Cell Signaling Flashcards
What are PTMs
Modifications that occur to a protein that alter its function in some way.
RTKs
Directly phosphorylate specific tyrosines on themselves and on a set of intracellular signalling proteins
Ligands are either ______ or ______
Soluble of membrane-bound
The three most commonly phosphorylated amino acids in eukaryotes
Serine, threonine and tyrosine
What group does phosphorylation occur on
Hydroxyl groups (OH)
________ phosphorylation is involved in signal transduction
Tyrosine
What side (intra or extracellular) contains ligand binding domain
Extracellular
What side (intra or extracelllular) is the effector side
Intracellular - effects a protein inside leads to signal cascade, cause alterations within cell
Where is kinase domain and what receptors contain it
Intracellular side, receptors that can phosphorylate themselves
Cysteine rich domains are able to
Be modified easily, (can respond to oxidative stress and be oxidized) can change ability to respond to ligands
What does ligand binding cause for RTKs
Receptor dimerization and cross-phosphorylation called trans-autophosphorylation
Each RTK subunit has a _______ ________ domain
How are tyrosine kinase domains activated
Domains need to come close enough together to phosphorylate eachother
2 ways the domains coming together (to phosphorylate eachother) is regulated
1.Ligand binding brings RTKs close together so kinase domains phosphorylate eachother)
2.Ligand binding triggers a conformational change activating kinase domain of receptor
Transfected means
Delivered an artificial construct to cells, cells now expressing receptor with mutation
Cells are transfected with DNA encoding a mutant form of the receptor that can
Dimerize but not phosphorylate
What happens with 2 subunits (1 with normal and 1 with mutant TKD) when ligand binds
Come together, no cross-phosphorylation since only one has a TKD
Dominant negative mutation
Only takes 1 mutant subunit to have mutated effect
Once bound, a signaling protein may become activated by:
1) itself becoming phosphorylated on tyrosines (eg IGF and IRS1),
2) binding alone may induce a conformational change
3) simply bringing it near the next protein in the signaling pathway
2 ways Trans-autophosphorylation contributes to RTK activation
Increases the kinase activity of the enzyme and creates high-affinity docking sites for intracellular signaling proteins
What happens with a fully active RTK
The ligand has bound, brought the 2 subunits close together to allow for trans-auto phosphorylation, that will lead to phosphorylation of tyrosine residues which with then act as docking stations for adaptor molecules
The phospho-tyrosine residue will always bind to some domain in specific proteins, this domain is called an
SH2 domain
Intracellular signalling (adaptor) proteins usually share highly conserved
phosphotyrosine binding domains SH2 (SRC Homology) domains or PTB (phosphotyrosine binding) domains.
Through these domains, these signaling proteins (SH2 and PTB) can bind to activated RTKs, but also to other phosphorylated intracellular signaling proteins.
Activated RTKs, and other phosphorylated intracellular signaling proteins.
SH3 domains bind
Proline rich motifs
Split tyrosine kinase domains cause
Phosphorylation at different sites
PDGF receptor
Start with a signal stimulus – ligand binding and then this signal is amplified, it recruits 3 different protein complexes, all of which are capable of recruiting and interacting with more proteins. These signals get further amplified and you will get the effect of cell proliferation or migration etc
c-Cbl protein decreases the signaling process by
Covalently adding a single ubiquitin (monoubiquitylation) to one or more sites on the RTK.
Monoubiquitylation promotes
endocytosis and degradation of RTKs by targeting to clathrin-coated vesicles and, ultimately, to lysosomes.
Poly-UUB targets
proteins for degredation directly
c-Cbl is an ________ of the RTK signalling pathway
Inhibitor
c-Cbl decreases the signaling process by
covalently adding a single ubiquitin (monoubiquitylation) to one or more sites on the RTK.
When the cell wants to downregulate response
the receptor can be internalized, once the pH changes in the endosome the ligand will dissociate from the receptor and the receptor can be recycled back to the PM
Adaptor proteins are composed almost entirely of
SH2 and SH3 domains
Adaptor proteins
couple activated RTKs to important signaling proteins that do not have their own SH2 domains, such as Ras.
Activated Ras is
Ras-GTP
MAPK (ERK) activated by EGF
Plateaus in 5 mins
MAPK (ERK) activated by NGF
Lasts for hours
EGF promotes _________ while NGF promotes __________
Proliferation, differentiation
How is cross-talk prevented
Adaptor proteins
The Rho family
regulate the cytoskeleton to control cell shape, polarity, motility and adhesion. (Rho, Rac, CDC42)
Unlike Ras, inactive Rho (with GDP bound) is often not membrane-bound, but bound to
guanine nucleotide dissociation inhibitors (GDIs) in the cytosol.
Is inactive Rho membrane bound
no
PI-3-kinase
PM-bound enzyme, phosphorylates inositol phospholipids instead of proteins
Phosphoinositol (PI) undergoes _________ phosphorylation at ______ ______ on its head group
Reversible, multiple sites
PI-3-K always leads to phosphorylation of carbon ______, results in formation of ____________
3, PI3 phosphates
PTEN
DEphosphorylates and inactivates PIP3 at C3
Akt
serine/threonine protein kinase, PH-domain containing
The PI-3-kinase-Akt pathway signals cells to grow through a more complex mechanism that depends on a large serine/threonine kinase called _______
mTOR
mTOR
Master regulator of growth, turns on protein translation causes growth
PDK1 in the PI-3-kinase-Akt signal pathway
phosphorylates and partially activates Akt
mTOR in the PI-3-kinase-Akt signal pathway
further phosphorylates Akt, causes dissociation from PIP3 dock site
Akt promotes ____
growth
Phosphorylation of Bad
releases apoptosis inhibitor
Src family
cytoplasmic tyrosine kinase
Src kinases contain ______ and are located on the ________ side of the PM
SH2 & SH3 domains, cytoplasmic side
The largest class of receptors that rely on cytoplasmic tyrosine kinases is the class of
cytokine receptors
Cytokine receptors are stably associated with cytoplasmic tyrosine kinases called ______
JAKs
JAKs phosphorylate and activate ________
STATS
STATS go where when activated
migrate to the nucleus
There are at least six STATs in mammals, each has a _______ domain
SH2
2 functions an SH2 domain in STATs performs
1) mediates the binding of STAT to activated receptor
2) upon STAT release from receptor, mediates the binding to another phosphorylated STAT.
TGFβ-receptor serine-threonine kinases signal more directly to the nucleus than the JAK-STAT pathway by directly activating latent gene regulatory proteins called
Smads
The human transforming growth factor-β superfamily consists of 30-40
secreted, dimeric proteins (hormones or local mediators
TGFβ-receptor serine-threonine kinases signal more or less directly to the nucleus than the JAK-STAT pathway?
More
How does TGFβ-receptor serine-threonine kinases signal more directly to the nucleus?
directly activating latent gene regulatory proteins called Smads
Latent gene regulatory proteins
The protein is there and ready to be activated to go directly the nucleus and exert its effects
TGFβ-receptor serine-threonine kinases signal
-2 monomers brought into close prox. by ligand binding
-TGFβ binding causes type-II receptor to phosphorylate type I receptor
-Type I phosphorylates Smad 2 or 3
-Smad 2 or 3, dissociates from receptor and dimerizes with Smad 4
-Smad 4 goes to nucleus, recruits gene regulatory elements to activate gene trascription
Inhibitory Smads (iSMADS) negatively regulate the Smad pathway through 4 various mechanisms:
- Compete with Smads for bind sites on rec.
- Recruit ubiquitin ligases called Smurfs, which ubiquitylate the rec.
- Recruit protein phosphatase- removes phosphate groups
- Bind to co-Smad (Smad4) prevents Smad 2/3 from binding or promoting its ubiquitylation
STATs and Smads are activated by
phosphorylation (not proteolysis)
Latent gene regulatory proteins activated by phosphorylation
STATs and Smads
Latent gene regulatory proteins activated by proteolysis
receptor protein Notch, the secreted Wnt proteins, the Hedgehog proteins, and the NFκB latent gene regulatory protein
the production of nerve cells in Drosophila arise from
single cells within an epithelial sheet of precursor cells
Notch is best known for its role in
the production of nerve cells in Drosophila
When a precursor cell commits to becoming a nerve cell, it signals to its immediate neighbors not to do the same by
displaying on its surface a transmembrane signal protein called Delta
What does the delta ligand do to prevent neighbouring cells from becoming a nerve cell
Delta ligand binds to Notch receptors on neighboring cells to prevent them from becoming neural.
Nerve cells differentiation is an example of ________ signalling called ________ ________
Contant-dependant, lateral inhibition
Notch is a ___________ protein that requires _________ processing to function.
transmembrane, proteolytic
What protein pathway provides the simplest most direct signaling to the nucleus of any known pathway.
Notch
What happens to Notch when activated by the binding of Delta
a protease cleaves the cytoplasmic tail of Notch, and the resulting fragment translocates into the nucleus to activate transcription
Wnt proteins bind to ______ receptors and inhibit the degredation of ______
frizzled, β-catenin
What are Wnt proteins
secreted signal molecules with a fatty acid chain covalently attached to their N-terminus to increase their binding to cell surfaces
3 intracellular signaling pathways Wnt activates
1) The Wnt/β-catenin pathway,
2) The planar polarity pathway,
3) The Wnt/Ca2+ pathway
All 3 intracellular signaling pathways Wnt activates begins with
-binding of Wnts to Frizzled
-When activated by Wnt proteins, Frizzled recruits the scaffold protein Dishevelled (relays signal down all 3 paths),
What is Frizzled
Family of cell-surface receptors
What is dishevelled
Scaffold protein
The Wnt/β-catenin pathway acts by regulating the
proteolysis of β-catenin, (functions both in cell-cell adhesion and gene regulation)
Wnt acts by binding both (2)
a frizzled protein and a LDL-receptor-related protein (LRP)
Cytoplasmic β-catenin is degraded by a large degradation complex that contains a
a serine/threonine kinase (CK1)
What does CK2 do in Wnt pathway
phosphorylates β-catenin priming it for further phosphorylation by another serine/threonine kinase (GSK3).
Final phosphorylation marks β-catenin for
ubiquitylation and rapid degradation in proteasomes.
What happens in absense of Wnt signal in terms of degredation and signaling
β-catenin degraded, no signalling
What protein results without Wnt signal and what does it do
Groucho, inhibiting the transcription of genes that would be activated in the presence of Wnt.
What does mutation of APC complex cause
Wnt genes to always be transcribed
WITH Wnt signal, what happens in nucleus
β-catenin displaces Groucho, allowing for transcirption of Wnt genes
What does dishevelled do in WITH Wnt signal
dissociates the APC complex by pulling Axin away, CK1 and GSK3 (now free kinases) phosphorylate LRP - how pathway can be turned off
Similarities between Hedgehog and Wnt proteins
-are secreted signal molecules that act as local mediators in development
-activate latent gene regulatory proteins by inhibiting their proteolysis
-trigger a switch from transcriptional repression to activation
What do hedgehog proteins bind and what does that do
bind to Patched, relieving its inhibition of Smoothened
On inside of cell in absense of hedgehog you get transcriptional __________ through the __________ of the ______ protein
repression, phosphorylation, Ci
What is Ci phosphorylated by in absense of hedgehog
PKA, GSK3, CK1
What does phosphorylation of Ci do
leads to the proteolytic processing of Ci resulting in cleavage and the small cleaved part of the Ci protein goes to the nucleus and inhibits transcription
what happens to Patched in presence of hedgehog
Internalization of patched, degraded in lysosome
What happens following degredation of Patched in presence of hedgehog
Patched WAS inhibiting Smoothened, now gets phosphorylated by PKA and CK1, inserted into PM
-Smoothened then recruits protein complex, inhibits Ci cleavage
What happens when Ci cleavage is inhibited in presence of hedgehog
full length Ci goes to nucleus and acts as a transcriptional activator
What can excess hedgehog signalling lead to
Cancer
Is Patched activated by Ci
Yes, increase in Patched on cell surface inhibits further Hedgehog signaling
Cell receptors that activate the NFkB signaling pathway in animal cells
-Toll-like receptors
-Tumor necrosis factor α (TNFα) and interleukin-1 (IL1) (vertebrate cytokines)
NFκB signaling pathway with TNFα
-TNFα binds to rec on PM, activated IKK complex
-goes to phosphorylate IkB, IkB then ubiquinated and degraded
-IkB was inhibiting NFkB, now is free to move to nucleus and activate transcription
Pathways that rely on phosphorylation
-PI3-K
-JAK/STAT
-TGFB/SMAD
Pathways that rely on proteolysis
-Notch/Delta
-Wnt/ Beta Catenin
-Hedgehog
-NFkB
