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The Ras – MAP kinase
pathway
I
The big
family of
Receptor
Tyrosine
Kinases
Receptor domain architecture and receptor activation by EGF (TGFa)
unusual mode of
allosteric activation, formation of an asymmetric dimer,
followed by trans-phosphorylation of the C-tails
Receptors cannot dimerize because their dimerization arms (domain-II)
are buried within the molecule (bound to domain IV)
II
membrane
cytosol
II
The dimerized receptor; interaction between II-domains
(« dimerization arms »)
II
membrane
II
Detail of the
transphosphorylation
reaction of Cterminal tails by
dimerized EGF
receptors
ATP
tyrosine
Phophorylated
tyrosines and
their
surrounding
amino acids
form docking
sites for
adaptor- and
effector proteins
Different ligands, different receptors and different receptor-combinations,
with different affinities for interaction with adaptor- or effector proteins
A) When large scale
micro-array protein
interaction protocoles
are applied, « the tough
gets going ».
This picture illustrates
the « interactome » of
different
phosphorylated EGF
receptor C-terminal tails
B) Note that with
ERBB2, interactions
vary greatly with
increasing
concentrations of
cytoplasmic tails (i.e.
overexpression of
ERBB2 may change
signalling patterns).
Image from: Jones et al.
Nature 2006;439:168174)
Domain
architecture
of adaptor-,
docking- and
effector
proteins
Different protein-protein interaction domains drive the assembly of
receptor signalling complexes
Branching of signal transduction pathway (signalling network)
Sos mediates guanine nucleotide exchange by widening the nucleotide
binding pocket and by hindering Ser 17 (inside the circle) to interact
with GDP.
Ras
GDP
glu942
hSos
Activation of Ras means; rearranging the switch regions through
the tight link between threonine 35 (in pink) and the third (g)phosphate of ATP. This rearrangement creates favourable
interaction sites with effectors
Ras inactive
GTP
GDP
Ras GDP
Ras active
Effector loop;
no interaction
Ras GTP
interaction with Raf-RBD: effector loop
Raf binds to the Ras effector loop (aa 32-40) with its Ras-binding
domain (RBD). The image below shows how the switch-II region of
RasGDP hinders the interaction with Raf
effector loop
Ras
Ras
GTP
GDP
Raf-RBD
Switch II region
switch regions
RasGTP induces
a cascade of
phosphorylation
reactions, in
which each
kinase activates
the other by
phosphorylation
of residues in
the activation
segment. ERK2
dimerizes and
enters the
nucleus
Exactly how Raf
is activated and
how ERK2 enters
the nucleus
remains
uncertain
MEK activates MAPkinase by phosphorylation of Thr183 and Tyr185.
MEK is a rather unusual protein kinase because of its highly restricted choice of
substrates
MAPKinase inactive
MAPKinase active
ATP
Pthr183
thr183
Ptyr185
tyr185
Activation segment
Substrate entry site
Activated ERKK2 enters
the nucleus and
phosphorylates
transcription factors. In
the case of Elk-1 this
leads to dimerization with
SRF whereas in the case
of c-Fos this results in a
prolonged half-life and
thus more effective
induction of transcription.
With respect to the cell
division cycle, expression
of cyclinD is one of the
consequences of the action
of ERK2.
ERK2 is a member of a large family of mitogen-activated protein kinases
(MAPK) which share sequence identity as well as mode of activation
ERK2, as well as other
members of the MAPK
family, phosphorylates
and activates yet other
protein kinases.
These too constitute a
family, the MAPKactivated protein
kinases.
MNK1, MAP-kinase
interacting kinase, is
phosphorylated and
activated by ERK2.
It binds the
eukaryotic initiation
factor eIF-4E and
eIF-4G.
MNK1
phosphorylates eIF4E and this
facilitates the
association of eIF-3,
an essential protein
for the assembly of
the ribosomal
complex
One of the genes induced by the MAPkinase pathway is the dual specificity
phosphatase MKP-1. It dephosphorylates and de-activates MAPK, both in the
nucleus and the cytoplasm, on its thr-tyr residues
The Ras-ERK signal transduction pathway in other species
Ras-ERK and development of the ommatidium in Drosophila (rhabdomere
formation)
Ras-ERK and the development of the vulva in
Caenorhabdites elegans