Map kinase

MAP Kinase

1. MAP kinase 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. MAP kinase 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. MAP kinase pathway 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”

5. Requirement for signal transduction 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

6. RAS A ctivation 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 GRB 2 contain an SH 2 domain binds to the phosphotyrosine residue of the receptor Now SOs protein (guanine nucleotide exchange factor) binds to the SH 2 domain of the GRB 2 protein and forms GRB 2 SOs complex . This GRB 2 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.

7. Kinase cascade 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

8. Inactivation of RAS

9. Activation of transcription factors by 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

10. Activation of transcription factors by 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

11. Activation of transcription factors by 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

12. JAK-STAT pathway 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 SH 2 related domain, a kinase domain and a pseudokinase domain Kinase domain allows JAK to phosphorylate proteins 7 STAT proteins STAT1, STAT2, STAT3, STAT4, STAT 5A, STAT 5B, STAT 6 STAT contain many different domains, most conserved region is the SH 2 domain

13. JAK-STAT pathway mechanism 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 STAT and its phosphorylation by JAK Phosphorylation STAT dimerize and translocate to the nucleus to regulate gene transcription

14. Movement of STATs from cytosol to the nucleus STAT dimers have to pass through nucleus pore complex (NPCs) which is present along the nucleus envelope To enable STATs to 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 STAT dimer is then free in the nucleus Specific STAT – specific importin proteins

15. Integration with other signalling pathways JAKs phosphorylate cytokine receptors which can bind Grb 2 protein Grb 2 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 .

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