Receptors in Metabolic Signalling Flashcards
Features of enzyme linked receptors?
- Mostly single transmembrane domain receptors
- Activation leads to activation of a receptor kinases
- Activation leads to activation of multiple signalling pathways
What’s tyrosine receptors kinase involved with?
Insulin
Epidermal growth factor (EGF)
Platelet derived growth factor (PDGF)
What’s JAK/STAT involved with?
Growth Hormone
Interferon
What’s Serine Threonine receptor kinase involved with?
TGFβ
What do enzyme linked receptors regulate?
Cell growth Division Differentiation Survival Migration
What does inappropriate activation of enzyme linked receptors associated with?
disease particularly cancer
Describe tyrosine kinase activity
- Dimerisation brings 2 receptor molecules together allowing phosphorylation of each other
- Not all tyrosine residues can be phosphorylated
- The phospho-tyrosine together with surrounding AA are recognised by SH2 domains of other proteins allowing them to bind + undergo activation
What’s Grb-2?
growth receptor bound protein 2
adaptor protein
Why is Ras an important protein?
regulates cellular activation + growth.
What are mutations in Ras are common in?
many tumours
Role of Grb-2?
recognises receptor via SH2 domain
Describe multiple pathways via activation of tyrosine kinase receptors
- phosphylated tyrosine activates Ras
- Grb-2 recognises receptor via SH2 domain
- activates Grb-2
- Ras GEF (guanine nucleotide exchange factor) binds to Grb-2 via SH3 domain
- Ras activated by phosphorylation
- cellular activation
-PI 3-kinase activated by phosphorylating inositide phosphates within plasma membrane
-allows other molecules to bind to inositide phosphates
eg PDK (phosphoinositide-dependent protein kinase)
Describe regulation of Ras activity
- exchange of GDP -> GTP when protein activated
- recruits protein RAF
- process sped up by protein guanine nucleotide exchange factor (GEF)
- GTP binding proteins have GTPase activity
- GTPase activity slowly de-phosphorylates Ras inactivating it
- aided by GTPase-activating proteins (GAP)
Role of GTPase-activating proteins (GAP)
aids de-phosphorylation of Ras from 30 mins to 10⁵ fold
to down-regulate this process so there isn’t overstimulation of the cell
Describe effect mutations of Ras
30% of human tumours
prevent hydrolysis of GTP -> GDP similar to cholera toxin + mutation in Gas results in pituitary tumours. Mutations in Ras-GAP also have the same effect.
Structure of insulin receptor?
2α + 2β subunits linked by disulphide bridges
has tyrosine kinase activity
Describe effect of binding of insulin
- binding of insulin results in auto-phosphorylation of 2β subunits
- activation of receptor leads to activation family of small protein substrates - IRS
- IRS recruited via phosphorylated tyrosines on β chain with SH2 domains on IRS –> phosphorylation
- generation of more phosphorylated tyrosines
- recruits more proteins:
- IRS activates PI3K
- activate PKB
- PKB stimulates glycogen synthesis, protein synthesis, increased expression of GLUT transporters on membrane
-insulin also recruits Ras + PLC (involved in generation of IP3)
Diff between insulin vs EGF receptor?
already dimerised so dimerisation is required but not sufficient for activation
How are receptor kinases inactivated?
- de-phosphorylation –> inactivation
- phosphatases activated due to receptor activation
- so signalling process sets in motion events that lead to signal termination
- small g-proteins have intrinsic GTPase activity
- receptor internalisation
What does loss of phosphatase activity mean?
cannot inactivate receptor –> excessive stimulation of the downstream pathways
What’s Herceptin?
monoclonal antibody that targets EGF receptor blocking its action
What happens if EGF receptor is mutated?
can dimerise in absence of EGF so not dependent on EGF
Ras may lose its GTPase activity, so it
What happens if Ras loses its GTPase activity?
can no longer turn off its activity –> inappropriate growth
Receptor tyrosine kinase summary
- Superfamily: Consist of a ligand binding domain + protein tyrosine kinase domain
- Primarily control cell growth + differentiation
- Consists of single transmembrane protein except insulin receptor
- Activation –> phosphorylation specific tyrosine residues in an AA sequence
- Phosphotyrosine interacts with proteins that contain SH2 domains
- SH2-containing proteins include PI-PLCg and PI3K which generate 2nd messengers
What’s the Jak-STAT signaling pathway activated by?
Growth hormone
Describe binding of GH receptor
- binding of GH to receptor
- Jaks cross-phosphorylate each other on tyrosines
- activated Jaks phosphorylate receptors on tyrosines
- phosphorylation of adjacent chain
- recruitment of STAT (signal transducer and activator of transcription) on phosphotyrosines
- Jaks phosphorylate them
- STAT dissasociate from receptor + dimerised via SH2 domain
- transported to nucleus
- causes regulation of various genes via transcription
Importance of GH receptor?
cytokine binding
What’s tyrosine kinase activity in GH receptor provided by?
recruitment JAK protein
What’s STAT?
signal transducer and activator of transcription
How GH affects GH receptor?
- Absence = receptors exist as monomers
- Binding = conformational changes permitting single GH to bind 2 receptor molecules -cooperative effect
- Intracellular domains of the receptor also brought together
Describe features of JAK2?
- Anchored to the membrane
- Has two kinase domains
- Dimerization leads to phosphorylation + activation of JAK
- Activation of JAK2 by cross phosphorylation leads to phosphorylation of other proteins
Role of phosphotyrosine?
acts as docking site for proteins that contain SH2 domain
Why’s STAT dimerised?
stable dimer has a strong affinity for specific DNA binding sites + regulates gene expression
Describe binding of TGF-β to receptor
- serine/threonine kinase domain phosphorylates serine/threonine
- receptor dimerisation
- phosphorylation
- interacts with smad 2 +3
- associated with smad 4
- migrates to nucleus
- binds to specific promotors
- specific activation of transcription
