Signal Transduction Flashcards
What different changes in cell behaviour can be done due to an extracellular signal?
- altered metabolism
- altered gene expression
- altered cell shape or movement
What are 4 different types of signalling?
- contact dependent signalling
- paracrine
- synaptic
- endocrine
What is contact dependent signalling?
- the extracellular signal cannot diffuse
- attached to signalling cell
- must be within close proximity
What is paracrine signalling?
- signals are operating over short ranges
- ligands have structural features that restrict their diffusibility
- important for local modifications (e.g. in wound healing)
What is synaptic signalling?
- nervous system
- neuronal cell types
- vesicle packets released at synapses
- signalling can only occur within synapses
What is endocrine signalling?
- specialised cell types that are releasing the signalling molecule into the blood stream
- made available to target cells
- long range signalling
- e.g. beta cells releasing insulin
What are some specifications that a signalling machine must meet?
- recognise the extracellular signal
- generate an inter-cellular signal
- elicits change in cell behaviour
What are some important properties of a signalling machine?
- specificity
- sensitivity (affinity for ligand)
- dynamic range (range of concentrations and what responses are given)
- duration (timing of the response)
- processing (timing + shape of the signal. example is a sigmoidal graph, increasing conc will increase response. Or hyperbolic or all-or-none response)
- integration (multiple signals impacting cells)
- feedback and noise (positive and negative feedback, can amplify a forward reaction)
What is dynamic range?
difference between the smallest and largest usable signal through a transmission or processing chain or storage medium
What are the three classes of receptors that respond to signals?
- ion-channel coupled receptors
- when ligand binds, conformational change occurs and ion is allowed in
- e.g. GABA-A receptors, ligand is GABA, a inhibitory neurotransmitter - g-protein coupled receptors
- function by switching mechanism that involves GTP and GDP - enzyme-coupled receptors
- when ligand binds, leads to activation of a protein/enzyme catalysis process
What are G protein coupled receptors?
- largest family of receptors
- conserved structure
- small molecule ligands (acetylcholine)
What is the structure of GPCR?
- 7 transmembrane helices
- extracellular ligand binding site
- hydrophobic pocket is created by the helices which the ligand can bind
What happens when a ligand binds to the G-protein coupled receptor?
- ligand binds in hydrophobic pocket
- this elicits a transmembrane conformational change in the conformation of the helices
- intracellular signalling is initiated by sensing the conformational switch
How does the GPCR signal via G protein switches?
- 3 subunits, alpha, beta and gamma
- GDP is bound to the alpha subunit (inactive state)
- the ligand binding induces G protein binding to receptor
- this induces the exchange of GDP to GTP
- breaks up subunits ti Galpha and Gbetagamma
- the GTP bound form can bind effectors (GTP binds to the alpha subunit)
- GTP is hydrolyses back to GDP
What are GPCR’s inhibited by?
Arrestins
- proteins which are recruited to the GPC when it is phosphorylated by GRKinase
- arrestin locks the GPCR in its inactive state and degrades signal
Name an enzyme coupled receptor.
Receptor Tyrosine Kinases (RTKs)
What are receptor tyrosine kinases?
- an enzyme coupled receptor
- have intrinsic kinase activity
- phosphorylate substrates on tyrosine residues
- ligand binding outside the cell activates the kinase inside the cell
What is the structure of the tyrosine kinase domain?
- have 2 domains linked together by a flexible region
- N terminal domain contains an ATP binding site
- C terminal domain contains substrate binding site and 2 alpha helices that determine specificity by the exposed amino acids in the helix
- activation loop in the C terminal domain
- contains one or more Tyr molecules
- binds to catalytic cleft
What 2 conformations is the activation loop in for tyrosine kinase domain?
unphosphorylated form - sits in the substrate binding site
phosphorylated form - the loop flips away from the binding site
How do drugs inhibit RTKs?
bind to the ATP binding site on the N terminal domain
How does the RTK become activated?
- receptor dimerisation and inter-molecular phosphorylation
- forced dimerisation activates RTK signalling in the absence of ligand
- ligand acts to dimerise receptors
What is achondoplasia?
- commonly known as dwarfism
- premature stopping of long bone elongation
- mutations in FGFR3
- causes transmembrane domain to dimerise in the absence of ligand
What is crozoun syndrome?
- genetic disease resulting in premature fusion of skull bones
- mutations in the extracellular domain of FGFR2 which produces cysteine residues
- leading to covalent dimerisation and activation of signalling in the absence of ligand
What happens during ‘dominant negative’ receptor mutants?
- a receptor is mutated to lack kinase domain
- unable to signal even though they do bind the ligand as usual
- can block a wild type receptor from signalling due to dimerisation
What does activation of kinase lead to in RTK?
- inter receptor phosphorylation of the tyrosine residues seen in C terminal tail of receptor
- auto phosphorylation
- recruitment of substrates
- this causes binding sites for signalling proteins
- activated signalling proteins relay signals downstream
How does RTK recruits its substrates?
- by specific recognition of pTYR by SH2 and PTB domains
- these domains have tyrosine phosphate specific binding functions
- binding site for phosphotyrosine and amino acid side chain
How does the signal start from when the RTK is activated?
- The SH2 domain of the adaptor protein (Grb2) is recruited to the activated RTK tails
- Recognition domain SH3 in Grb2, recruits a protein called Sos (Ras-GEF)
- When Sos is recruited, it interacts with inactive Ras protein (which is a G-protein) and displaces GDP with GTP and activates Ras
- Can interact with downstream pathways
What downstream pathway is activated by Ras GTPase?
- MAP kinase cascade
- sequence of kinases that phosphorylate each other
- terminal phosphorylase (Erk) phosphorylates and initiates a range of changes of behaviour
What is the MAP kinase pathway?
- Activated Ras protein binds to Raf
- Raf has one substrate which it phosphorylates- (Mek)
- Activated Mek has one substrate phosphorylates- (Erk)
- Erk phosphorylates many other proteins that undergo changes
What is the PI3 Kinase pathway?
PI3 Kinase is an enzyme that phosphorylates a specific membrane lipid
- increasing the conc of PI-3,4,5-triphosphate
- that is a docking site for some proteins
- so is recruiting proteins to the membrane hence changing its behaviour
What does recruitment of effectors to the membrane due to PI-3,4,5-triphosphate do?
- recruits 2 protein kinases (PDK1 and Akt) with PH domains (domains that recognise PI-3,4,5-triphosphate)
- PDK1 phosphorylates Akt
- Akt activates mTOR (regulator of biosynthesis and energy balance)
- Akt also phosphorylates a protein called Bad
- inhibits apoptosis in proteins
What are cytokine receptors?
- family of structurally related ligands and receptors
- ligand recognition mechanism
- act via receptor dimerisation
- signal via activation of JAK (just another kinase lol) family through phosphorylation of the STAT family
Why are cytokines medically important?
erythropoetin: - induces proliferation of erythroblasts - treatment for anemia G-CSF: - induces the proliferation of neutrophils - treatment with chemo
What is the structure of the receptor that growth hormones (and cytokines) bind to?
- has cytokine homology domain where 2 modules are at right angle to each other
What is the structure of the growth hormone?
- 4 alpha helices
What is the activation mechanism for growth hormones to their receptors?
- receptor dimerises through interactions between specific residues in the ligand with residues found in the loops of the 2 modules
- binding of the ligand locks the dimerisation of the receptor
- activates the downwards signalling
What happens once the JAK kinase has been activated?
- JAKs have 2 catalytic domains
- the dimerisation results in phosphorylation of the C terminal region of the receptor
- The C terminal region recruits STAT
- STAT has an SH2 domain that binds to the phosphorylated receptor
- STAT then becomes phosphorylated by the JAK Kinase
- Then dissociates from the receptor and forms a dimer through the SH2 domains binding to the phosphorylated sites
- STATs then relocate to the nucleus where is interacts with DNA
