of MEK kinase interactions
Introduction
Regulatory mechanisms controlling the proliferation, differentiation, or apoptosis of cells involve intracellular protein kinases that can transduce signals detected on the cell's surface into changes in gene expression. Most prominent amongst the known signal transduction pathways that control these events are the mitogen-activated protein kinase (MAPK) cascades, whose components are evolutionarily highly conserved in structure and organisation, each consisting of a module of three cytoplasmic kinases: a mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK), an MAP kinase kinase (MAPKK), and the MAP kinase (MAPK) itself. The MAPKKK is a serine–threonine kinase that receives activating signals from a membrane-spanning receptor and then phosphorylates and activates its substrate, an MAPKK. This enzyme is a dual-specificity kinase with the potential to phosphorylate critical threonine and tyrosine residues in its substrate protein, the MAPK. MAPKs represent a family of serine–threonine kinases with the potential to phosphorylate other cytoplasmic proteins and to translocate from the cytoplasm to the nucleus, where they can directly regulate the activity of transcription factors controlling gene expression.
The best-investigated MAPK cascades are those of the budding yeast Saccharomyces cerevisiae. This organism uses five different MAPK modules to regulate mating and sporulation and to respond to environmental changes such as high osmolarity, cell integrity, starvation, and filamentous growth (reviewed in Ref. [1]). In the case of the mating response, pheromone stimulation of haploid cells of the opposite sex (a and α) causes cellular responses that prepare the cell for mating, specifically fusion with the mating partner to form a diploid. These responses include polarised growth towards the mating partner, an arrest of the cell cycle in G1, an increased expression of proteins required for cell adhesion and cell and nuclear fusion. Pheromone ligand activates a seven transmembrane-spanning receptor that in turn stimulates GTP binding to a heterotrimeric G protein (Fig. 1). Subsequent dissociation of the G protein liberates βγ subunits (Ste4p/Ste18p) that bind to a scaffold protein, Ste5p, which assembles the MAPK module by simultaneous interaction with the MAPKKK, Ste11p, the MAPKK, Ste7p, and the MAPK, Fus3p. The scaffold protein, Ste5p, also has the potential to homo-oligomerise and to interact with elements of the cytoskeleton via other associated proteins, thereby forming a large signalling complex (Fig. 1). G protein βγ subunits also activate Ste20p, an MAPKKK kinase that phosphorylates the MAPKKK, Ste11p. For activation of Ste11p, binding of another protein, Ste50p, is also necessary. These events allow the signal to be transduced via the MAPKK, Ste7p, to the MAPK Fus3p. In unstimulated cells, Fus3p appears to form a complex with three other factors, Dig1p, Dig2p, and Ste12p. Ste12p is a transcription factor and Dig1p and Dig2p are associated inhibitory proteins. Pheromone stimulation causes phosphorylation of these three factors by Fus3p, leading to release of Ste12p from Dig1p and Dig2p, thereby allowing Ste12p to interact with other proteins of the transcriptional machinery and to activate transcription [2].
The best understood MAPK signal transduction pathway of mammalian cells is that formed by the Raf–MAPK/ERK kinase (MEK)–extracellular regulated kinase (ERK) module (Fig. 1). Proliferative signals like growth factors cause activation and autophosphorylation of their cognate receptor tyrosine kinases. The phosphorylated tyrosine residues of the receptor serve as docking sites for the adaptor protein Grb2, which is complexed with Sos. Sos is a GTP exchange factor that activates the small G protein Ras, which thereby recruits the MAPKKK Raf to the plasma membrane where it is activated. The details of Raf activation are not yet fully understood, but activated Raf signals via its substrate MEK to the MAPK, ERK (reviewed in [3], [4]).
Whereas Fus3p and Ste7p in S. cerevisiae are direct homologues of mammalian ERK and MEK, respectively, there are no structural homologues of Raf kinases present in yeast; the MAPKKK's that perform the function of Raf in S. cerevisiae and Schizosaccharomyces pombe are the protein kinases Ste11p and Byr2p, respectively. In order to identify homologues of Ste11p and Byr2p in mammalian cells, a degenerate PCR-based strategy was used, and this led to the cloning of a mammalian MAPKKK, termed MEK kinase 1 (MEKK1) [5], [6]. Subsequently, the same strategy was used to identify three additional homologues, designated MEKK2, MEKK3 [7], and MEKK4 [8]. This review will focus on these four mammalian MAPKKKs of the MEKK family, with emphasis on those proteins shown to interact with them and serve as their upstream regulators or downstream substrates.
Section snippets
The MEKK family
MEKK1, 2, 3, and 4 were cloned based on their similarity to the yeast MAPKKKs Ste11p and Byr2p in their respective catalytic domains. They are serine–threonine kinases with a C-terminal catalytic domain and an N-terminal regulatory domain (Fig. 2). Whereas the kinase domain of MEKK1 shares 75% similarity and 35% identity with the kinase domains of Byr2p and Ste11p [5], it is approximately 50% homologous to the corresponding domains of MEKK2, 3, and 4 [8]. The N-terminal regulatory domains of
MEKK-regulated MAPK cascades
The first cloned mammalian Ste11p/Byr2p homologue, MEKK1, received its name based on early biochemical studies that demonstrated its potential to phosphorylate and activate the MAPKK MEK, a property previously thought to be exclusive to Raf [5]. MEKK1 has been shown to interact via its catalytic domain with MEK, phosphorylating it on serines 218 and 222 in vitro and in vivo [11], [12]. These sites are identical to those phosphorylated by Raf and this mechanism accounts for full MEK activation
Other functions of MEKKs
Recent work has revealed that MEKKs not only regulate MAPK signalling cascades, but that they have a broader spectrum of functions, some of which are independent of MAPK activation.
How specificity is achieved
The likelihood that MEKK isozymes activate distinct signalling pathways in a stimulus-specific manner, yet show the potential to participate in many overlapping functions, raises the question of how specificity in MEKK signalling is achieved. Subcellular localisation might be one answer. As already discussed, caspase cleavage of membrane-associated full-length MEKK1, which appears to promote survival, releases a pro-apoptotic kinase domain into the cytoplasm where it may phosphorylate a
Activation of MEKKs
As broad as the downstream effects of MEKKs are the stimuli activating them: EGF- [49], [85], TNFα- [46], Fcε- [86], CD28- [87], and IL1- [44] receptor-mediated stimulation of cells have been shown to increase MEKK1 activity, as does DNA- [36] and microtubule-damaging agents [32], [33]. Little is known about the extracellular stimuli that activate MEKK2, 3, and 4, and only a few reports suggest mechanisms for MEKK regulation. In the case of growth factor receptors, effector kinases are in many
Prospects
This review summarises our current knowledge about the regulation and function of MEKK family members, with emphasis on factors that are capable of directly interacting with individual isoforms. Although much is already known about the regulation and function of MEKK1, further studies will be necessary to establish the functionally important signal transduction pathways regulated by this MAPKKK in response to specific cellular stimuli. It is clear that caspase-mediated cleavage of MEKK1 is
References (107)
- et al.
Cell
(1997) - et al.
Trends Biochem Sci
(1994) - et al.
Exp Cell Res
(1999) - et al.
J Biol Chem
(1996) - et al.
J Biol Chem
(1997) - et al.
J Biol Chem
(1995) - et al.
J Biol Chem
(1994) - et al.
J Biol Chem
(1997) - et al.
J Biol Chem
(1996) - et al.
Exp Cell Res
(1999)
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
Cell
J Biol Chem
J Biol Chem
J Biol Chem
Curr Opin Genet Dev
Cell
Curr Opin Cell Biol
Cell
Cell
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
Cell
J Biol Chem
J Biol Chem
Immunity
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
Cell
FEBS Lett
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
J Biol Chem
Cell
Cell
Cell
Curr Biol
Cited by (251)
An insight into crosstalk among multiple signalling pathways contributing to the pathophysiology of PTSD and depressive disorders
2024, Progress in Neuro-Psychopharmacology and Biological PsychiatryA systems and computational biology perspective on advancing CAR therapy
2023, Seminars in Cancer BiologyIL-1β induced IL-8 and uPA expression/production of dental pulp cells: Role of TAK1 and MEK/ERK signaling
2018, Journal of the Formosan Medical AssociationAeromonas hydrophila-induced alterations in cytosolic calcium activate pro-apoptotic cPKC-MEK1/2-TNFα axis in infected headkidney macrophages of Clarias gariepinus
2017, Developmental and Comparative ImmunologyJNK inhibition enhances cell–cell adhesion impaired by desmoglein 3 gene disruption in keratinocytes
2024, Histochemistry and Cell BiologyDachshund Homolog 1: Unveiling Its Potential Role in Megakaryopoiesis and Bacillus anthracis Lethal Toxin-Induced Thrombocytopenia
2024, International Journal of Molecular Sciences