Download - Chapter Sixteen Cell Signaling
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Chapter 16Cell Communication
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Extracellular signal molecules can act overshort or long distances
Figure 16-3
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Overview of signal initiation and responses
1. Receptor recognizesstimulus (3 typesshown)
2. Signal transferred tocytoplasmic surface ofreceptor
3. Signal transmitted toeffector molecule
4. Cessation of response
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Signal transduction - signal conversion
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Figure 16-13
First messenger
Second messengerEffector
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Figure 16-5
Same signal molecule can induce differentresponses in different target cells
Although heart and salivary gland have samereceptor, different intracellular effector proteinsare activated
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Extracellular signals bind to cell-surface or intracellularreceptors
Carrier protein
Figure 16-8
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Animal cells get multiple signals
Figure 16-6
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Extracellular signals can act rapidly or slowly
Fig. 16-7
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signal transduction -- stimulusreceived by cell-surfacereceptor is different thansignal released in cell interior
signal pathways -- series ofdistinct proteins that alter theconformation of thedownstream protein
Figure 16-13
Signaling proteins can relay, amplify, integrateand distribute incoming signal
First messenger
Second messenger
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Second messengers
small, nonprotein intermediaries acting in signaltransduction
e.g., cAMP, calcium, lipid-derived
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Figure 16-17a
3 classes of cell surface receptors
1. Ion channel coupled receptors
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Figure 16-17b
3 classes of cell surface receptors
2. G-protein-coupled receptors
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3 classes of cell surface receptors
3. Enzyme-coupled receptors
Figure 16-17c
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Basic characteristics ofcellsignaling systems
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Many intracellular signaling molecules actas molecular switches
shape change by phosphorylation
Figure 16-15
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may cause a conformationalchange in the protein
may be part of a proteinbinding site
may be added to serine,tyrosine, or threonine
may inc. or dec. proteinsactivity
Protein phosphorylation
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most substrates are otherenzymes
Protein kinases/phosphataseschange the shape/activities ofthe proteins they modify
continual balance of kinaseand phosphatase activities
human genome ~500different kinases and ~100different phosphatases
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Many intracellular signaling molecules actas molecular switches
shape change by GDP/GTP exchangeG-proteins are NOT kinases
Figure 16-15
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The activity of monomeric GTP-binding proteins iscontrolled by two types of regulatory proteins
Figure 16-16
GAP - GTPase-activating proteinsGEF - Guanine nucleotide-exchange factors
G-proteins areNOT kinases
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2 alternate types ofsignal transductionpathways
1. G protein-linkedreceptor
2. Protein kinase receptor
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Figure 16-18
All GPCRs have a similar structure
7 transmembrane -helices
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e.g., adenylyl cyclase
e.g., cAMP
e.g., glucagon receptor
G protein-coupled receptors (GPCRs)
GPCR are largest superfamily of proteins in animal genome(e.g., nematode has 19,000 genes of which 1000 are GPCRs)
Target of ~40% of modern medicinal drugsSee Figure 16-19
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Heterotrimeric G proteins
1. signal causes shape changein receptor
2. G protein binds to receptor
3. GTP binds to G protein,causing shape change
4. subunit and subunitsseparate
G-protein activation
Fig. 16-19
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Signal relay to effector
activated subunitchanges effectorshape (on)
Fig. 16-20
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1. GTP hydrolyzed and subunit changes shape(off)
2. G protein no longerbinds effector, butreforms trimer
3. Effector changesshape (off)
Ending the responseTurning off theeffector
Fig. 16-20
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Ending the response - II
1. GRK (G protein-coupled receptor kinase)phosphorylates receptor
2. arrestin binds, blocks G protein binding3. receptor endocytosed from PM
Receptordesensitization
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G-protein signaling
--Shows that response can be ended by GTP hydrolysisand endocytosis of receptor
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Specificity of G protein-coupled responses
not all parts of signal transduction machinery identical inall cells
multiple forms of receptors (e.g., 9 different isoformsof epinephrine receptors) with different ligand and Gprotein affinities
multiple G proteins ( 20 G; 5 G; 11 G ); variouscombinations
Gs stimulatory and Gi inhibitory
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G proteincoupled receptors
--some G proteins directly regulate ion channels
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16_19_open_K_chan.jpg
Some G proteins directly regulate ion channels
subunits activeslows heart beat
K+
See Fig. 16-21
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G proteincoupled receptors
--some G proteins activate membrane-bound enzymes
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Figure 16-20
Enzymes activated by G proteins catalyze production ofsmall intracellular signaling molecules
e.g. cAMP; IP3/DAG
e.g. Adenyl cyclase and phospholipase C
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Glucose is stored in animal cells asglycogen (polymer of glucose)
(glycogen) phosphorylase makes more glucose
glycogen synthase makes more glycogen
Glucagon & epinephrine
Insulin
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Glucagon (pancreas)
glucagon and epinephrine blood glucose byinhibiting glycogen synthase and activatingphosphorylase
the 2 hormones bind to different receptors, yetlead to same intracellular response, How?? Bothuse the same secondary messenger cAMP.
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Adenylyl cyclase
integral membrane proteinthat makes cAMP
an effector
See Fig. 16-23
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Figure 16-25
Adrenaline stimulates glycogen breakdown inskeletal muscle cells
Protein kinase A (PKA) isactivated by cAMP
PKA also phosphorylatesglycogen synthase, whichinhibits glycogen synthesis
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Figure 16-26
Adrenaline in the same cells also stimulates glucose synthesis
Enzymes needed forgluconeogenesis
cAMP can activategene transcription
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cAMP Signaling
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Signal amplification
Each GPCR activatesmultiple G proteins
Each active adenylylcyclase make many cAMPs
Each active PKA canphosphorylate multiplephosphorylase kinases
Etc..
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Reversal of signalphosphatase-1 removes thephosphates that were added byPKA
cAMP is destroyed by cAMPphosphodiesterase
Figure 16-23
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Phosphatidylinositol (PI) and phosphoinositides (PIPs)
Numberingthe carbons ininositol
Phosphorylationof OH in inositol
PI PIPs
Named according toring position (inparentheses) andnumber of phosphategroups (in subscript)
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Animal cells have several PI and PIP kinasesand phosphatases
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PI headgroups are recognized by proteindomains that discriminate the different forms
Proteins are recruited to regions of themembrane where these PIs are present
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Figure 16-20
Enzymes activated by G proteins catalyze production ofsmall intracellular signaling molecules
e.g. cAMP; IP3/DAG
e.g. Adenyl cyclase and phospholipase C
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Lipid-derived second messengers
phospholipases --hydrolytic enzymes thatsplit phospholipids
PLA -- phospholipase A
PLD -- phospholipase D
PLC -- phospholipase C
Phosphatidylinositol-derivedare best studied
inositol
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phospholipase Ccleaves PIP2 into DAGand IP3
Phosphatidylinositol (PI)- mediated responses
See Fig. 16-25
DAG IP3
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How does PLC get activated?
See Fig. 16-25
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Diacylglycerol (DAG)
lipid molecule remains in PM
recruits and activates protein kinase C (PKC)
PKC is a serine/threonine kinase that is imp. in manycellular events (e.g., cell growth, differentiation,metabolism and transcriptional activation)
phorbol esters -- plant compounds that mimic DAGand activate PKC; cells lose growth control andbehave as malignant cells
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IP3 (inositol 1,4,5-trisphosphate)
small, water soluble
binds to a Ca++ channel receptoron smooth endoplasmic reticulum(SER); Ca++ release
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Figure 16-27
Phospholipase C releases lipid-derived second messengers
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Ca++ as an intracellular messengerCa++ release from intracellular stores acts as a secondmessenger
First messenger: hormones, neurotransmitters, electricalactivation (muscle)
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Calcium Signaling
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Ca++ is not made enzymatically [Ca++] controlled by pumps and channels Free intracellular [Ca++] ~ 0.1 M [Ca++] within lumen of ER ~ 1 mM (10,000-fold
higher)
channels intocytoplasm normallyclosed
out of cytoplasmpumps usually active
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Where are Ca++ release channels found?
IP3Rs: present onsmooth ER of mostcell types
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Unlike cAMP, Ca++ modulates a broad rangeof effectors
cAMP acts on kinases; primarily PKA
Ca++ can activate kinases, ion pumps,proteases, phosphatases, Ca ++-bindingproteins, etc.
Calmodulin -- best known Ca ++-bindingprotein
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Calmodulin (CaM)
binds Ca++ only instimulated cells(affinity too low)
Ca++ binding changesconfirmation andaffinity for otherproteins
Activated CaMactivates Ca++ pumpto reduce cytosoliclevels
Figure 16-29
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Calmodulin
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Role of GPCR in vision
Receptor - rhodopsin
G protein -transducin
Effector - cGMPphosphodiesterase
response 20 msec(20/1000th of a sec)
cGMP keepsopen
See Figure 16-30
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16_29_amplifies_light.jpg See Fig. 16-31
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2 alternate types ofsignal transductionpathways
1. G protein-linkedreceptor
2. Protein kinase receptor
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transmembrane receptorsenzymes that adds phosphate groups totyrosine aa on proteinseach RTK monomer crosses PM once (GPCR7Xs) > 50 different RTKs have been identified
Receptor tyrosine kinases (RTKs)
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Typical RTK
1. Inactive receptor monomers
2. Ligand induces dimerization andautophosphorylation
3. Signaling protein bind to specificphosphorylated Tyr
Figure 16-32
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phosphotyrosine motifs -- onlytyrosines surrounded by certain aa arephosphorylated
SH2 domains (src homology 2) andPTB domains (phosphotyrosine binding);sites in proteins that recognize thephosphorylated tyrosines; found inmany cell signaling proteins
Important concepts
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Interaction between SH2 domain andphosphotyrosine
~ 100 aa long
Found in ~110 different human proteins
SH2 compact plug-in module can be inserted nearlyanywhere in a protein sequence and not disturb proteinfolding
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Role of RTK in cellular activities
many types, but best studied:
-- hormones - e.g. insulin, growth hormone-- growth factors - epidermal growthfactor (EGF), fibroblast growth factor(FGF), platelet-derived growth factor(PDGF)
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tetramer of two and two polypeptide chainsinsulin binding to changes conformation of activated receptor phosphorylates self(autophosphorylation) and soluble proteins (insulinreceptor substrates; IRSs)
Insulin receptor
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PTB domain of IRS-1binds to insulin receptor
receptor phosphorylatesdocking sites on IRS-1
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IRS-1 (insulin receptor substrate-1)
pTyr binding oninsulin receptor
docking sites
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SH2 domain
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RTK activate PI-3-kinase
Fig. 16-33
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phosphorylated inositol rings bind to modules on manyproteinsrecruits specific protein to cytoplasmic face of PMPH: pleckstrin homology domains targets proteins tomembranes by binding PIP2 or PIP3 when they areproduced
PI PIP PIP2
Phosphoinositides direct major signaling cascades
PIP3 -- PI(3,4,5)
-can act as docking sites and as secondary messengers
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PIP3 Produced in Response to Stimulation of a RTK
-GFP fused to a PH (PIP3 binding domain from a tyr kinase)-PDGF causes the production of PIP3 at the plasma membrane
Dr. Seth Field
e.g., PDGF or Insulin
PH = pleckstrin homology
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PI-3-kinase and PKB activation
Akt = Protein kinase BFig. 16-33
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PI3K generates PIP2 and PIP3protein kinase B (PKB)binds/partially-activated by PIP2and PIP3 via its PH domainsfully activated by anotherkinase(s)signal reversed bydephosphorylating receptorInc. protein syn.Inc. glucose uptakeInc. glycogen syn.
thus, insulinleads to lowerblood glucose
PH domains --pleckstrin homologydomain -- bindsPIP2/PIP3
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The importance of Ras
Ras is a monomeric G protein held to PM bylipid group
Monomeric G proteins mimic G in function
nearly all RTK activate Ras
30% of human tumors have a mutated ras (orras-like) gene
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Figure 16-33
Most RTK activate the monomeric G protein Ras
Ras mutations leading to tumorsprevent GTP hydrolysis; Ras alwayson
Ras GEF MAP kinasepathway
Grb2Sos
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Ras
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MAP kinase cascadeMitogen-activated proteinkinase
growth factor binding leads toinc transcription of genesinvolved in growth responses
Ras GTP hydrolysis is inducedby a GTPase activating protein(30 min dec to 1/10 sec)
Ras mutations leading totumors prevent GTP hydrolysis;Ras always on
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16_32_MAP-kinase.jpg
MAP kinase pathway
See Fig. 16-34
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Signals that originate from ECM
integrins transmit signals thatinfluence cell growth, shape,migration, differentiation and survival
normal cells do not grow insuspension
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Signals induce cell growth
1. clustering ofintegrins inducesprotein binding
2. P-tyr activation ofsrc and FAK kinases
3. Ras activated
4. MAP kinase cascadeinc. transcription ofgenes involved ingrowth
FAK = Focal adhesion kinasesrc is an oncogene
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Actin
P-tyr proteins
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Extracellular signals bind to cell-surface or intracellularreceptors
Carrier protein
Figure 16-8
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Role of nitric oxide as an intracellular messenger
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Mode of action for Viagra
NO (nitric oxide) released by nerve and endothelial cells in penis
NO --> higher cGMP levels --> muscle relaxation and increasedblood flow --> erection
Viagra inhibits PDE5 (form of cGMP phosphodiesterase; found onlyin penis) keeping cGMP levels high
smooth muscle cellaround arterialblood vessels
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Animal cells get multiple signals
Figure 16-6
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Figure 16-40
Signaling cascades are not just linear
convergence - 2 ligands, 1 pathdivergence - 1 ligand, many paths
convergence divergence
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Scaffolds help organize cascades
structural not enzymatic components
provide spatial localization & substratespecificity