Module 6 Flashcards
cell signalling
the pathways where cells receive signals, interpret them and then act biologically
Signal transduction
the mechanisms cells use to “see” ligands that bound to receptors on other side of cells
- info is converted from a ligand to a biological response
Why is signal transduction important
because it can cause cells to:
- die
- mature + differentiate
- move
- acquire/lose specific functions
plays a role in:
- fight or flight
- insulin/blood glucose regulation
How do cells receive signals from outside the cell?
- Shape change in receptor - initiated signalling
- Signal relay and amplification - until signal received by effector protein
- Effector protein mediates biological response
- Signal shutdown - feedback inhibition
general features of pathways
a) specificity
b) amplification
c) modularity
d) desensitisation
e) integration
types of signals
a) endocrine signalling (hormones)
b) paracrine signalling (cells signal to adjacent cells)
c) autocrine signalling (cell controls its own functions)
d) plasma-membrane attached signalling (ligand attaches to one cell and need physical touch to relay message)
Juxtacrine
Receptors
- most receptors are found on cell surface
- cytoplasmic + nuclear receptors can bind lipophilic that pass through membrane
- some receptors can detect light and heat
- magnitude of cells response can be limited by no. of receptors expressed by cell
- ligand binding causes structural change in receptor - initiation of signalling
second messengers
- hormones and ligands are first messengers
- first messengers bind + release second messengers
- second messengers are transient (can be easily inactivated) + serve to rapidly amplify signals
Effectors
- signalling proteins and second messengers move the ‘message’ around the cell and convert it to a way that the effector proteins can understand it
- the ones that act on the signal
- terminal component of pathway
- more sm and effector proteins increase cells ability to amplify signals (small signal = large response)
Allosteric modification
molecule alters the conformation of a protein when it binds non-covalently to protein (alternative site) e.g. calmodulin/Ca2+
Covalent modification
modification of chemical structure of protein
- reversible
e.g. phosphorylation, ubiquitination etc
proteolysis
- protein cleavage by a protease
- can activate or inactivate a protein
- not reversible
e.g. insulin (?)
how do modifications alter protein function?
- activates enzymatic activity
- unmasks active sites
- alters localisation of protein
- facilitated protein:protein interactions
- alters protein stability
signals and their pathways are: ?
modular
- proteins have small, conserved domains capable of interacting with other proteins
- interaction domains are essential
- many proteins have multiple interaction domains that allow interaction with many different proteins at once
Signalling cascade
- multiple similar steps
- more steps = more amplification
- individual components used in multiple pathways
- can be activated by multiple different receptor families = allows pathways to talk and coordinate response
gene expression is controlled by:
transcriptional regulators
Transcriptional regulators
modulate gene expression initiation by RNApolymerase
IL-1 receptor signal intiation
- when IL-1 has bound, the intracellular tails of receptor come together to form a docking site for signalling proteins
E3 ubiquitin ligase
effector proteins that can attach ubiquitin groups to target proteins
- ubiquitination can lead to
1. protein degradation
2. generate signal cascade
causes polyubiquitination
TRAF6 Summary
- TRAF6 generates K63 polyUB chains upon itself
- these form a scaffold to recruit TAK1 kinase and the 1-kB kinase complex
- TAK1 activated 1akB kinase by phosphorylation
- 1-aB kinase then phosphorylates 1-aB.
- 1KKB phosphorylates 1KBa and when it is phosphorylated it can now bind to the E3 ligase. generates K48 chain.
- every time E3 adds a ubiquitin to the chain of ubiquitins it uses the ligase 48 of the previous ubiquitin
signalling proteins:
- MyD88 (signal adaptor)
- IRAK (kinase)
- TRAF6 (E3 ubiquitin ligase)
NF-kB
a master regulator of immune
- urgent stress responses - rapid mechanism
activated by:
- infection
- inflammatory cytokinesis produced during infection
ubiquitination (K48+K63) is central to NF-kB signalling
Signal inhibition for IL-1
a) turning off IL-1 receptor
- negative feedback inhibition
- induction of IL-1RA, and antagonist of the IL-1R
b) turning off NFkB
- negative feedback inhibition
- resynthesis of 1kB
Switch protein
allosteric binding to other molecules results in a switch to an “on” or “off” state
- g-proteins
- calcium binding proteins
G-protein coupled receptors
- largest protein in human genome
- half of GPCRs are olfactory receptors (smell)
- all GPCR signalling initiates the activation of a molecular switch G-protein
- all GPCRs are common targets fir current drugs
have:
- 7 transmembrane a helical domains
- 4 cytosolic domains
- 4 extracellular domains
2 configurations:
1. GDP bound “off”
2. GTP bound “on”
GPCR activation
- starts with everything off
- GDP binds receptor (GEF tells it to kick off GDP, take up GTP = activates)
- once active, it leaves receptor
- finds effector activates it
GPCR activators (G alpha)
- GaS (activates adenylyl cyclase)
- Gai (inhibits adenylyl cyclase)
- Gaq and GaO - stimulates phospholipase C
G a S
stimulatory hormones cause G-protein to have GaS
- GaS stimulates AC to produce cAMP
- GaS induces a conformational change allowing two domains to interact together and form an active site
Gai
Inhibitory hormones (e.g. adenosine) causes protein to have Gai. Gai inhibits Adenylyl cyclase
- Gai pushes those two domains away from each other (cant form active site)
cAMP
activates Protein Kinase A (PKA)
- when inactive, PKA has 4 subunits (2 catalytic that are inhibited by 2 regulatory units)
- cAMP binds to the 2 regulatory units = conformation change causes them to release the catalytic units = PKA activated
PKA in fight and flight
phosphorylates (turns off) glycogen synthase (stops glucose = glycogen)
phosphorylates (turns on) glycogen phosphorylase (activates glycogen = glucose)
GPCRs that interact with Gaq and GaO
- activate phospholipase
- phospholipase C hydrolyses the phosphoester bond in phospholipid (PIP2).
- PIP2 cleavage generates 2 second messengers: DAG and IP3
- IP3 promotes release of calcium stores
- DAG + Ca2+ activates PKC
- PKC cleaves PIP2 to give IP3 + membrane anchored DAG
IP3 goes to calcium channel + instructs it to open. channel opens and flood cytosol with Ca2+ and Ca2+ is regonised by PKC. It instructs PKC to go to plasma membrane to look for DAG. Found DAG = activates PKC and then PKC can phosphorylate various targets
Calcium signalling
Ca2+ functions as second messenger in all cells
major Ca2+ sensing proteins include PKC and calmodulin
Signal Inhibition for GPCR
a) turning off GPCR
- arrestins direct GPCR for internalisation, leading to either recycling of dephosphorylated inactive GPCR to plasma membrane, or degradation
b) turning off activated G-proteins
- interaction between active Ga and its effector protein increases its GTPase activity (GTP - GDP +P)
- returns trimerase G-protein to its “off” state
c) turning off second messengers: cAMP + IP3
Pyroptosis
Programmed necrosis initiated by inflammatory caspases
Messy form of cell suicide that deliberately activates immune system and triggers inflammation
Pyroptosis vs apoptosis
Pyroptosis is mediated by inflammatory caspases activated by an inflammatory
- cell growing and bursting
Apoptosis is mediated by apoptotic caspases activated by death receptor signalling and/or apoptosome
- cell shrinking
- immunologically silent death
Mammalian caspases
Cycle in proteases that cleave specific sequences after an aspartic acid residue
Expressed as zymogens (inactive proteases)
Cleavage can activate or inactivate a caspase substrate
Tyrosine kinases
- JAKs phosphorylate each other - activating kinase function, receptor chains a docking site for proteins containing phosphotyrosine - binding motif (SH2 domain)
- STAT monomers recruited and become tyrosine phosphorylated by JAKs
- Triggers STAT dissociation from the receptor and self interaction
- Dimerisation reveals a nuclear localisation sequence - nuclear entry
- STATs bind to specific DMA motifs and regulate gene expression
Receptor tyrosine kinases
Dimer, ligand binding change brings together receptor COO- tails.
- kinase domains in receptor activate/phosphorylate residue chains a docking site for proteins containing phosphotyrosine-binding motif (SH2 and PTB)
- residues adjacent to phosphotyrosine confer specificity of SH2 domain
Cell division MAPk pathway
- binding of GRB2 and sos inactivate Ras
(Ras is the switch) - Ras activation GDP-GTP
Feedback inhibition
Turn off kinase = phosphates SHP1 dephosphorylates JAK
Turn off receptor = phosphatase SOCS dephosphorylates receptor tails and JAK degradation
Turn off 2nd messenger = PI3-phosphates removed by phosphatase (PTEN)
