Ch. 6: Cytoplasmic Signaling Circuitry Flashcards
signal transduction
- generally the transmission of a molecular signal outside of a cell to the inside of a cell.
-A way for cells to communicate with
each other. - One cell can produce a ligand which causes a neighboring cell to have a
response.
How do you get from the input layer to the final output layer?
Ex- growth factor binding to activation of cell division
- Signal processing (intracellular signaling, signal relay, middle-men, cascade)?
What happens after receptor trans-phosphorylation?
Specific proteins are able to bind to specific phosphorylated parts
on the intracellular side of the RTK.
What does the phosphorylated intracellular domain do?
serves as a docking site for specific proteins
Structure of GRB2
- SH2 domain
- SH3 domain
What does SH2 domain do?
binds to specific phosphorylated tyrosine amino acids
What does SH3 domain do?
Binds to proline-rich parts of proteins
What is Sos? What does it do?
- a guanine nucleotide exchange factor
- activates RAS proteins by replacing GDP with GTP
What happens when RAS is activated?
binding of GTP causes RAs to change shape and release the effector loop. Once released, the effector loop can bind downstream signaling proteins.
What are the three pathways of active RAS?
3 pathways
- MAPK (proliferation)
- PI3K (inhibition of apoptosis)
- RaI-GEFs (motility)
MAPK pathway
RAS activates MAPKKK
- Ras binds to MAPKKK, activating it and MAPKKK is a kinase that phosphorylated MAPKK
MAPKK -> MAPK
- phosphorylation of MAPKK activates the kinase domain so MAPKK can then phosphorylate MAPK
Phosphorylation of MAPK causes activation of its kinase domain so it can then phosphorylate further downstream molecules
Importance of scaffold protein
- Ensures all members of
pathway are physically
located near one another - KSR scaffold organizes this
highly efficient signal
transduction
what is downstream of ERK?
- activated ERK can then enter
the nucleus and phosphorylate
transcription factors (ex: ETS). - Activated TFs can then cause the transcription of genes that promote cell division (ex: HB-
EGF, cyclin D1, FOS, P21Waf1) - Activated ERK also activates
widespread protein synthesis by activating a translation initiation factor (eIF4E)
What is RAF?
- a common
oncogene - vRAF murine and ckn retrovirus
- 50% human melanomas
- B-RAF, single aa substitution, moves an inhibitory part of the protein out of the way, causes RAF
to be constitutively active
What would happen if RAF was constitutively active?
it would cause constant proliferation, leading to tumorigenesis
PI3K pathway
- PI3K itself is recruited to the membrane when receptor is activated/phosphorylated
- At the membrane, activated
RAS is also able to bind PI3K
and enhance its function (Causes PI3K to become close to its PI substrates present at the plasma membrane) - Activation of PI3K enhances formation of PIP3
- Proteins that carry a PH
domain can bind to PIP3 - Recruitment of AKT to the membrane, causes
activation of AKT - At the membrane AKT is
phosphorylated by other
proteins (PDK1 and
mTORC2) and activated - AKT can then
phosphorylate a variety
of substrates
Inositol biology: Can be phosphorylated by? can be cleaved by?
- PI (inositol) can be phosphorylated by PI kinases to form PIP2
- PIP2 can be phosphorylated by PI3K to form PIP3
- can be cleaved by PLC to form DAG and IP3
AKT
Serine/Threonine kinase
Downstream effects of activated AKT
inhibition of cell death (apoptosis)
* At the membrane AKT is
phosphorylated by other
proteins (PDK1 and
mTORC2) and activated
* AKT can then phosphorylate a variety of
substrates
How is AKT regulated?
Normally highly regulated
- PTEN causes PIP3 to turn
back into PIP2, this
causes the inactivation of
AKT
* Protein phosphatases de-
phosphorylate AKT as
well
How is AKT deregulated in cancer cells?
- Loss of PTEN
- Hyperactivity of PI3K
What do activated Rho proteins promote?
invasive qualities
- Re-organization of actin
cytoskeleton
- filopodia
- lamellipodia
Filopodia
Small finger like extensions from the plasma membrane, cell uses to explore environment and adhere to the extracellular matrix
Lamellipodia
Broad ruffles at the leading
edge of motile cells
What is the second most common mutated oncogene pathway in all human cancers?
PI3K pathway
RAL GEFs pathway
RAS activates RAL proteins via RAL GEFs
- Active Ras can bind to Ral-GEFs and recruit them to the membrane.
- Ras binding to Ral-GEFs also changes Ral-GEF shape and activates the guanine
nucleotide exchange function.
- RAL proteins promote cell motility/invasion
What is the #1 most commonly mutated oncogene in human cancers?
RAS
One of the mechanisms to stop intracellular signaling molecules so proliferation only occurs when necessary?
CBL
1) CBL ubiquitinates receptor and tags it for degradation by the proteasome. (Book)
2) CBL ubiquitinates receptor which induces endocytosis and subsequent sorting to a
lysosome for ultimate
destruction. (Literature)
CBL destruction via activated SRC oncogene causes…
an increase in RTK at the cell surface
