BIOL 142 Lecture 29: Cell Signaling II - Hydrophilic Signals
30 Apr 2018
School
Department
Course
Professor
1.#When%a%chemical%messenger%(also%called%a%"ligand”%or%a%“signal”)%reaches%a%target%
cell,%it%binds%to%a%receptor.%In%general,%a%hydrophobic%ligand%binds%to%a(n)%__________,%
whereas%a%hydrophilic%ligand%binds%to%a(n)%__________.%
A)#intracellular#receptor;#cell-surface#receptor#
B)%cell-surface%receptor;%intracellular%receptor%
C)%intracellular%receptor;%intracellular%receptor%
D)%cell-surface%receptor;%cell-surface%receptor%
E)%none%of%the%above%answers%are%correct.%
Small/hydrophobic%signals%can%cross%the%plasma%membrane%to%intracellular%receptors
Large%class%of%hydrophilic%signals%are%on%the%cell's%surface
Outline:#Cell#Signaling#
How%cells%communicate:%signal%transduction%and%2nd%messengers.%
1.
Small%signaling%molecules%enter%target%cells%and%bind%intracellular%receptors.%
Nitric-oxide%as%a%signal%
a.
Steroid%hormonal%cues%
b.
2.
Most#signaling#molecules#bind#cell-surface#receptors.
G-protein#linked#receptors
a.
Ion-channel#linked#receptors#
b.
Enzyme%linked%receptors%
c.
3.
Hydrophobic%steroid%hormones%cross%the%plasma%membrane%of%the%target%cell%
Steroid%hormones%all%pass%through%the%plasma%membrane%of%the%target%cell.%
•
In%the%cell,%the%hormones%bind%to,%and%activate%receptors%found%in#the#cytoplasm#
or#nucleus.%
•
Upon%activation,%the%hormone%receptors%are%capable%of%regulating%gene%
transcription.%
•
Learning%Objectives
List%the%series%of%events%required%to%activate%and%inactivate%the%G-protein%of%a%G-
protein%linked%receptor
1.
Compare%the%activation%of%the%enzymes
Adenylyl%cyclasei.
Phospholipase%C%by%the%GTP-bound%α%subunit%of%an%activated%G-proteinii.
2.
Describe%the%function%of%ion%channel>linked%receptors3.
Extracellular% signal%molecules%fall%into%one%of%two%classes%
Hydrophilic%signaling%molecules%bind%cell-surface%receptors.%•
1.%The%G-protein%linked%receptor%family%is%the%largest%family%of%cell-surface%receptors%
Vital%to%physiology
There%are%toxins%that%work%by%modifying%the%activity%of%these%receptors
○
Cancer%can%hijack%this%process
○
•
Several%types%of%G-proteins#exist—each%binds%a%specific%type%of%receptor%and%set%
of%downstream%targets.%All%G-proteins#have%a%similar%structure%and%operate%in%a%
similar%way%
•
The%trimeric#G-proteins#are%composed%of%α,#β,#and#γ subunits#and%are%tethered%
to%the%membrane%inside%the%cell.%
•
Inactive%G-proteins#are%bound#by#GDP#at%the%α%subunit#
Receptor%is%not%bound%to%G-protein%receptor%here
○
Inactive%G%protein%is%bound%to%GDP
○
•
A%hydrophilic#signal#binds%to%the%extracellular%side%of%the%receptor%and%changes%
the%shape%of%the%receptor%on%the%cytoplasmic%side%
G%protein%can%physically%interact%with%receptor%
○
•
G%protein%activation%triggers%a%detachment%of%the%α,#β,#and#γsubunits#from%the%
receptor%and%each%other%to%roam%the%plasma%membrane.%
G%protein%exchanges%GDP%for%GTP
○
Dropping%off%GDP%and%picking%up%a%new%molecule%
○
When%G-protein%is%bound%to%GTP,%α-subunit#and#can#physically#interact#
with#and##activate#downstream#targets
○
•
2.#Trimeric%G)proteins,%that%act%as%molecular%switches%inside%cells:%
A)%are%active%when%cAMP%is%bound.%
B)%are%active%when%GDP%is%bound.%
C)%are%active%when%ATP%is%bound.%
D)%are%active%when%ADP%is%bound.%
E)#are#active#when#GTP#is#bound.#
The%effector%protein%may%catalyze%many%reactions,%amplifying%the%signal.%
The%activated#αand#βγ protein#subunits#can%both%interact%directly%with%target%
proteins%in%the%plasma%membrane%that%can%relay%the%signal%to%other%
destinations.
•
The%α and#βγ complex#proteins%remain%dissociated)and)active)until%the%α%
subunit#hydrolyses#the%bound#GTP#to%GDP.#
Needs%to%be%able%to%be%turned%on%or%off%to%have%effect
If%it's%a%cell%related%to%growth,%will%form%a%tumor
§
○
G-protein%turned%on%by%GDP%-->%GTP
○
Once%subunit%binds%to%GTP,%a%little%timer%turns%on%and%then%the%a%subunit%
will%hydrolyze%GTP%for%GDP
○
When%the%a%subunit%is%bound%to%GDP,%it%re-associates%with%βγ
○
•
GTP#hydrolysis#causes%the%α%subunit#to%re-associate%with%the%βγ subunits#and%
the%signal%is%shut#off.%
•
3.#If%a%G%protein%were%unable%to%release%its%bound%nucleotide%but%could%hydrolyze%it,%
signal%transduction%would:
A.#not#move#beyond#this#point.
B.%be%continuous%beyond%this%point.%
C.%be%unaffected.
D.%be%constantly%switching%on%and%off.%
E.%be%unpredictable.%
Activation%of%a%G%protein%involves%trading%GDP%for%GTP%to%turn%it%on
Here%is%a%G%protein%that%can%hydrolyze,%but%can't%release%the%nucleotides•
If%you%can't%release%them,%you%can't%trade%them%and%can't%be%turned%on•
Signal%transduction%pathway%with%a%roadblock•
4.#If%a%G%protein%could%release%its%bound%nucleotide%but%was%not%able%to%hydrolyze%it,%
signal%transduction%would:
A.%not%move%beyond%this%point.
B.#be#continuous#beyond#this#point.#
C.%be%unaffected.
D.%be%constantly%switching%on%and%off.%
E.%be%unpredictable.%
Mutation%effects%ability%to%hydrolyze
Hydrolysis%inactivates%G%protein•
G%protein%bound%to%GTP%is%turned%off%by%hydrolyzing%GTP%for%GDP•
Will%have%excessive% signaling•
2.%What%are%the%effectors%of%G/Proteins?
The%G/protein%GS#that%activates%the%enzyme,%adenylyl%cyclase%(also%called%
adenylate%cyclase)%to%make%cAMP
i.
The%G/protein%GQ#that%activates%the%enzyme,%phospholipase%C.%ii.
i.%GTP-bound%GS%activates%Adenylyl)cyclase,%an%enzyme%that%catalyzes%cyclic%AMP%
(cAMP)%formation.%
The%cAMP%2nd%messenger%activates%the%enzyme%Protein%Kinase%A,%involved%in%a%
wide-range%of%biological%processes%including%long-term%memory
Regulates%downstream%targets
○
•
Has%one%phosphate%group%attached%to%the%figure%in%two%places%that%forms%a%ring•
cAMP%is%quickly%degraded%by%Phosphodiesterase#(PDE)
Fast%cAMP%degradation%is%necessary%to%maintain%sensitivity%of%the%cell%to%
extracellular% signal%molecules
Needs%to%be%able%to%turn%off%or%on%signals%to%respond%to%extracellular%
clues
§
○
A%lack%of%cAMP%degradation%results%in%a%failure%of%response%to%further%
extracellular% signals
○
One%of%the%many%physiological%actions%of%caffeine%is%to%act%as%a%
Phosphodiesterase%(PDE)%inhibitor%
Caused%cAMP%to%hang%around%for%a%longer%period%of%time%for%more%
cellular% signaling
§
Viagra%is%also%PDE%inhibitor
§
○
•
Arise%in%intracellular% cAMP%can%activate%gene%transcription
In%the%cytosol,%cAMP%activates%PKA%which%then%moves%into%the%nucleus%and%
phosphorylates%specific%gene%regulatory%proteins.
Protein%kinase%A
A%comes%from%being%activated%from%cAMP□
Adds%phosphate%groups%onto%specific%target%proteins□
§
Protein#kinases phosphorylates%specific%targets%by%adding%a%
phosphate%group
§
○
Once%phosphorylated,%these%proteins%stimulate%the%transcription%of%a%
whole%set%of%target%genes
○
PKA%can%also%phosphorylate%proteins%in%the%cytosol%
Once%activated,%can%activate%other%targets%in%the%cytosol%or%go%inthe%
nucleus%to%phosphorylate%transcription%factors
§
Affects%behavior,%water%absorption,%determining%when%we%
synthesize%steroid%hormones
All%depend%on%when%and%where%it's%turned%on□
§
○
•
5. Cholera%toxin%can%affect%cells%by%inhibiting%the%ability%of%the%G%protein%α%subunit%to%
hydrolyze%GTP%to%GDP%.%On%a%cellular%level,%which%one%of%the%following%would%be%
helpful%in%reducing%the%harmful%effect%of%cholera%toxin?%
Increasing%the%amount%of%intracellular%cAMP.%A.
Inhibiting#the#activity#of#adenylyl#cyclase#in#the#cell.#B.
Adding%ligand%for%the%G-protein%coupled%receptor%in%this%pathway.%C.
Increasing%the%amount%of%protein%kinase%A%activity%in%the%cell.%D.
Inhibiting%the%activities%of%the%β%and%γ%G-protein%subunits.%E.
Cholera%toxin%binds%to%GS%and%locks%it%in%"on"%position,%especially%in%the%epithelia%of%the%
gut
When%you%have%cholera%bacteria,%you%produce%excessive%G%protein%signaling%that%
leads%to%an%excess%of%salt%being%moved%into%the%gut%interior,%causing%water%to%be%
moved%out
•
Without%treatment,%die%of%dehydration•
Too%much%cAMP%is%formed%and%to%much%PKA%is%signaled•
Need%to%downregulate%the%activity%caused%by%the%toxin•
ii)%GTP-bound%GQ%activates%Phospholipase#C#
Phospholipids%can%be%hydrolyzed%into%components%that%act%as%second%
messengers%
•
Activation%of%GQ%-signal%binds%a%receptor%that%causes%the%receptor%to%change%in%
shape%so%the%G%protien%can%bind%to%the%receptor%and%triggers%a%change%from%GDP%
to%GTP%so%alpha%subunit%can%dissociate
•
Phosphatidyl%inositol-bisphosphate%(PIP2)%is%hydrolyzed%into%inositol%
triphosphate%(IP3)%and%diacylglycerol%(DAG).%
PhosC%cuts%PIP2%into%2%parts%
○
Molecules%continue%downstream%signaling
○
Can%open%calcium%channels%in%the%endoplasmic%reticulum%and%release%
calcium%to%the%cytoplasm
ER%is%a%storage%place%for%the%calcium%and%Ca%goes%to%areas%of%lower%
concentration
§
○
•
The%two%parts%each%become%second%messengers,%with%IP3%moving%into%the%
cytoplasm%and%DAG%remaining%in%the%membrane.%
Ca%with%DAG%activate%protein%kinase%C
Activated%by%calcium%and%DAG
§
Then%phosphorylates%targets%to%generate%a%cellular% response
§
○
•
3.%Ion%channel-linked%receptors%convert%hydrophilic%chemical%signals%into%electrical%
signals.%
Span%plasma%membrane•
Have%closed%and%open%state
Closed:%nothing%can%pass%through%the%pore%of%the%receptor
○
When%hydrophilic%signals%bind,%channel%opens%and%particular%ions%can%pass%
the%plasma%membrane
○
•
Ion#channel/linked#receptors#can%open%to%let%certain%ions%(for%e.g.%Na+,%K+,%Ca2
+,%or%Cl,)%in%or%out%of%a%cell% and%are%responsible%for%rapid-transmission%of%signals%
across%synapses%in%the%nervous%system.%
•
Ion%channels%transduce%a%chemical%signal,%in%the%form%of%a%pulse%of%
neurotransmitter%(a%hydrophilic#chemical)signal)%delivered%to%the%outside%of%the%
target%cell,%directly%into%an%electrical)signal,%in%the%form%of%a%change%in%voltage%
across%the%plasma%membrane%of%the%target%cell%
Moving%ions%from%one%compartment%(outside)%to%inside%the%cell,%you%
change%the%difference%in%charge%on%that%plasma%membrane,%changing%its%
voltage
Voltage%changes%are%important%for%electrical%communication%of%cells
§
○
Effect%on%physiology
How%neural%cells%in%the%spinal%cells%communicate
§
Cause%contraction%of%heart%cells%in%unison%to%drive%pressure%for%
circulation
§
Muscle%contraction%at%the%appropriate%time%
§
○
•
Which%of%the%following%statements%is CORRECT about%the%chemical,%
caffeine? Caffeine:
Primarily%inhibits%phospholipase%CA.
Primarily%activates%phosphodiesteraseB.
Primary%inhibits%adenylyl%cyclaseC.
Primarily%activate%adenylyl%cyclaseD.
Primarily#inhibits#phosphodiesteraseE.
Testosterone%mediates%its%effect%by%which%of%the%following%mechanisms
Activates%a%receptor%that%increases%the%permeability%of%the%membrane%cationsA.
Activates%an%intracellular%enzyme%when%combined%with%a%membrane%receptorB.
Activates#genes#by#binding#with#an#intracellular#receptorC.
Activates%the%phosphatidylinositol%systemD.
Cell$Signaling$II$-Hydrophilic$Signals
Wednesday,% April%25,%2018
11:56%AM
1.#When%a%chemical%messenger%(also%called%a%"ligand”%or%a%“signal”)%reaches%a%target%
cell,%it%binds%to%a%receptor.%In%general,%a%hydrophobic%ligand%binds%to%a(n)%__________,%
whereas%a%hydrophilic%ligand%binds%to%a(n)%__________.%
A)#intracellular#receptor;#cell-surface#receptor#
B)%cell-surface%receptor;%intracellular%receptor%
C)%intracellular%receptor;%intracellular%receptor%
D)%cell-surface%receptor;%cell-surface%receptor%
E)%none%of%the%above%answers%are%correct.%
Small/hydrophobic%signals%can%cross%the%plasma%membrane%to%intracellular%receptors
Large%class%of%hydrophilic%signals%are%on%the%cell's%surface
Outline:#Cell#Signaling#
How%cells%communicate:%signal%transduction%and%2nd%messengers.%1.
Small%signaling%molecules%enter%target%cells%and%bind%intracellular%receptors.%
Nitric-oxide%as%a%signal%a.
Steroid%hormonal%cues%b.
2.
Most#signaling#molecules#bind#cell-surface#receptors.
G-protein#linked#receptorsa.
Ion-channel#linked#receptors#b.
Enzyme%linked%receptors%c.
3.
Hydrophobic%steroid%hormones%cross%the%plasma%membrane%of%the%target%cell%
Steroid%hormones%all%pass%through%the%plasma%membrane%of%the%target%cell.%•
In%the%cell,%the%hormones%bind%to,%and%activate%receptors%found%in#the#cytoplasm#
or#nucleus.%
•
Upon%activation,%the%hormone%receptors%are%capable%of%regulating%gene%
transcription.%
•
Learning%Objectives
List%the%series%of%events%required%to%activate%and%inactivate%the%G-protein%of%a%G-
protein%linked%receptor
1.
Compare%the%activation%of%the%enzymes
Adenylyl%cyclase
i.
Phospholipase%C%by%the%GTP-bound%α%subunit%of%an%activated%G-protein
ii.
2.
Describe%the%function%of%ion%channel>linked%receptors
3.
Extracellular% signal%molecules%fall%into%one%of%two%classes%
Hydrophilic%signaling%molecules%bind%cell-surface%receptors.%
•
1.%The%G-protein%linked%receptor%family%is%the%largest%family%of%cell-surface%receptors%
Vital%to%physiology
There%are%toxins%that%work%by%modifying%the%activity%of%these%receptors
○
Cancer%can%hijack%this%process
○
•
Several%types%of%G-proteins#exist—each%binds%a%specific%type%of%receptor%and%set%
of%downstream%targets.%All%G-proteins#have%a%similar%structure%and%operate%in%a%
similar%way%
•
The%trimeric#G-proteins#are%composed%of%α,#β,#and#γ subunits#and%are%tethered%
to%the%membrane%inside%the%cell.%
•
Inactive%G-proteins#are%bound#by#GDP#at%the%α%subunit#
Receptor%is%not%bound%to%G-protein%receptor%here
○
Inactive%G%protein%is%bound%to%GDP
○
•
A%hydrophilic#signal#binds%to%the%extracellular%side%of%the%receptor%and%changes%
the%shape%of%the%receptor%on%the%cytoplasmic%side%
G%protein%can%physically%interact%with%receptor%
○
•
G%protein%activation%triggers%a%detachment%of%the%α,#β,#and#γsubunits#from%the%
receptor%and%each%other%to%roam%the%plasma%membrane.%
G%protein%exchanges%GDP%for%GTP
○
Dropping%off%GDP%and%picking%up%a%new%molecule%
○
When%G-protein%is%bound%to%GTP,%α-subunit#and#can#physically#interact#
with#and##activate#downstream#targets
○
•
2.#Trimeric%G)proteins,%that%act%as%molecular%switches%inside%cells:%
A)%are%active%when%cAMP%is%bound.%
B)%are%active%when%GDP%is%bound.%
C)%are%active%when%ATP%is%bound.%
D)%are%active%when%ADP%is%bound.%
E)#are#active#when#GTP#is#bound.#
The%effector%protein%may%catalyze%many%reactions,%amplifying%the%signal.%
The%activated#αand#βγ protein#subunits#can%both%interact%directly%with%target%
proteins%in%the%plasma%membrane%that%can%relay%the%signal%to%other%
destinations.
•
The%α and#βγ complex#proteins%remain%dissociated)and)active)until%the%α%
subunit#hydrolyses#the%bound#GTP#to%GDP.#
Needs%to%be%able%to%be%turned%on%or%off%to%have%effect
If%it's%a%cell%related%to%growth,%will%form%a%tumor
§
○
G-protein%turned%on%by%GDP%-->%GTP
○
Once%subunit%binds%to%GTP,%a%little%timer%turns%on%and%then%the%a%subunit%
will%hydrolyze%GTP%for%GDP
○
When%the%a%subunit%is%bound%to%GDP,%it%re-associates%with%βγ
○
•
GTP#hydrolysis#causes%the%α%subunit#to%re-associate%with%the%βγ subunits#and%
the%signal%is%shut#off.%
•
3.#If%a%G%protein%were%unable%to%release%its%bound%nucleotide%but%could%hydrolyze%it,%
signal%transduction%would:
A.#not#move#beyond#this#point.
B.%be%continuous%beyond%this%point.%
C.%be%unaffected.
D.%be%constantly%switching%on%and%off.%
E.%be%unpredictable.%
Activation%of%a%G%protein%involves%trading%GDP%for%GTP%to%turn%it%on
Here%is%a%G%protein%that%can%hydrolyze,%but%can't%release%the%nucleotides•
If%you%can't%release%them,%you%can't%trade%them%and%can't%be%turned%on•
Signal%transduction%pathway%with%a%roadblock•
4.#If%a%G%protein%could%release%its%bound%nucleotide%but%was%not%able%to%hydrolyze%it,%
signal%transduction%would:
A.%not%move%beyond%this%point.
B.#be#continuous#beyond#this#point.#
C.%be%unaffected.
D.%be%constantly%switching%on%and%off.%
E.%be%unpredictable.%
Mutation%effects%ability%to%hydrolyze
Hydrolysis%inactivates%G%protein•
G%protein%bound%to%GTP%is%turned%off%by%hydrolyzing%GTP%for%GDP•
Will%have%excessive% signaling•
2.%What%are%the%effectors%of%G/Proteins?
The%G/protein%GS#that%activates%the%enzyme,%adenylyl%cyclase%(also%called%
adenylate%cyclase)%to%make%cAMP
i.
The%G/protein%GQ#that%activates%the%enzyme,%phospholipase%C.%ii.
i.%GTP-bound%GS%activates%Adenylyl)cyclase,%an%enzyme%that%catalyzes%cyclic%AMP%
(cAMP)%formation.%
The%cAMP%2nd%messenger%activates%the%enzyme%Protein%Kinase%A,%involved%in%a%
wide-range%of%biological%processes%including%long-term%memory
Regulates%downstream%targets
○
•
Has%one%phosphate%group%attached%to%the%figure%in%two%places%that%forms%a%ring•
cAMP%is%quickly%degraded%by%Phosphodiesterase#(PDE)
Fast%cAMP%degradation%is%necessary%to%maintain%sensitivity%of%the%cell%to%
extracellular% signal%molecules
Needs%to%be%able%to%turn%off%or%on%signals%to%respond%to%extracellular%
clues
§
○
A%lack%of%cAMP%degradation%results%in%a%failure%of%response%to%further%
extracellular% signals
○
One%of%the%many%physiological%actions%of%caffeine%is%to%act%as%a%
Phosphodiesterase%(PDE)%inhibitor%
Caused%cAMP%to%hang%around%for%a%longer%period%of%time%for%more%
cellular% signaling
§
Viagra%is%also%PDE%inhibitor
§
○
•
Arise%in%intracellular% cAMP%can%activate%gene%transcription
In%the%cytosol,%cAMP%activates%PKA%which%then%moves%into%the%nucleus%and%
phosphorylates%specific%gene%regulatory%proteins.
Protein%kinase%A
A%comes%from%being%activated%from%cAMP□
Adds%phosphate%groups%onto%specific%target%proteins□
§
Protein#kinases phosphorylates%specific%targets%by%adding%a%
phosphate%group
§
○
Once%phosphorylated,%these%proteins%stimulate%the%transcription%of%a%
whole%set%of%target%genes
○
PKA%can%also%phosphorylate%proteins%in%the%cytosol%
Once%activated,%can%activate%other%targets%in%the%cytosol%or%go%inthe%
nucleus%to%phosphorylate%transcription%factors
§
Affects%behavior,%water%absorption,%determining%when%we%
synthesize%steroid%hormones
All%depend%on%when%and%where%it's%turned%on□
§
○
•
5. Cholera%toxin%can%affect%cells%by%inhibiting%the%ability%of%the%G%protein%α%subunit%to%
hydrolyze%GTP%to%GDP%.%On%a%cellular%level,%which%one%of%the%following%would%be%
helpful%in%reducing%the%harmful%effect%of%cholera%toxin?%
Increasing%the%amount%of%intracellular%cAMP.%A.
Inhibiting#the#activity#of#adenylyl#cyclase#in#the#cell.#B.
Adding%ligand%for%the%G-protein%coupled%receptor%in%this%pathway.%C.
Increasing%the%amount%of%protein%kinase%A%activity%in%the%cell.%D.
Inhibiting%the%activities%of%the%β%and%γ%G-protein%subunits.%E.
Cholera%toxin%binds%to%GS%and%locks%it%in%"on"%position,%especially%in%the%epithelia%of%the%
gut
When%you%have%cholera%bacteria,%you%produce%excessive%G%protein%signaling%that%
leads%to%an%excess%of%salt%being%moved%into%the%gut%interior,%causing%water%to%be%
moved%out
•
Without%treatment,%die%of%dehydration•
Too%much%cAMP%is%formed%and%to%much%PKA%is%signaled•
Need%to%downregulate%the%activity%caused%by%the%toxin•
ii)%GTP-bound%GQ%activates%Phospholipase#C#
Phospholipids%can%be%hydrolyzed%into%components%that%act%as%second%
messengers%
•
Activation%of%GQ%-signal%binds%a%receptor%that%causes%the%receptor%to%change%in%
shape%so%the%G%protien%can%bind%to%the%receptor%and%triggers%a%change%from%GDP%
to%GTP%so%alpha%subunit%can%dissociate
•
Phosphatidyl%inositol-bisphosphate%(PIP2)%is%hydrolyzed%into%inositol%
triphosphate%(IP3)%and%diacylglycerol%(DAG).%
PhosC%cuts%PIP2%into%2%parts%
○
Molecules%continue%downstream%signaling
○
Can%open%calcium%channels%in%the%endoplasmic%reticulum%and%release%
calcium%to%the%cytoplasm
ER%is%a%storage%place%for%the%calcium%and%Ca%goes%to%areas%of%lower%
concentration
§
○
•
The%two%parts%each%become%second%messengers,%with%IP3%moving%into%the%
cytoplasm%and%DAG%remaining%in%the%membrane.%
Ca%with%DAG%activate%protein%kinase%C
Activated%by%calcium%and%DAG
§
Then%phosphorylates%targets%to%generate%a%cellular% response
§
○
•
3.%Ion%channel-linked%receptors%convert%hydrophilic%chemical%signals%into%electrical%
signals.%
Span%plasma%membrane•
Have%closed%and%open%state
Closed:%nothing%can%pass%through%the%pore%of%the%receptor
○
When%hydrophilic%signals%bind,%channel%opens%and%particular%ions%can%pass%
the%plasma%membrane
○
•
Ion#channel/linked#receptors#can%open%to%let%certain%ions%(for%e.g.%Na+,%K+,%Ca2
+,%or%Cl,)%in%or%out%of%a%cell% and%are%responsible%for%rapid-transmission%of%signals%
across%synapses%in%the%nervous%system.%
•
Ion%channels%transduce%a%chemical%signal,%in%the%form%of%a%pulse%of%
neurotransmitter%(a%hydrophilic#chemical)signal)%delivered%to%the%outside%of%the%
target%cell,%directly%into%an%electrical)signal,%in%the%form%of%a%change%in%voltage%
across%the%plasma%membrane%of%the%target%cell%
Moving%ions%from%one%compartment%(outside)%to%inside%the%cell,%you%
change%the%difference%in%charge%on%that%plasma%membrane,%changing%its%
voltage
Voltage%changes%are%important%for%electrical%communication%of%cells
§
○
Effect%on%physiology
How%neural%cells%in%the%spinal%cells%communicate
§
Cause%contraction%of%heart%cells%in%unison%to%drive%pressure%for%
circulation
§
Muscle%contraction%at%the%appropriate%time%
§
○
•
Which%of%the%following%statements%is CORRECT about%the%chemical,%
caffeine? Caffeine:
Primarily%inhibits%phospholipase%CA.
Primarily%activates%phosphodiesteraseB.
Primary%inhibits%adenylyl%cyclaseC.
Primarily%activate%adenylyl%cyclaseD.
Primarily#inhibits#phosphodiesteraseE.
Testosterone%mediates%its%effect%by%which%of%the%following%mechanisms
Activates%a%receptor%that%increases%the%permeability%of%the%membrane%cationsA.
Activates%an%intracellular%enzyme%when%combined%with%a%membrane%receptorB.
Activates#genes#by#binding#with#an#intracellular#receptorC.
Activates%the%phosphatidylinositol%systemD.
Cell$Signaling$II$-Hydrophilic$Signals
Wednesday,% April%25,%2018 11:56%AM
1.#When%a%chemical%messenger%(also%called%a%"ligand”%or%a%“signal”)%reaches%a%target%
cell,%it%binds%to%a%receptor.%In%general,%a%hydrophobic%ligand%binds%to%a(n)%__________,%
whereas%a%hydrophilic%ligand%binds%to%a(n)%__________.%
A)#intracellular#receptor;#cell-surface#receptor#
B)%cell-surface%receptor;%intracellular%receptor%
C)%intracellular%receptor;%intracellular%receptor%
D)%cell-surface%receptor;%cell-surface%receptor%
E)%none%of%the%above%answers%are%correct.%
Small/hydrophobic%signals%can%cross%the%plasma%membrane%to%intracellular%receptors
Large%class%of%hydrophilic%signals%are%on%the%cell's%surface
Outline:#Cell#Signaling#
How%cells%communicate:%signal%transduction%and%2nd%messengers.%1.
Small%signaling%molecules%enter%target%cells%and%bind%intracellular%receptors.%
Nitric-oxide%as%a%signal%a.
Steroid%hormonal%cues%b.
2.
Most#signaling#molecules#bind#cell-surface#receptors.
G-protein#linked#receptorsa.
Ion-channel#linked#receptors#b.
Enzyme%linked%receptors%c.
3.
Hydrophobic%steroid%hormones%cross%the%plasma%membrane%of%the%target%cell%
Steroid%hormones%all%pass%through%the%plasma%membrane%of%the%target%cell.%•
In%the%cell,%the%hormones%bind%to,%and%activate%receptors%found%in#the#cytoplasm#
or#nucleus.%
•
Upon%activation,%the%hormone%receptors%are%capable%of%regulating%gene%
transcription.%
•
Learning%Objectives
List%the%series%of%events%required%to%activate%and%inactivate%the%G-protein%of%a%G-
protein%linked%receptor
1.
Compare%the%activation%of%the%enzymes
Adenylyl%cyclasei.
Phospholipase%C%by%the%GTP-bound%α%subunit%of%an%activated%G-proteinii.
2.
Describe%the%function%of%ion%channel>linked%receptors3.
Extracellular% signal%molecules%fall%into%one%of%two%classes%
Hydrophilic%signaling%molecules%bind%cell-surface%receptors.%•
1.%The%G-protein%linked%receptor%family%is%the%largest%family%of%cell-surface%receptors%
Vital%to%physiology
There%are%toxins%that%work%by%modifying%the%activity%of%these%receptors
○
Cancer%can%hijack%this%process
○
•
Several%types%of%G-proteins#exist—each%binds%a%specific%type%of%receptor%and%set%
of%downstream%targets.%All%G-proteins#have%a%similar%structure%and%operate%in%a%
similar%way%
•
The%trimeric#G-proteins#are%composed%of%α,#β,#and#γ subunits#and%are%tethered%
to%the%membrane%inside%the%cell.%
•
Inactive%G-proteins#are%bound#by#GDP#at%the%α%subunit#
Receptor%is%not%bound%to%G-protein%receptor%here
○
Inactive%G%protein%is%bound%to%GDP
○
•
A%hydrophilic#signal#binds%to%the%extracellular%side%of%the%receptor%and%changes%
the%shape%of%the%receptor%on%the%cytoplasmic%side%
G%protein%can%physically%interact%with%receptor%
○
•
G%protein%activation%triggers%a%detachment%of%the%α,#β,#and#γsubunits#from%the%
receptor%and%each%other%to%roam%the%plasma%membrane.%
G%protein%exchanges%GDP%for%GTP
○
Dropping%off%GDP%and%picking%up%a%new%molecule%
○
When%G-protein%is%bound%to%GTP,%α-subunit#and#can#physically#interact#
with#and##activate#downstream#targets
○
•
2.#Trimeric%G)proteins,%that%act%as%molecular%switches%inside%cells:%
A)%are%active%when%cAMP%is%bound.%
B)%are%active%when%GDP%is%bound.%
C)%are%active%when%ATP%is%bound.%
D)%are%active%when%ADP%is%bound.%
E)#are#active#when#GTP#is#bound.#
The%effector%protein%may%catalyze%many%reactions,%amplifying%the%signal.%
The%activated#αand#βγ protein#subunits#can%both%interact%directly%with%target%
proteins%in%the%plasma%membrane%that%can%relay%the%signal%to%other%
destinations.
•
The%α and#βγ complex#proteins%remain%dissociated)and)active)until%the%α%
subunit#hydrolyses#the%bound#GTP#to%GDP.#
Needs%to%be%able%to%be%turned%on%or%off%to%have%effect
If%it's%a%cell%related%to%growth,%will%form%a%tumor
§
○
G-protein%turned%on%by%GDP%-->%GTP
○
Once%subunit%binds%to%GTP,%a%little%timer%turns%on%and%then%the%a%subunit%
will%hydrolyze%GTP%for%GDP
○
When%the%a%subunit%is%bound%to%GDP,%it%re-associates%with%βγ
○
•
GTP#hydrolysis#causes%the%α%subunit#to%re-associate%with%the%βγ subunits#and%
the%signal%is%shut#off.%
•
3.#If%a%G%protein%were%unable%to%release%its%bound%nucleotide%but%could%hydrolyze%it,%
signal%transduction%would:
A.#not#move#beyond#this#point.
B.%be%continuous%beyond%this%point.%
C.%be%unaffected.
D.%be%constantly%switching%on%and%off.%
E.%be%unpredictable.%
Activation%of%a%G%protein%involves%trading%GDP%for%GTP%to%turn%it%on
Here%is%a%G%protein%that%can%hydrolyze,%but%can't%release%the%nucleotides•
If%you%can't%release%them,%you%can't%trade%them%and%can't%be%turned%on•
Signal%transduction%pathway%with%a%roadblock•
4.#If%a%G%protein%could%release%its%bound%nucleotide%but%was%not%able%to%hydrolyze%it,%
signal%transduction%would:
A.%not%move%beyond%this%point.
B.#be#continuous#beyond#this#point.#
C.%be%unaffected.
D.%be%constantly%switching%on%and%off.%
E.%be%unpredictable.%
Mutation%effects%ability%to%hydrolyze
Hydrolysis%inactivates%G%protein•
G%protein%bound%to%GTP%is%turned%off%by%hydrolyzing%GTP%for%GDP•
Will%have%excessive% signaling•
2.%What%are%the%effectors%of%G/Proteins?
The%G/protein%GS#that%activates%the%enzyme,%adenylyl%cyclase%(also%called%
adenylate%cyclase)%to%make%cAMP
i.
The%G/protein%GQ#that%activates%the%enzyme,%phospholipase%C.%ii.
i.%GTP-bound%GS%activates%Adenylyl)cyclase,%an%enzyme%that%catalyzes%cyclic%AMP%
(cAMP)%formation.%
The%cAMP%2nd%messenger%activates%the%enzyme%Protein%Kinase%A,%involved%in%a%
wide-range%of%biological%processes%including%long-term%memory
Regulates%downstream%targets
○
•
Has%one%phosphate%group%attached%to%the%figure%in%two%places%that%forms%a%ring•
cAMP%is%quickly%degraded%by%Phosphodiesterase#(PDE)
Fast%cAMP%degradation%is%necessary%to%maintain%sensitivity%of%the%cell%to%
extracellular% signal%molecules
Needs%to%be%able%to%turn%off%or%on%signals%to%respond%to%extracellular%
clues
§
○
A%lack%of%cAMP%degradation%results%in%a%failure%of%response%to%further%
extracellular% signals
○
One%of%the%many%physiological%actions%of%caffeine%is%to%act%as%a%
Phosphodiesterase%(PDE)%inhibitor%
Caused%cAMP%to%hang%around%for%a%longer%period%of%time%for%more%
cellular% signaling
§
Viagra%is%also%PDE%inhibitor
§
○
•
Arise%in%intracellular% cAMP%can%activate%gene%transcription
In%the%cytosol,%cAMP%activates%PKA%which%then%moves%into%the%nucleus%and%
phosphorylates%specific%gene%regulatory%proteins.
Protein%kinase%A
A%comes%from%being%activated%from%cAMP□
Adds%phosphate%groups%onto%specific%target%proteins□
§
Protein#kinases phosphorylates%specific%targets%by%adding%a%
phosphate%group
§
○
Once%phosphorylated,%these%proteins%stimulate%the%transcription%of%a%
whole%set%of%target%genes
○
PKA%can%also%phosphorylate%proteins%in%the%cytosol%
Once%activated,%can%activate%other%targets%in%the%cytosol%or%go%inthe%
nucleus%to%phosphorylate%transcription%factors
§
Affects%behavior,%water%absorption,%determining%when%we%
synthesize%steroid%hormones
All%depend%on%when%and%where%it's%turned%on□
§
○
•
5. Cholera%toxin%can%affect%cells%by%inhibiting%the%ability%of%the%G%protein%α%subunit%to%
hydrolyze%GTP%to%GDP%.%On%a%cellular%level,%which%one%of%the%following%would%be%
helpful%in%reducing%the%harmful%effect%of%cholera%toxin?%
Increasing%the%amount%of%intracellular%cAMP.%A.
Inhibiting#the#activity#of#adenylyl#cyclase#in#the#cell.#B.
Adding%ligand%for%the%G-protein%coupled%receptor%in%this%pathway.%C.
Increasing%the%amount%of%protein%kinase%A%activity%in%the%cell.%D.
Inhibiting%the%activities%of%the%β%and%γ%G-protein%subunits.%E.
Cholera%toxin%binds%to%GS%and%locks%it%in%"on"%position,%especially%in%the%epithelia%of%the%
gut
When%you%have%cholera%bacteria,%you%produce%excessive%G%protein%signaling%that%
leads%to%an%excess%of%salt%being%moved%into%the%gut%interior,%causing%water%to%be%
moved%out
•
Without%treatment,%die%of%dehydration•
Too%much%cAMP%is%formed%and%to%much%PKA%is%signaled•
Need%to%downregulate%the%activity%caused%by%the%toxin•
ii)%GTP-bound%GQ%activates%Phospholipase#C#
Phospholipids%can%be%hydrolyzed%into%components%that%act%as%second%
messengers%
•
Activation%of%GQ%-signal%binds%a%receptor%that%causes%the%receptor%to%change%in%
shape%so%the%G%protien%can%bind%to%the%receptor%and%triggers%a%change%from%GDP%
to%GTP%so%alpha%subunit%can%dissociate
•
Phosphatidyl%inositol-bisphosphate%(PIP2)%is%hydrolyzed%into%inositol%
triphosphate%(IP3)%and%diacylglycerol%(DAG).%
PhosC%cuts%PIP2%into%2%parts%
○
Molecules%continue%downstream%signaling
○
Can%open%calcium%channels%in%the%endoplasmic%reticulum%and%release%
calcium%to%the%cytoplasm
ER%is%a%storage%place%for%the%calcium%and%Ca%goes%to%areas%of%lower%
concentration
§
○
•
The%two%parts%each%become%second%messengers,%with%IP3%moving%into%the%
cytoplasm%and%DAG%remaining%in%the%membrane.%
Ca%with%DAG%activate%protein%kinase%C
Activated%by%calcium%and%DAG
§
Then%phosphorylates%targets%to%generate%a%cellular% response
§
○
•
3.%Ion%channel-linked%receptors%convert%hydrophilic%chemical%signals%into%electrical%
signals.%
Span%plasma%membrane•
Have%closed%and%open%state
Closed:%nothing%can%pass%through%the%pore%of%the%receptor
○
When%hydrophilic%signals%bind,%channel%opens%and%particular%ions%can%pass%
the%plasma%membrane
○
•
Ion#channel/linked#receptors#can%open%to%let%certain%ions%(for%e.g.%Na+,%K+,%Ca2
+,%or%Cl,)%in%or%out%of%a%cell% and%are%responsible%for%rapid-transmission%of%signals%
across%synapses%in%the%nervous%system.%
•
Ion%channels%transduce%a%chemical%signal,%in%the%form%of%a%pulse%of%
neurotransmitter%(a%hydrophilic#chemical)signal)%delivered%to%the%outside%of%the%
target%cell,%directly%into%an%electrical)signal,%in%the%form%of%a%change%in%voltage%
across%the%plasma%membrane%of%the%target%cell%
Moving%ions%from%one%compartment%(outside)%to%inside%the%cell,%you%
change%the%difference%in%charge%on%that%plasma%membrane,%changing%its%
voltage
Voltage%changes%are%important%for%electrical%communication%of%cells
§
○
Effect%on%physiology
How%neural%cells%in%the%spinal%cells%communicate
§
Cause%contraction%of%heart%cells%in%unison%to%drive%pressure%for%
circulation
§
Muscle%contraction%at%the%appropriate%time%
§
○
•
Which%of%the%following%statements%is CORRECT about%the%chemical,%
caffeine? Caffeine:
Primarily%inhibits%phospholipase%CA.
Primarily%activates%phosphodiesteraseB.
Primary%inhibits%adenylyl%cyclaseC.
Primarily%activate%adenylyl%cyclaseD.
Primarily#inhibits#phosphodiesteraseE.
Testosterone%mediates%its%effect%by%which%of%the%following%mechanisms
Activates%a%receptor%that%increases%the%permeability%of%the%membrane%cationsA.
Activates%an%intracellular%enzyme%when%combined%with%a%membrane%receptorB.
Activates#genes#by#binding#with#an#intracellular#receptorC.
Activates%the%phosphatidylinositol%systemD.
Cell$Signaling$II$-Hydrophilic$Signals
Wednesday,% April%25,%2018 11:56%AM
