Differential regulation and properties of MAPKs
M Raman, W Chen and MH Cobb
Department of Pharmacology, The University of Texas Southwestern Medical Center, Dallas, TX, USA
Mitogen-activated protein kinases (MAPKs) regulate
diverse cellular programs including embryogenesis, pro-
liferation, differentiation and apoptosis based on cues
derived from the cell surface and the metabolic state and
environment of the cell. In mammals, there are more than
a dozen MAPK genes. The best known are the
extracellular signal-regulated kinases 1 and 2 (ERK1/2),
c-Jun N-terminal kinase (JNK(1–3)) and p38(a, b, c and
d) families. ERK3, ERK5 and ERK7 are other MAPKs
that have distinct regulation and functions. MAPK
cascades consist of a core of three protein kinases.
Despite the apparently simple architecture of this path-
way, these enzymes are capable of responding to a
bewildering number of stimuli to produce exquisitely
specific cellular outcomes. These responses depend on the
kinetics of their activation and inactivation, the subcel-
lular localization of the kinases, the complexes in which
they act, and the availability of substrates. Fine-tuning of
cascade activity can occur through modulatory inputs to
cascade component from the primary kinases to the
scaffolding accessory proteins. Here, we describe some of
the properties of the three major MAPK pathways and
discuss how these properties govern pathway regulation
Oncogene (2007) 26, 3100–3112. doi:10.1038/sj.onc.1210392
Keywords: kinase; MAPK; ERK; JNK; p38; phosphory-
The ERK1/2 MAPKs
The protein kinase cascade
Extracellular signal-regulated kinase 1 (ERK1) and
ERK2 were identified as growth factor-stimulated
protein kinases phosphorylating microtubule-associated
protein-2 (MAP-2) and myelin basic protein (MBP) 20
years ago. They are 43 and 41kDa, share 83% identity,
and are ubiquitously expressed. ERK1/2 phosphorylate
the protein kinases p90 ribosomal S6 kinase (RSK),
mitogen and stress activated kinase (MSK) and MAPK
interacting kinase (MNK) proteins involved in cell
attachment and migration, including paxillin, focal
adhesion kinase and calpain; and the transcription
factors Elk1, c-Fos, c-Myc and Ets domain factors,
among many others. ERK1/2 are activated to varying
extents by growth factors, serum, phorbol esters and
ligands for heterotrimeric G protein-coupled receptors,
cytokines, transforming growth factors, osmotic and
other cell stresses, and microtubule depolymerization.
Activation of ERK1/2 is involved in many cellular
responses such as cell motility, proliferation, differentia-
tion and survival (Lewis et al., 1998; Chen et al.,
2001; Johnson and Lapadat, 2002; Yoon and Seger,
The mitogen-activated protein kinase (MAPK) ki-
nases (MAP2Ks) MEK1 and -2 are the upstream dual-
specificity ERK1/2 kinases that phosphorylate tyrosine
and threonine residues in the ERK1/2 activation loops
(Figure 1). The MAP2K kinases or MAP3Ks phosphor-
ylate two serine residues or a serine and a threonine
residue in the activation loop of the MAP2Ks to activate
them. In the ERK1/2 pathway Raf isoforms are the
primary MAP3Ks. Mos, Tpl2, and perhaps other
MAP3Ks are utilized in a more restricted cell type and
Raf isoforms, A-Raf, B-Raf and C-Raf or Raf-1, are
the best-studied MAP3Ks and those that most fre-
quently and selectively regulate the ERK1/2 pathway.
Raf is activated by a combination of binding of small
G proteins of the Ras family to its N-terminus and
phosphorylation. Raf-1 was initially identified as the
MAP3K in the ERK1/2 pathway through its ability
to activate MEK1 and shortly thereafter as the
effector through which Ras controls the cascade (Rapp
et al., 2006).
Mos is the MAP3K utilized during oocyte matura-
tion. Mos activates ERK1/2 in Xenopus cell-free extracts
and intact oocytes. Activation of the pathway requires
translation of Mos, distinguishing its regulation from
that of other MAP3Ks in this pathway. In immature
oocytes, Mos mRNA is present but not translated into
protein. In response to steroid receptor stimulation, an
increase in polyadenylation of Mos transcript is
observed. This increases Mos protein production, which
activates MEK1 and subsequently ERK1/2. Active
ERK1/2 positively influence Mos protein production
and amplify activation of the pathway leading to CDK1
activation and germinal vesicle breakdown. This germ-
cell-specific mechanism of ERK1/2 activation has been
known for a number of years, but factors that regulate
this cascade have not been identified. The scaffold
paxillin, often found in focal adhesions, is required for
Mos-mediated ERK2 activation in Xenopus oocytes
Correspondence: Dr MH Cobb, 6001 Forest Park Road, Dallas, TX
Oncogene (2007) 26, 3100–3112
& 2007 Nature Publishing Group All rights reserved 0950-9232/07 $30.00
(Rasar et al., 2006). Activated ERK2 positively impacts
Mos production at least in part by phosphorylation of
paxillin, which is indirectly or directly due to ERK2.
The ERK2-dependent phosphorylation of paxillin ac-
celerates Mos protein production thereby driving
meiotic progression. Paxillin may regulate Mos via
effects on processing or translation of the mRNA and
on protein stability.
The MAP3K Tpl2, related to yeast protein kinases in
the mating pathway, was originally identified as the Cot
ERK1/2, c-Jun N-terminal kinase (JNK), p38 and
ERK5 via its ability to phosphorylate and activate
MAP2Ks upstream of these MAPKs (Salmeron et al.,
1996; Chiariello et al., 2000). The signaling role of Tpl2
depends on both the cell type and stimulus. Studies in
Tpl2 knockout mice have shown that Tpl2 is required
for ERK1/2 but not for JNK activation in lipopoly-
saccharide (LPS)-stimulated macrophages and B cells
(Dumitru et al., 2000). On the other hand, Tpl2 is
required for the activation of ERK1/2 and JNK by
tumor necrosis factor-alpha (TNF-a) in mouse embryo-
nic fibroblasts (MEFs) (Das et al., 2005). Additionally,
in macrophages and B cells, Tpl2 is an essential
mediator of ERK1/2 activation in response to diverse
Toll-like receptor (TLR) signals (Cho and Tsichlis, 2005;
Babu et al., 2006). Activation of Tpl2 is dependent on
several steps. In contrast to Raf and Mos, Tpl2 is
stabilized by association with p105 of the nuclear factor-
kappa B (NF-kB) pathway. Its release from p105 is
required to activate MAPK cascades (Kolch, 2005).
Much of what we know currently about ligand-
induced activation of MAPK cascades has been inferred
from single ligand treatments in specific cell types.
However, in vivo, the cell is sensing hundreds of different
cues at any given time. Several efforts have been
undertaken to assess responses of cells to combinations
of ligands and to elucidate how responses to multiple
stimuli are integrated to produce discrete outcomes. For
example, depending on the cell type and stimulating
ligand, cyclic AMP (cAMP) can either inhibit or activate
ERK1/2. Phorbol ester-induced activation of ERK1/2 is
whereas epidermal growth factor (EGF) is capable of
activating ERK1/2 independent of changes in cAMP
concentration (Pearson et al., 2006). Aberrant interac-
tions between the ERK1/2 and cAMP pathways are
involved in diseases ranging from polycystic kidney
disease to melanoma (Yamaguchi et al., 2003; Rapp
et al., 2006). cAMP also differentially regulates the
MAPK ERK5 in a cell-type- and context-dependent
manner. cAMP inhibits EGF-induced ERK5 activation
and subsequently decreases cell proliferation. cAMP-
dependent protein kinase phosphorylates MEKK2,
a key MAP3K upstream of ERK5 activation (see
Johnson, this issue), thus inhibiting activation of this
cascade by growth factors (Pearson et al., 2006). Thus,
it appears that cells with elevated cAMP are more
refractory to growth factor stimulation owing to
some re-wiring of signaling interactions upstream of
of cAMP accumulation,
Regulation of ERK1/2 interactions via docking motifs
Docking motifs in MAPKs, as well as reciprocal
interaction motifs in regulators and substrates, are
critical specificity determinants in MAPK signaling
(Table 1). A motif recognized by most MAPKs is the
docking or D motif. These motifs are found in
substrates as well as MAP2Ks, phosphatases and
scaffolds. Frequently, these motifs appear N-terminus
to the typical serine/threonine–proline phosphorylation
site preferred by ERK1/2 and are characterized by a
cluster of positively charged residues with two or more
nearby hydrophobic residues (Sharrocks et al., 2000;
Tanoue et al., 2000). In addition to the D motif, the
MEK1MEK2MEK4MEK7 MEK3 MEK6 MEK5
Raf Mos Tpl MEKK MUK
SPRK MLK TAK ASK
Elk-1 Elk-1Elk-1Sap-1 Sap-1 bHLH Sap-1
Figure 1MAPK cascades. Illustration of the three-tiered MAPK cascades for ERK, JNK and p38 family members.
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