Chapter 15: Cell signaling and signal transduction Flashcards
in biology, extracellular messengers are also called ________
ligands
3 types of signaling
- autocrine
- paracrine
- endocrine
Autocrine vs paracrine vs endocrine signaling
- Autocrine Signaling: the cell has RECEPTORS on its surface that respond to the messenger. Ligand secreted by receptor cell itself.
- Paracrine Signaling: Messenger molecules travel SHORT DISTANCES through extracellular space.
- Endocrine Signaling: messenger molecules reach their target through the BLOODSTREAM.
What is signal transduction?
it is a process by which a cell converts one kind of signal or stimulus to another.
most processes in signal transduction involve ordered sequences of reactions inside the cell, which are carried out by ______, activated by _________, resulting in a SIGNAL TRANSDUCTION PATHWAY.
most processes in signal transduction involve ordered sequences of reactions inside the cell, which are carried out by ENZYMES, activated by SECOND MESSENGERS, resulting in a SIGNAL TRANSDUCTION PATHWAY.
What is the most common method of activating/deactivating enzymes?
via phosphorylation/dephosphorylation
function of protein kinases
phosphorylate a protein
Function of protein phosphatases
remove an Pi from a protein
What is a second messenger?
- Second messengers are small substances that activate or inactivate specific proteins.
- generated by the transmittance of a signal from a receptor on the cytoplasmic domain to a nearby enzyme
cell surface receptors generate an intracellular SECOND MESSENGER through an enzyme called an ________
EFFECTOR
2 pathways to activate a target protein via signal pathways
- signaling pathway activated by a DIFFUSABLE SECOND MESSENGER
- signaling pathway activated by RECRUITMENT of PROTEINS to the plasma membrane.
Describe the signal transduction pathway that starts with Protein kinase 1 and ends in the activation of a transcription factor.
What enzyme reverses activation steps?
- PROTEIN KINASE 1 activates PROTEIN KINASE 2 via PHOSPHORYLATION.
- PROTEIN KINASE 2 activates PROTEIN KINASE 3 via PHOSPHORYLATION.
- PROTEIN KINASE 3 activates a TRANSCRIPTION FACTOR via PHOSPHORYLATION, increasing its affinity for a site on DNA.
- Binding of a transcription factor to the DNA affects the transcription of the gene in question.
NOTE: each activation step in this pathway can be reversed by a PHOSPHATASE
EXTRACELLULAR messengers include:
- small molecules such as amino acids and their derivatives.
- Gases such as NO and CO.
- Steroids.
- Eicosanoids
- peptides and proteins
what are eicosanoids?
lipids derived from fatty acids
Receptor types include:
- GPCRs (G-protein coupled receptors)
- RTKs (receptor protein-tyrosine kinases)
- ligand-gated channels
- steroid hormone receptors
- specific receptors (ex. B- and T-cell receptors)
How many transmembrane domains do GPCRs have?
seven
3 amino acids that can undergo phosphorylation
Ser, Thr, Tyr
Where are GPCRs located?
embedded in the plasma membrane
What is the G protein called which has 3 subunits? what are the subunits called?
HETEROTRIMERIC G PROTEINS.
-alpha, beta, and gamma subunits
Which subunit of the heterotrimeric G protein interacts with GTP/GDP?
alpha subunit
When is a heterotrimeric G protein recruited by a receptor?
When the receptor changes conformation due to its binding to a ligand.
Mechanism of receptor-mediated activation of effectors by means of heterotrimeric G proteins
Steps:
- LIGAND BINDS TO RECEPTOR, altering its conformation and INCREASING ITS AFFINITY FOR THE G PROTEIN to which it binds.
- Upon binding of the heterotrimeric G protein to the receptor, the α subunit RELEASES ITS GDP , which is then REPLACED BY GTP.
- The Gα subunit then DISSOCIATES FROM THE Gβγ complex and BINDS TO AN EFFECTOR (in this case, ADENYLYL CYCLASE), activating the effector.
- activation of adenylyl cyclase PRODUCED cAMP.
- The GTPase activity of the Gα hydrolyses the bound GTP, DEACTIVATING Gα.
- Gα associates with Gβγ, REFORMING THE TRIMERIC G PROTEIN and the effector ceases its activity.
- Receptor is phosphorylated by a GRK.
- phosphorylated receptor is bound to an ARRESTIN molecule, which inhibits the ligand-bound receptor from activating additional G proteins
In the mechanism of receptor-mediated activation of effectors by means of heterotrimeric G proteins, what is the SECOND MESSENGER?
cAMP
In the mechanism of receptor-mediated activation of effectors by means of heterotrimeric G proteins, what is the EFFECTOR?
adenylyl cyclase
T/F: Gβγ acts as GTPase
False. Gα functions as a GTPase
3 fates of an internalized GPCR
- travel along endocytotic pathway into endosomea
- degraded in lysosomes
- dephosphorylated and returned to plasma membrane. Recycled for further use (resensitized).
How is the GPCR protein response terminated?
- by DESENSITIZATION: blocking active receptors from turning on additional G proteins.
- proteins called ARRESTINS compete with G proteins to bind GPCRs
- termination of response is accelerated by RGSs (regulators of G protein signaling)
How are GPCRs internalized?
ARRESTIN-bound GPCRs are internalized when they are trapped in CLATHRIN-COATED pits which bud into the cytoplasm.
Effect of toxins such as cholera on G proteins
bacterial toxins such as cholera target G proteins and PERMANENTYL ACTIVATE them. If the Gα protein is always on, CELLS WON’T STOP SECRETING MUCUS.
What are PIs?
“phosphoinositides”: derivatives of phosphotidylinositol.
-Second messengers
role of a phospholipase
lipid-splitting enzyme
role of phospholipid kinase
lipid-phosphorylating enzyme
role of phospholipid phosphatases
lipid-dephosphorylating enzymes
When the neurotransmitter _________ binds to the smooth muscle cell, the cell is stimulated to contract
acetylcholine
What does PI4K stand for? what does it do?
- PI4K stands for “PI 4-kinase”
- it phosphorylates PI and generates PIP1 (or PI-4-P)
What is the role of PIP5K
“PIP 5-kinase” phosphorylates PIP1 to generate PIP2 (PI-4,5-bP)
Role of PI3K
“PI 3-kinase” phosphorylates PIP2 to generate PIP3
PH domain
lipid-binding domains formed by PHOSPHORYLATED PHOSPHOINOSITIDES
role of PI-PCLβ
“PI-specific phospholipase C-β” catalyses the reaction in which PIP2 os split into DAG and IP3
What activates the PI-PCLβ enzyme?
when a stimulus is received by a receptor, the ligand-bound heterotrimeric G protein activates the enzyme PI-PCLβ
What does DAG stand for and what does it do?
“Diacylglycerol” is derived from the splitting of PIP2 by PI-PCLβ and it RECRUITS PROTEIN KINASE PKC to the membrane and activates the enzyme.
PKC phosphorylates ser and thr residues on target molecules.
What does IP3 stand for and what does it do?
“inositol 1,4,5-triosphosphate” diffuses into the cytosol where it binds to an IP3 receptor and Ca2+ receptor channel in the membrane of the SER, causing the RELEASE OF Ca2+ IONS INTO THE CYTOSOL
General rule:
Enzymes in ANABOLIC processes are activated via ___________ and enzymes in CATABOLIC processes are activated via _________
General rule:
Enzymes in ANABOLIC processes are activated via DEPHOSPHORYLATION and enzymes in CATABOLIC processes are activated via PHOSPHORYLATION
T/F: glucagon and epinephrine bind to the same receptor on the cell.
FALSE. Epinephrine and glucagon bind to DIFFERENT RECEPTORS on the SAME CELL.
Epinephrine and glucagon stimulate glycogen _____ and inhibit its ______
Epinephrine and glucagon stimulate glycogen BREAKDOWN and inhibit its SYNTHESIS
Glycogen breakdown occurs via __________. Show reaction
glycogen phosphorylase mutase
Glycogen ————————————> G1P G6P
Pi^
Glycogen synthesis occurs via ___________. Show reaction
UTP\ /PPi glycogen synthase
G1P—————–>UDP-glc—————————>glycogen
2 fates of G6P produced by breakdown of glycogen
- converted to F6P and continues into glycolysis
2. converted into glc + Pi and transferred into the blood
cAMP is synthesized from ATP by _____________
adenylyl cyclase
What breaks down cAMP into AMP?
phosphodiesterase
Steps of liver cell response to glucagon or epinephrine
- glucagon/epinephrine binds to receptor, activating a Gα subunit, which ACTIVATES AN ADENYLYL CYCLASE EFFECTOR.
- The activated enzyme catalyses the FORMATION OF cAMP.
- cAMP molecules diffuse into the cytosol where they BIND TO PKA, activating it.
- PKA PHOSPHORYLATES/ACTIVATES PHOSPHORYLASE KINASE
- phosphorylase kinase PHOSPHORYLATES/ACTIVATES GLYCOGEN PHOSPHORYLASE
- glycogen phosphorylase catalyses the formation of G1P molecules
- G1P converted to glc and dumped into blood.
3 fates of active PKA:
- phosphorylates/inactivates GLYCOGEN SYNTHASE
- phosphorylates/activates PHOSPHORYLASE KINASE
- can enter the nucleus and phosphorylate CREB.
What is CREB?
When phosphorylated (by PKA), CREB-P is a TRANSCRIPTION FACTOR, which promotes the synthesis of anabolic enzymes required to SYNTHESIZE GLUCOSE from smaller precursors.
effect of cAMP in:
- the liver
- fat cells
- smooth muscle cells
- liver: breakdown of glycogen
- fat: breakdown of triacylglycerols
- smooth muscle: muscle relaxations
T/F: depending on the different AKAP roteins, PKA has a different response
true
Name of the GPCR photosensitive protein responsible for black and white vision
rhodopsin
What kind of receptors are odorant receptors and receptors for bitter and sweet tastes?
GCPRs
cGMP (another second messenger) becomes linear via ________. Becomes GMP.
PDE (phosphodiesterase)
What phosphorylate tyrosine residues on target proteins?
protein-tyrosine kinases
What do protein-tyrosine kinases regulate?
cell growth, division, differentiation, survival and migration
RTK stands for
receptor protein-tyrosine kinase
What causes the dimerization of RTK molecules?
binding of ligand to receptor
2 pathways of RTK dimerization
- ligand-mediated dimerization
2. receptor-mediated dimerization
Describe ligand-mediated dimerization and activation of RTKs
- in its nonactive state, the receptors are present in the membrane as monomers.
- binding of a ligand leads to DIMERIZATION of the receptor and activation of its KINASE activity.
- One receptor adds phosphate groups to the cytoplasmic domain of the other receptor subunit.
- the newly formed phosphotyrosine residues of the receptors serve as binding sites for target proteins containing either SH2 or PTB domains.
- Target protein becomes activated as a result of interactions with the receptor.
Describe receptor-mediated dimerization and activation of RTKs
- a separate ligand molecule binds to each of the inactive receptor monomers.
- Binding of each ligand causes a conf. change in the receptor that creates a DIMERIZATION INTERFACE.
- ligand-bound monomers interact through interface and become an active dimer (able to activate SH2 or PTB domains)
describe trans autophosphorylation
process by which one RTK phosphorylates the other in a dimer to activate it
groups of signaling proteins that can interact with activated RTKs
ALL CONTAIN EITHER SH2 OR PTB
- adaptor proteins
- docking proteins
- transcription factors
- enzymes
what do adaptor proteins do?
bind other proteins
what do docking proteins do?
supply receptors with other tyrosine phosphorylation sites
describe Grb2 protein function
- Grb2 are ADAPTOR PROTEINS
- they function as a LINK between other proteins, such as the growth factor RTK and Sos. Sos activates the downstream protein named Ras.
Describe the function of IRS
“insulin receptor substrate” is both a DOCKING and an ADAPTOR protein.
- able to bind to receptor because it contains PTB domain.
- once bound, tyrosine residues on the protein are phosphorylated by the receptor and act as binding sites for other signaling proteins.
STAT family transcription factor interaction with RTK
binding of a transcription factor to RTK leads to its phosphorylation and and activation.
-active STAT translocates to the nucleus
Ras (a G protein) is active when bound to ____ and inactive when bound to ___
Ras is ACTIVE when bound to GTP and INACTIVE when bound to GDP
3 main proteins that interact with G proteins
- GEF: guanine nucleotide exchange factor
- GAP: GTPase activating protein
- GDI: guanine nucleotide dissociation inhibitor
What happens when a G protein interacts with GEF?
- its activity INCREASES
- GDP is EXCHANGED (not phosphorylated) with GTP, rendering the molecule active.
What happens when a G-protein interacts with GDI?
-the protein can interact with GEF
-inhibits release of GDP
(If GDP is bound to protein, it is able to interact with GEF and be exchanged for GTP)
What happens when a G-protein interacts with GAP?
GTP is hydrolyzed to GDP
The Ras-MAP kinase cascade is a cascade of enzymes which leads to the activation of ______________
transcription factors
What is Ras?
Ras is a small GTPase that is anchored to the inner surface of the plasma membrane by covalently attached lipid groups
- single subunit
- Ras-GTP = active
- ras-GTP binds and activates DOWNSTREAM SIGNALING PROTEINS
Generalized MAP kinase cascade steps
- binding of growth factor to its receptor leads to AUTOPHOSPHORYLATION of the tyr residues on the receptor and subsequent recruitment of Grb2-Sos proteins.
- This complex causes the GTP-GDP exchange of Ras
- Ras-GTP phosphorylates/activates Raf protein (MAPKKK)
- Phosphorylated Raf protein phosphorylates MEK (MAPKK)
- MEK phosphorylates ERK (MAPK)
- active MAPK translocates into the nucleus where it phosphorylates transccription factors such as Elk-1, INCREASING THEIR AFFINITY FOR REGULATORY SITES ON THE DNA, LEADING TO AN INCREASE IN TRANSCRIPTION OF SPECIFIC GENES.
Insulin regulates blood glucose levels by _______ cellular uptake of glucose
increasing
what kind of protein is an insulin receptor?
a protein-tyrosine kinase
IRS bind proteins with ___ domains
SH2
SH2 domain-containing proteins are kinases that phosphorylate a lipid, _____
PI3K.
PI3K in turn converts PIP2 into PIP3
How many alpha and beta subunits are present in an insulin receptor?
2 alpha and 2 beta subunits (tetramer)
What does the binding of an insulin molecule to the alpha subunits of an insulin receptor do?
changes the conformation of the beta subunits, which activates the tyrosine-kinase activity of the B subunits.
The activates B subunits phosphorylate tyrosine residues in the cytoplasmic domain of the receptor and on several IRSs
IRS-1 has a ___ domain
PTB
PI3K converts ____ into ______
PIP2 to PIP3
____ binds to PIP3 and undergoes conf. change
AKT
The two sites of phosphorylation exposed on AKT upon conf. change due to binding with PIP3:
T308 and S473
T308 is phosphorylated via ____
PDK1 (kinase)
What phosphorylates S473 of AKT?
mTORC2
Cellular responses as a result of complete activation of AKT?
- glycogen metabolism
- cell proliferation
- apoptosis
Does insulin activate or deactivate glycogen synthase?
activates it to DECREASE BLOOD GLUCOSE concentrations
INTRACELLULAR Ca2+ concentration can increase due to:
- voltage-gated Ca2+ channel due to nerve impulse
- Ca2+ exchange channel proteins. Exchange for Na+ ions
- Receptor proteins in SER
How does PKB decrease blood glucose levels?
by recruiting vesicle-boung GLUT4 transporters to the plasma membrane. GLUT4 transporters transport glucose out of blood.
T/F: calcium levels are low in the cytosol
true, because it is pumped out into the extracellular side and the membrane is highly impermeable to Ca2+ ions
PI–>PIP–>PIP2–>PIP3 enzymes
- PI–>PIP (PIP4-kinase)
- PIP–>PIP2 (PIP5-kinase)
- PIP2–>PIP3 (PIP3-kinase)
What is SOCE?
“store-operated calcium entry”
-during SOCE, depleted calcium levels trigger a response that leads to OPENING OF CALCIUM CHANNELS
The mechanism responsible for SOCE is a signaling system between the _______ and _______
ER and plasma membrane
Converge vs diverge vs crosstalk
- Signals from UNRELATED receptors can CONVERGE to activate a COMMON effector.
- IDENTICAL signals can DIVERGE to activate a variety of effectors
- Signals can be passed BACK AND FORTH between pathways as a result of CROSSTALK.
Example of convergence
Different ligands bind to RTK and GPCR, but both can activte phospholipases. PLC beta (in GPCRs) and PLC gamma (in RTKs) both break down PIP2 into DAG and IP3 (calcium channel opener)
Example of divergence
One receptor activates 3 different pathways:
RTK can activate either PLC gamma, PI3K, or GAP via IRS reruitment
Example of crosstalk
Ca2+ ions can inhibit PKC activity or regulate activities such as mitosis. ie. one pathway interacts with another
What enzyme produces Nitric oxide?
nitric oxide synthase
How does NO affect cGMP?
NO stimulates guanylyl cyclase, which generates cGMP
cGMP decreases _________ and ______ smooth muscle
cGMP decreases CYTOSOLIC CALCIUM and RELAXES smooth musce
Describe the mechanism of male arousal
- Ach binds to receptor
- rise in cytosolic Ca2+ concentration
- Ca2+ ACTIVATES NO SYNTHASE
- NO diffuses into plasma membrane into smooth muscle cells
- NO stimulates guanylyl cyclase
- cGMP binds to cGMP-dependent kinase which results in muscle relaxation/dilation of blood vessels
How does viagra work?
viagra inhibits the breakdown of cGMP via phosphodiesterase
What type of cells produce NO?
endothelial cells that line the inside of blood vessels
Define apoptosis
the ordered process involving cell shrinkage, loss of adhesion to other cells, dissection of chromatin, and engulfment by phagocytosis
Effect of caspase targeting protein kinases during apoptosis
inactivation of FAK (focal adhesion kinase), disrupting cell adhesions.
0-0-0-0-0-0
*dashes holding above cells together are FAKs
Effect of caspase targeting lamins during apoptosis
dissasembly of nuclear lamina
Effect of caspase targeting cytoskeleton proteins
change in cell shape due to effects on intermediate filaments, actin, and tubulin
Effect of caspase targeting endonuclease
endonuclease is activated and it CLEAVES DNA
2 pathways of apoptosis
extrinsic and intrinsic
Steps of extrinsic pathway of apoptosis
- tumor necrosis factor binds to TNF receptor
- TNF receptor binds to FAD and TRADD adaptor proteins and procaspase 8
- Caspase 8 is activated (initiator caspase)
- Caspase 8 activates downstream executioner caspase enzymes
What are caspases?
peptidases tat bind to and break down proteins
Steps of intrinsic pathway of apoptosis
- In reponse to cellular damage, Bcl-2 type proteins increase mitochondrial membrane permeability
- Cytochrome c is released from mito into cytosol
- Cytc associates with Apaf-1 and procaspase 9, forming an APOPTOSOME.
- caspase 9 activates downstream executioner caspases
What kind of G protein is Sos? What does it do?
Sos is a GEF. It exchanged the GDP on Ras to GTP
T/F: Ras is a G protein
True
AKT also known as
PKB
Activation of PI3K/PKB pathway leads to an increased likelihood of what?
that the cell will survive a stimulus which would usually lead to its destruction
What binds to AKT to change its conformation and expose T308 and S473 domains?
PIP3
