The Ras – MAP kinase pathway

I

The big family of ReceptorTyrosine Kinases

Receptor domain architecture and receptor activation by EGF (TGF)

unusual mode of allosteric activation, formation of an asymmetric dimer,

followed by trans-phosphorylation of the C-tails

membrane

cytosol

II II

Receptors cannot dimerize because their dimerization arms (domain-II) are buried within the molecule (bound to domain IV)

The dimerized receptor; interaction between II-domains(« dimerization arms »)

II II

membrane

Detail of the trans-phosphorylation reaction of C-terminal 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;439:168-174)

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.

glu942

Ras

GDP

hSos

Effector loop; no interaction

interaction with Raf-RBD: effector loop

Activation of Ras means; rearranging the switch regions through the tight link between threonine 35 (in pink) and the

third ()-phosphate of ATP. This rearrangement creates favourable interaction sites with effectors

GDPGTP

Ras inactive Ras active

Ras GDP Ras GTP

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

Raf-RBD

Ras Ras

GDP GTP

switch regions

effector loop

Switch II region

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

Ptyr185

thr183

tyr185

Pthr183

MAPKinase inactive MAPKinase active

ATP

Substrate entry siteActivation segment

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 MAPK-activated 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 eIF-4E 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