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. Enzyme-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.
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)
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 of 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-(FGF
(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
19. 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
20. 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.
22. 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?
23. 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