Enzyme linked cell surface receptors

A good detail presentation on tyrosine kinase receptors

1. En z yme- linked Cell Surface Receptors BY: FOZIYA KHAN PHARMACOLOGY BRANCH SEM I

2. CONTENTS Introduction Classification Receptor Tyrosine Kinases (RTKs) Tyrosine Protein Phosphorylation Subfamilies of RTKs Protein Tyrosine Kinases Signaling from Tyrosine Kinase Receptors Ras

3. INTRODUCTION Also known as a catalytic receptors transmembrane receptor , where the binding of an extracellular ligand causes enzymatic activity on the intracellular side. integral membrane protein possessing both enzymatic catalytic and receptor functions. Upon ligand binding a conformational change is transmitted which activates the enzyme, initiating signaling cascades.

4. En z yme- linked Cell Surface Receptors Classification Receptor Tyrosine kinases: phosphorylate specific tyrosines Tyrosine kinase associated receptors: associate with intracellular proteins that have tyrosine kinase activity. Receptor like tyrosine phosphatases: remove phosphate group Receptor Serine/ Threonine kinases: phosphorylate specific Serine/ Threonine Receptor guanylyl cyclases: directly catalyzes the production of cGMP Histidine kinase associated receptors: kinase phoshorylates itself on Histidine and then transfers the phosphate to a second intracellular signaling protein.

5. Receptor Tyrosine Kinases (RTKs) Intrinsic tyrosine kinase activity Soluble or membrane-bound ligands: Nerve growth factor, NGF Platelet-derived growth factor, PDGF Fibroblast growth factor, EGF Epidermal growt h factor, EGF Insulin Downstream pathway activation: Ras-MAP kinase pathway

6. TYROSINE KINASE RECEPTORS these receptors traverse the membrane only once respond exclusively to protein stimuli cytokines mitogenic growth factors: platelet derived growth factor epidermal growth factor

7. Functions include: Cell proliferation, differentiation Cell survival Cellular metabolism Some RTKs have been discovered in cancer research Her2, constitutively active form in breast cancer EGF-R overexpression in breast cancer Other RTKs have been covered in studies of developmental mutations that block differentiation

8. Outline Activated RTKs transmit signal to Ras protein Ras transduces signal to downstream serine-threonine kinases Ultimate activation of MAP kinase Activation of transcription factors

9. Ligand binding to RTKs Most RTKs are monomeric ligand binding to EC domain induces dimerization FGF binds to heparan sulfate enhancing its binding to receptor: dimeric receptor-ligand complex Some ligands are dimeric: direct dimerization of receptors Insulin receptors occur naturally as a dimer Activation is due to the conformational change of the receptor upon ligand binding

10. Tyrosine Protein Phosphorylation Eukaryotic cells coordinate functions through environmental signals - soluble factors, extracellular matrix, neighboring cells. Membrane receptors receive these clues and transduce signals into the cell for appropriate response. Tyrosine kinase signaling is the major mechanism for receptor signal transduction. Tyrosine protein Phosphorylation is rare (1%) relative to serine/Threonine Phosphorylation. TK pathways mediate cell growth, differentiation, host defense, and metabolic regulation. Protein tyrosine Phosphorylation is the net effect of protein tyrosine kinases (TKs) and protein tyrosine phosphatases (PTPs).

11. Substrate + ATP Substrate -P + ADP Protein Tyrosine Kinase Protein Tyrosine Phosphatase (PTP)

12. Subfamilies of Receptor Tyrosine Kinases

13. Protein Tyrosine Kinases (TKs) Receptor tyrosine kinases (RTK) insulin receptor EGF receptor PDGF receptor TrkA Non-receptor tyrosine kinases (NRTK) c-Src Janus kinases (Jak) Csk (C-terminal src kinase) Focal adhesion kinase (FAK)

14. TABLE 15–4 Some Signaling Proteins That Act Via Receptor Tyrosine Kinases SIGNALING LIGAND RECEPTORS SOME RESPONSES Epidermal growth factor (EGF) EGF receptor stimulates proliferation of various cell types Insulin insulin receptor stimulates carbohydrate utilization and protein synthesis Insulin-like growth factors IGF receptor-1 stimulate cell growth and survival (IGF-1 and IGF-2) Nerve growth factor (NGF) Trk A stimulates survival and growth of some neurons Platelet-derived growth factors PDGF receptors stimulate survival, growth, and proliferation o f various cell types Macrophage-colony-stimulating M-CSF receptor stimulates monocyte/macrophage factor (M-CSF) proliferation and differentiation Fibroblast growth factors FGF receptors stimulate proliferation of various cell (FGF- ( FG F (FGF-R1to FGF-R4 ) (FGF-R1–FGFR4 ) R1-FGFR4) types ; inhibit differentiation of some precursor cells; inductive signals in development Vascular endothelial growth VEGF receptor stimulates angiogenesis factor (VEGF) Ephrins (A and B types) Eph receptors (A and B) stimulate angiogenesis; guide cell and axon migration

15. Signaling from tyrosine kinase receptors Ligand induced dimerization Autophosphorylation Phosphorylation in the catalytic domain increase the kinase activity Phosphorylation outside the catalytic domain creates specific binding for other proteins. Autophosphorylated receptors bind to signaling proteins that have SH2 (phosphotyrosine residues) domains

16. Receptor Dimerization and Kinase Activation

17. Ras Monomeric GTPase switch protein Its activation is enhanced by GEF GDP-GTP exchange Deactivation of Ras-GTP complex requires GAP, which increases intrinsic GTPase activity 100 fold Lifetime of Ras-GTP is higher than that of G  Ras is a small protein (170 aa. Vs 300 aa of G  ) G  has a domain that functions like GAP

18. Mutant ras proteins are associated with many cancers Mutant ras can bind GTP but can not hydrolyze it, and thus remain constitutively in “on” state Most oncogenic ras proteins contain a mutation in codon 12 (Gly) This blocks the binding of GAP to ras, and prevents GTP hydrolysis.

19. Ras upstream and downstream signaling

20. Linking ras to RTKs Experimental evidences Fibroblasts were induced to proliferate with FGF and EGF Anti-ras antibody microinjected: cell proliferation arrest Injection of mutant ras proteins allows cell to proliferate in the absence of growth factors. Ligand-bound RTKs activate ras! How?

21. Two cytosolic proteins are involved: GRB2, Sos SH2 domain in GRB2 binds to a P*-tyrosine residue in the activated receptor Two SH3 domains of GRB2 bind to and activate Sos Sos is GEF protein and convert inactive GDP-ras into active GTP-ras Developmental studies elucidated the role of GRB2 and Sos in linking RTKs to ras activation

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