Module 8 - Receptor mediated cell signaling, metabolism Flashcards
Describe the steps of RTK receptor signaling
- growth factor binds to inactive RTK receptor
- receptors dimerize and RTK becomes active
- cytoplasmic tails of RTK are autophosphorylated
- proteins bind to RTK phosphotyrosines
- target proteins are phosphorylated
- downstream signaling pathways are activated
Describe the steps of EGF receptor signaling through Ras activation
- EGF binds to EGFR1
- EGFR1 and EGFR2 dimerize and EGFR1 phosphoryltes the EGFR2 Tyr
- conformational change leads to EGFR2 phosphorylating EGFR1 Tyr
- GRB2 adaptor (an SH2 adaptor that also contains SH3) binds to phosphorylated Tyr residues
- SOS (a GEF protein) binds to the SH2/SH3 adaptor
- Ras (a G protein) binds to the SOS adaptor
- GTP exchange for GDP activates Ras downstream signaling pathways
- OR -
- RasGAP stimulates GTPase in Ras and deactivates signaling
What is the difference between SH2 and SH3?
SH2 binds pTyr and Asn parts of the receptor
SH3 binds Pro parts of the SOS protein
(having both is what creates the bridge)
The Raf-MEK-ERK pathway is a type of…
EGF receptor signaling
What are the two kinds of EFG defects and how do they lead to cancer?
dominant gain of function mutations: one affected gene copy in the GTPase domain causes uncontrolled cell growth (oncogenes) by stimulating MAPK and being insensitive to GAP regulation
recessive loss of function mutations: two affected gene copies inactivate tumor suppressors
What kind of cancer is commonly associated with Ras mutations? How much? What mutations?
80% of pancreatic cancers
missense mutations G12D, G12V, and G12R which all decrease the GTPase activity of oncogenic Ras protein
Regarding GTP binding, GAP binding, and GTPase activity, what combinations of strong and weak produce normal response vs. cancer?
if all are strong, there will be normal response
if GTP is strong but GAP and GTPase are weak, there will be cancer (no regulation, only stimulated growth)
if GTP and GTPase are weak but GAP is strong, there will be cancer but not as bad (less regulation but also less growth)
Describe the structure of the insulin receptor
a2B2 tetrameric complex linked by disulfide bonds
Describe the insulin receptor signaling pathway through PI-3K activation
- one molecule of insulin binds to the alpha subunit
- conformational changes in the beta subunit activate intracellular kinase domains
- autophosphorylation of tyrosines causes a final conformational change in the beta subunits
- SPLIT:
- one branch activates GRB2, SOS, and Ras for MAPK signaling and altered gene expression and cell division
- Other branch is the Insulin specific pathway (cont. below)
- phosphorylated tyrosine recruits and phosphorylates IRS
- phosphorylated IRS binds to PI-3K
- PI-3K signaling leads to glucose uptake and glycogen synthesis
What is the difference in SHC and IRS proteins recruited to the insulin receptor pathway?
SHC: recruited for MAPK pathway, binding domain is Asn-Pro-Gln-pTyr
IRS: recruited for PI-3K pathway, binding domain is Asn-Pro-Ala-pTyr
Describe the PI-3K pathway starting with activated PI-3K
- PI-3K phosphorylates PIP2 to PIP3 (terminated by PTEN)
- PIP3 recruits PDK1
- PDK1 phosphorylates Akt
- Akt phosphorylates downstream proteins and causes glucose uptake and glycogen synthesis in liver cells
What is the difference between IP3 and PIP3
PIP3 is 3, 4, 5
IP3 is 1, 4, 5
PI-3K phosphorylates PIP2 to PIP3
PLC breaks PIP2 into DAG and IP3
How do PDK1 and Akt stay at the membrane?
PDK1 and Akt have pleckstrin homology domains that bind to membrane associated PIP3 lipids in the insulin signaling pathway
TNF pathways do not involve any…
kinases
TNF receptor signaling initiates two pathways that lead to….
What determines the chosen outcome?
cell death (apoptosis) or cell survival
pathway and outcome is determined by relative abundance of downstream signaling proteins (FADD takes apoptotic pathway, TRAF2 takes cell survival pathway)
TNF is mediated by adaptors that have what types of domains?
death domains or death effector domains
Describe TNF receptor signaling up to when the two pathways diverge
- 3 molecules of TNF-a bind to the TNF receptor trimer
- binding stimulates the exchange of SODDs for TRADDs (SODD inhibits signaling)
- TRADD proteins initiate adaptor complex formation and downstream signaling (paths diverse at the level of TRADD)
Describe the TNF receptor signaling starting when the two pathways diverge
Apoptotic pathway:
- DD and DED complexes assemble
- procaspase 8 autocleaves
- CASP8 cleaves procaspase 3
- CASP3 cleaves cellular proteins leading to cell death
Cell survival pathway:
- NIK/RIP mediate IKK phosphorylation
- IKK phosphorylates IKBa, activating p50/p65
- active p50/p65 form the active NFkB complex which moves to the nucleus
- NFkB increases expression of anti-apoptotic genes that inhibit CASP8 and CASP3 (stop apoptosis pathway)
What is the structure of death domains of TNF?
six alpha helices
How is CASP3 activated by CASP8?
CASP8 cleaves caspase 3 at Asp175 to a partially active procaspase 3
an inactive procaspase 3 finds another and noncovalent interactions form between the two subunits to form a partially active enzyme
procaspase 3 cleaves at Asp28 and Asp9 (extras off the ends)
two sets of procaspase subunits (p12 and p17) assemble into a heterotetramer to form a fully active CASP3
What are nuclear receptors? What are the basic steps?
ligand regulated transcription factors that bind to specific DNA sequences in target genes and recruit coregulatory proteins to module transcriptional initiation rates
- ligand binds
- complex binds DNA
- coregulatory proteins recruited and activate RNA polymerase
What are the two types of nuclear receptors and their similarities and differences?
steroid and metabolite receptors
same:
- bind DNA specifically
- recruit regulatory proteins
different:
- DNA sequence is different (steroid bind inverted repeat sequences as homodimers, while metabolite bind direct repeat DNA sequences at heterodimers)
- steroid bind head to head while metabolite bind head to tail
If two cell types are exposed to a steroid molecule but only one type had activated gene expression, what is the difference between the two cell types?
only one expresses the cognate steroid receptor
Where do the zinc fingers on nuclear receptor homodimers bind?
cysteines in the major groove of DNA
