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Lecture 27
Cell Signalling Pathways
Performance enhancing drugs
Anabolic steroids/ Growth hormones/ EPO: drugs of choice in
sports dependent on strength + muscle mass (they increase bulk).
Anabolic steroids: mimic testosterone and can disrupt hormonal
equilibrium by affecting muscle growth + behaviour (amongst other
things) and bind to androgen receptors triggering binding of receptor
to DNA and transcription of androgen responsive targets. GH:
Peptide that binds to receptors + exerts effects by activating
responsive genes that change cell growth rate + increase muscle
mass. EPO: Peptide involved in the development of various blood
lineages from haematopoietic precursors.
Cytokines
Structure : Similar, points to evolutionary relationship (common ancestral molecule)
Functioning: binding promotes dimerization of cognate cell surface receptors. Transduction of
cell signal is conserved among cytokines. Stoichiometry is 1:2 in terms of ligand to receptor.
Specificity is achieved through TFs present in cell + genes responsive in cell type.
Epo binds to receptor: monomers are always associated with JAK (just
another kinase). Kinase activity low when not bound. When ligand bound
a 1) conformational change will lead to homodimerisation and JAKs
phosphorylate each other at tyrosines (Y) and become highly activated.
2) SH domain proteins (part of STAT): recognize phosphorylated
tyrosines and STAT (a strong transcription factor) is phosphorylated by
JAK. 3) STATs recognize each other in end to end manner and
homodimerize. This exposes the NLS (Nuclear localization signal) and
will take STAT into nucleus. THIS IS SIGNAL TRANSDUCTION
Attenuation of signal: constitutive signalling
can be dangerous. Two pathways to
attenuate it. Short term: SH2 domains of
SHP1 recognize phosphotyrosine, and
unplug phosphatase domain.
Phosphotyrosines compete for SH2
domains, gets rid of phosphates on JAK
and bring back down to ground state.
Long term: In event of increased
cytokine signalling, SOCS transcribed
from STAT. They bind to phosphotyrosines in activated receptors or
via SH2 domain + target them for ubiquitin mediated degradation.
Receptor Tyrosine Kinases (RTKs)
Function: cell growth + proliferation. Examples: EGF, insulin
fibroblast growth factor all work through RTKs Stoichiometry = 2:2 or
1:1. 1) EGF binds to receptor 2) conformational change
homodimerization (weak kinase when separate, strong when
dimerized) 3) Kinase within receptor (no JAK) phosphorylates
tyrosine residues in intracellular domain. 4) Y residues recognized by
adapter proteins (GRB2) that contains SH2 and SH3 domains. 5)
GRB binds to Sos-a Guanine exchange factor (GEF) 6) Complex acts
on membrane bound inactive GDP-bound Ras and exchanges GDP
for GTP (Ras is in active form when bound to GTP) Ras is also one of
the main targets of cancer
Majenta Whyte Potter-Mäl
1 of 2
Molecular Biology
Lecture 27
7) Active Ras dissociates from complex (RTK/ Sos/GRB) and
contacts downstream effectors to transduce cell signal. Sos
plays role in carrying this out.
Kinase cascade
Activated Ras in GTP bound found will bind with Raf (kinase
binding protein). Raf becomes activated. When activated it
sets up a kinase cascade (phosphorylates another kinase, and
then another kinase and then you end up phosphorylated a
MAP kinase. Map kinase in phosphorylated form will
homodimerize, will go to nucleus, and activation of
transcription. Raf is an oncogene. You can always tell when
this path is being activated by looking at phosphorylated form of
map kinase in nucleus.
RTKs and Ras in developmental
processes
Sevenless: In a gene 7 mutation called
sevenless, don’t form R7 photoreceptor.
Found that a signalling event that is
initiated in R8 will activate sevenless (an
RTK receptor). When mess up
sevenless, get cone cell. If no
signalling no sevenless (R7 becomes
unspecified cone cell). If you have
activated Ras that activates sevenless,
we have mutants for all of those proteins
and learned this in drosophila genetics.
If you have activated RAS, don’t have to
worry about RTK. NEED RAS ONLY.
GEFs and GAPs regulate Ras activity through modifying
its GTP/GDP bound state: These dominants form of RAS are
associated with specific mutations. Most common mutation is
ras occurs at particular region called glycine 12. GAPs help
Ras hydrolize GTP to GDP. Hyperactivation of Ras that
doesn’t know how to turn it off.
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Spleen Focus Forming virus (SFFV): expresses viral
protein gp55 and causes erythroleukemia via the EPO
Too much Ras can lead to
problems like rough eyes. A wild receptor. It infects progenitor cells, interacts with epo
receptors. You get constitutive dimer receptors and
type C dominant Ras
constitutive activation.
Mutations in signalling molecules: often associated with cancers. Ras and RTKs are some of
#1 targets of cancer. Important for normal cell homeostasis and must be regulated in a very tight
manner. GTPase inactivating mutations are common in Ras-associated cancers. Other RTKs and
TKs also seem to play an important role in cancer (HER2,TRK and ABL)
Majenta Whyte Potter-Mäl
2 of 2
Molecular Biology