Chapter 16Cell Communication
Extracellular signal molecules can act overshort or long distances
Figure 16-3
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
Signal transduction - signal conversion
Figure 16-13
First messenger
Second messengerEffector
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
Extracellular signals bind to cell-surface or intracellularreceptors
Carrier protein
Figure 16-8
Animal cells get multiple signals
Figure 16-6
Extracellular signals can act rapidly or slowly
Fig. 16-7
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
Second messengers
small, nonprotein intermediaries acting in signaltransduction
e.g., cAMP, calcium, lipid-derived
Figure 16-17a
3 classes of cell surface receptors
1. Ion channel coupled receptors
Figure 16-17b
3 classes of cell surface receptors
2. G-protein-coupled receptors
3 classes of cell surface receptors
3. Enzyme-coupled receptors
Figure 16-17c
Basic characteristics ofcellsignaling systems
Many intracellular signaling molecules actas molecular switches
shape change by phosphorylation
Figure 16-15
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
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
Many intracellular signaling molecules actas molecular switches
shape change by GDP/GTP exchangeG-proteins are NOT kinases
Figure 16-15
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
2 alternate types ofsignal transductionpathways
1. G protein-linkedreceptor
2. Protein kinase receptor
Figure 16-18
All GPCRs have a similar structure
7 transmembrane -helices
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
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
Signal relay to effector
activated subunitchanges effectorshape (on)
Fig. 16-20
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
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
G-protein signaling
--Shows that response can be ended by GTP hydrolysisand endocytosis of receptor
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
G proteincoupled receptors
--some G proteins directly regulate ion channels
16_19_open_K_chan.jpg
Some G proteins directly regulate ion channels
subunits activeslows heart beat
K+
See Fig. 16-21
G proteincoupled receptors
--some G proteins activate membrane-bound enzymes
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
Glucose is stored in animal cells asglycogen (polymer of glucose)
(glycogen) phosphorylase makes more glucose
glycogen synthase makes more glycogen
Glucagon & epinephrine
Insulin
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.
Adenylyl cyclase
integral membrane proteinthat makes cAMP
an effector
See Fig. 16-23
Figure 16-25
Adrenaline stimulates glycogen breakdown inskeletal muscle cells
Protein kinase A (PKA) isactivated by cAMP
PKA also phosphorylatesglycogen synthase, whichinhibits glycogen synthesis
Figure 16-26
Adrenaline in the same cells also stimulates glucose synthesis
Enzymes needed forgluconeogenesis
cAMP can activategene transcription
cAMP Signaling
Signal amplification
Each GPCR activatesmultiple G proteins
Each active adenylylcyclase make many cAMPs
Each active PKA canphosphorylate multiplephosphorylase kinases
Etc..
Reversal of signalphosphatase-1 removes thephosphates that were added byPKA
cAMP is destroyed by cAMPphosphodiesterase
Figure 16-23
Phosphatidylinositol (PI) and phosphoinositides (PIPs)
Numberingthe carbons ininositol
Phosphorylationof OH in inositol
PI PIPs
Named according toring position (inparentheses) andnumber of phosphategroups (in subscript)
Animal cells have several PI and PIP kinasesand phosphatases
PI headgroups are recognized by proteindomains that discriminate the different forms
Proteins are recruited to regions of themembrane where these PIs are present
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
Lipid-derived second messengers
phospholipases --hydrolytic enzymes thatsplit phospholipids
PLA -- phospholipase A
PLD -- phospholipase D
PLC -- phospholipase C
Phosphatidylinositol-derivedare best studied
inositol
phospholipase Ccleaves PIP2 into DAGand IP3
Phosphatidylinositol (PI)- mediated responses
See Fig. 16-25
DAG IP3
How does PLC get activated?
See Fig. 16-25
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
IP3 (inositol 1,4,5-trisphosphate)
small, water soluble
binds to a Ca++ channel receptoron smooth endoplasmic reticulum(SER); Ca++ release
Figure 16-27
Phospholipase C releases lipid-derived second messengers
Ca++ as an intracellular messengerCa++ release from intracellular stores acts as a secondmessenger
First messenger: hormones, neurotransmitters, electricalactivation (muscle)
Calcium Signaling
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
Where are Ca++ release channels found?
IP3Rs: present onsmooth ER of mostcell types
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
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
Calmodulin
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
16_29_amplifies_light.jpg See Fig. 16-31
2 alternate types ofsignal transductionpathways
1. G protein-linkedreceptor
2. Protein kinase receptor
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)
Typical RTK
1. Inactive receptor monomers
2. Ligand induces dimerization andautophosphorylation
3. Signaling protein bind to specificphosphorylated Tyr
Figure 16-32
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
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
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)
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
PTB domain of IRS-1binds to insulin receptor
receptor phosphorylatesdocking sites on IRS-1
IRS-1 (insulin receptor substrate-1)
pTyr binding oninsulin receptor
docking sites
SH2 domain
RTK activate PI-3-kinase
Fig. 16-33
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
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
PI-3-kinase and PKB activation
Akt = Protein kinase BFig. 16-33
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
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
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
Ras
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
16_32_MAP-kinase.jpg
MAP kinase pathway
See Fig. 16-34
Signals that originate from ECM
integrins transmit signals thatinfluence cell growth, shape,migration, differentiation and survival
normal cells do not grow insuspension
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
Actin
P-tyr proteins
Extracellular signals bind to cell-surface or intracellularreceptors
Carrier protein
Figure 16-8
Role of nitric oxide as an intracellular messenger
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
Animal cells get multiple signals
Figure 16-6
Figure 16-40
Signaling cascades are not just linear
convergence - 2 ligands, 1 pathdivergence - 1 ligand, many paths
convergence divergence
Scaffolds help organize cascades
structural not enzymatic components
provide spatial localization & substratespecificity