The MAP kinase pathway involves signal transduction from activated RAS protein to MAP kinase. The activated RAS starts a kinase cascade involving RAF, MEK and MAPK. MAP kinase then translocates to the nucleus and activates transcription factors like C-Fos. This leads to expression of genes involved in cell cycle progression. The JAK-STAT pathway involves phosphorylation of STAT proteins by JAK kinases upon cytokine binding. Phosphorylated STATs form dimers, enter the nucleus and regulate gene transcription. Both pathways integrate with other signaling cascades and are regulated by phosphatases and inhibitory proteins.

1. MAPkinase
BHAGYA SIRIPALLI,
Pharmacology, SVIPS
PSG COLLEGE OF
PHARMACY
1

2. Contents
MAP kinase pathway
Requirement for signal transduction
RAS activation
Kinase cascade
Inactivation of RAS
Activation of transcription factors by MAP
kinase

3. MAPkinase
Map kinase is an enzyme that translocate the signal to the nucleus and activates many
transcriptional factors by phosphorylating many different proteins.
A mitogen-activated protein kinase (MAPK or MAP kinase) is a type of protein
kinase that is specific to the amino acids serine and threonine (i.e., a serine/threonine-
specific protein kinase). MAPKs are involved in directing cellular responses to a diverse
array of stimuli, such as mitogens, osmotic stress, heat shock and proinflammatory
cytokines.
They regulate cell functions including proliferation, gene
expression, differentiation, mitosis, cell survival, and apoptosis.
MAP kinases are found in eukaryotes only, but they are fairly diverse and encountered in
all animals, fungi and plants, and even in an array of unicellular eukaryotes.

4. MAPkinasepathway
Signal passes from activated RAS to cascade of protein kinases
This cascade transmits signals downstream from activated RAS protein to MAP kinase
Then MAP kinase translocate the signal to the nucleus and activates transcriptional factors
This whole process is called “MAP kinase pathway”

6. Requirement for signaltransduction
Signal – that is to be passed
Receptor- to which the ligand bind
Adaptor proteins – form link between membrane bound receptor and protein is to be
activated
Protein cascade – lead to the activation of transcriptional factors
Transcriptional factors – which carry out transcription process

7. RAS Activation
Epidermal growth factor receptor (EGFR) is a receptor linked tyrosine kinase which is
activated by extracellular ligand Epidermal growth factor (EGF)
Binding of EGR to EGFR leads to the EGFR phosphorylation on tyrosine residues
Docking proteins such as GRB2 contain an SH2 domain binds to the phosphotyrosine residue
of the receptor
Now SOs protein (guanine nucleotide exchange factor) binds to the SH2 domain of the GRB2
protein and forms GRB2 SOs complex.
This GRB2 SOs complex promotes the phosphorylation so the GDP in RAS is replaced with GTP.
Now that the GDP in RAS is converted to GTP, the RAS become active.
This activated RAS starts the kinase cascade.

9. Kinasecascade
Activated RAS activates the RAF kinase
RAF kinase phosphorylates and activate MEK (MEK1 and
MEK2)
MEK phosphorylates and activate MAPK
RAF and MAPK (ERK) are serine/threonine selective
protein kinases
MEK- serine/tyrosine/threonine kinase

10. Inactivation of RAS

11. Activation of transcription factorsby
MAP-kinase
Early response genes (genes which are induced before cells enter the S-phase and
replicate their DNA )
One such gene encodes the transcription factor “C-Fos”
C-Fos with other factors induces expression of many genes encoding proteins which are
necessary for cells to progress through the cell cycle
Enhancers that regulate the C-Fos genes contain a serum response factor (SRF) and
ternary complex factor (TCF)
By modifying these two factors MAPK induces transcription of C-Fos

12. Activation of transcription factorsby
MAP-kinase contd.,
In Cytosol:
MAPK phosphorylates and activates a kinase P90 RSK, which translocate to the nucleus and
phosphorylates a specific serine in SRF
In Nucleus:
MPAK directly phosphorylates specific serine in TCF
Association of phosphorylated TCF with two molecules of phosphorylated SRF forms an
active trimeric factor

13. Activation of transcription factorsby
MAP-kinase contd.,
Scaffold proteins
Scaffold strategy prevent cross-talk between different MAPK modules
Atleast 5 parallel MAPK modules can operate in a mammalian cell
These modules make use of atleast 12 MAP kinases, 7 MAP kinase kinases and 7 MAP kinas
kinase kinase

15. JAK-STATpathway
JAK Janus kinase, STAT– Signal Transducer and Activation of Transcription
This signalling pathway transmits information from chemical signals outside the cell,
which cause DNA transcription and activity
4 JAK proteins – JAK1, JAK 2, JAK 3, TYK2
JAKs contains a FERM domain, an SH2 related domain, a kinase domain and a
pseudokinase domain
Kinase domain allows JAK to phosphorylate proteins
7 STATproteins STAT1,STAT2,STAT3,STAT4,STAT5A, STAT5B, STAT6
STATcontain many different domains, most conserved region is the SH2 domain

16. JAK-STATpathwaymechanism
Agonist induced dimerization alters the intracellular
domain conformation to increase the affinity for JAK
On binding, JAK gets activated and phosphorylates
tyrosine residues of receptor
Binding of STATand its phosphorylation by JAK
Phosphorylation STATdimerize and translocate to
the nucleus to regulate gene transcription

17. Movement of STATsfrom cytosol to the
nucleus
STATdimers have to pass through nucleus pore complex (NPCs) which is present along the
nucleus envelope
Toenable STATsto move into the nucleus, an amino acid sequence on STATs, called nuclear
localisation signal (NLS) is bound by proteins called importins
After entering, a protein called Ran binds to the importins releasing them from the STAT dimer
STATdimer is then free in the nucleus
Specific STAT– specific importin proteins

18. Integration with other signalling
pathways
JAKs phosphorylate cytokine receptors which can
bind Grb2 protein
Grb2 activates Sos proteins MAPK
signalling
Phosphorylated cytokine receptors ca also bound
by P13k
P13k activates AKT
Thus JAK-STAT is able to interconnect with other
cell-signalling pathways such as P13k/ AKT/ mTOR/
pathway.

19. Regulation
1. Protein inhibitors of activated STATs (PIAS)
2. Protein tyrosine phosphatases (PTPs)
3 major PTPs – SHP-1, HP-2, CD45
3. Suppressors of cytokine signalling (SoCS)