Signaling: Call Tracing Flashcards
Lecture 22
1
Q
What is signal transduction?
A
ability of a cell to translate a receptor-ligand interaction to changes in its behavior or gene expression
2
Q
What happens if there is an absence of a signal?
A
Can indicate time of cell death
3
Q
What are the 6 classes of cell signals?
A
- Peptide hormones
- Steroid hormones
- Growth factors
- Cytokines
- Eicosanoids
- Neurotransmitters
4
Q
What is the difference between growth factors and cytokines?
A
Growth factors signal when it is time for a cell to grow. Cytokines work similarly, but signal when it is time for a cell to proliferate.
5
Q
What do cytokines do?
A
signal when it’s time for a cell to proliferate (divide, multiply, and populate)
6
Q
What are eicosanoids?
A
cell signals; polyunsaturated fatty acids (20 carbons in length)
7
Q
What do neurotransmitters do?
A
move signals through a chemical synapse, from one neuron to another
8
Q
What are the four broad signaling classes?
A
- Paracrine
- Autocrine
- Endocrine
- Juxtacrine
9
Q
What is paracrine signaling?
A
A cell releases a molecule into the external environment via exocytosis; a close, adjacent cell receives the signal on its surface; ex. neurotransmitters
10
Q
What is juxtacrine signaling?
A
signaling where cells are in physical contact with each other at the plasma membrane; one cell presents a signaling molecule on its surface and the other cell binds directly with receptors for the signaling molecule
11
Q
What is an examine of juxtacrine signaling?
A
immune cell recognition
12
Q
What is an example of paracrine signaling?
A
neurotransmitters
13
Q
What is autocrine signaling?
A
a single cell signals to itself; exocytoses a molecule that binds to receptors on the surface of the same cell
14
Q
What is endocrine signaling?
A
hormones are dropped into the bloodstream, travel great distances from the source of the signal
15
Q
What are the 2 main mechanisms for how signals get into cells? Which is more common? When is the less commonly used one used?
A
- Cell-surface receptors on the plasma membrane (common)
- Intracellular receptor (used when the signaling molecule is hydrophilic)
16
Q
Why is the intracellular receptor route used when the signaling molecule is hydrophilic?
A
Because hydrophilic molecules don’t cross the plasma membrane (easily)
17
Q
Why can hormones bind intracellular receptors to regulate transcription?
A
Because they are hydrophobic and able to pass through the plasma membrane
18
Q
Describe the mechanism(s) for the flow of information during signal transduction utilizing cell surface receptors.
A
- A ligand/primary messenger binds to a receptor.
- Step 1 causes a conformational change in the structure of the receptor, triggering downstream events, producing secondary messengers.
3a. Signals interact with molecules in the cytoplasm that provoke an immediate cellular response.
3b. Signals enter the nucleus and change gene expression (activate or inactivate transcription).
19
Q
How does intracellular signaling work?
A
Receptor is soluble and intracellular. The signaling molecule-intracellular receptor complex enters the nucleus and binds to a specific region of DNA to affect the transcription of a gene.
20
Q
What are the two possible outcomes for the flow of information using cell surface receptors?
A
- Immediate cellular response in the cytosol
- Changes in gene expression in the nucleus by influencing transcription
21
Q
What are the 3 different ways signals can be integrated?
A
- One receptor activates multiple pathways
- Different receptors activate the same pathway
- Different receptors activate different pathways; one pathway affects the other
22
Q
Is altering gene expression or protein function with signaling faster?
A
Altering protein function is much faster.
23
Q
What are the 3 classes of receptors on the cell surface that facilitate signal transduction?
A
- ion-channel linked receptor (ligand-gated channels)
- G-protein linked receptor
- enzyme-linked receptor
24
Q
How do ligand-gated channels work?
A
Channels are activated when a ligand binds to the channel proteins (ex. neurotransmitters)
25
How many different G-protein linked receptors does the human genome code for?
2000
26
What's the difference between G-protein linked receptors and other GTPase families?
G-protein linked receptors are heterotrimeric GTPases. They have 3 subunits that differ from one another. In contrast, the other G proteins are monomeric.
27
How are heterotrimeric G proteins activated?
activated when the heterotrimeric G-protein binds a G-protein coupled/linked receptor; the alpha subunit releases GDP and acquires GTP, separating from the beta-gamma complex
28
Describe the structure of G-protein linked receptors.
Large polypeptides
7 transmembrane alpha-helices
N terminus is exposed to extracellular fluid, C-terminus is in the cytosol
Extracellular portion has unique messenger-binding site
29
What does it mean when GDP is bound to the heterotrimeric G protein?
All 3 subunits are held together and the G protein is inactive.
30
Where does the nucleotide bind in the heterotrimeric G protein?
the alpha subunit
31
What causes the heterotrimeric G protein to bind to the receptor?
A signal molecule arrives at the receptor, causing the G protein to bind at a different site.
32
What happens to the heterotrimeric G protein after the signal molecule binds to the receptor?
GDP will come off the alpha subunit and will be replaced by GTP. The hterotrimeric G protein dissociates from the receptor and the beta-gamma complex of the two subunits separate from the activated GTP-bound alpha receptor
33
What happens when heterotrimeric GTPases hydrolyze GTP?
The three subunits come together and the cycle restarts.
34
How are secondary messengers created with G-linked protein receptors?
Activated alpha subunit binds and activates specific enzymes to produce specific second messengers.
35
What are 3 types of secondary messengers?
1. cyclic mononucleotides (cAMP, cGMP)
2. phosphoinositides (IP3)
3. diacylglycerol
36
What protein catalyzes cAMP formation?
adenylyl cyclase
37
What does adenylyl cyclase do?
removes two phosphates, catalyzing cAMP formation from Atp
38
What does phospholipase C do?
cleaves PIP2 to yield IP3 and diacylglycerol (DAG)
39
When does adenylyl cyclase work?
when the alpha subunit of the heterotrimeric G-protein is GTP bound
40
When does phoshpodiesterase function in a cell?
when cells want to reduce the amount of signaling they receive
41
What does phosphodiesterase do?
hydrolyzes cAMP to AMP
42
Which subunit of the heterotrimeric G protein activates phospholipase C?
alpha subunit
43
What happens to diacylglycerol after it is produced?
The second messenger remains associated with the plasma membrane due to its two acyl chains. It can activate the protein kinase C.
44
What happens to the IP3 after it is produced?
The second messenger is released into the cytosol.
45
What do second messengers do?
relay signals from one location in the cell to another, causing a cascade of changes within the receiving cell
46
How do second messengers interact with protein kinases?
If a signal molecule binds with an activated receptor and adenylyl cyclase is activated, along with the alpha subunit of the heterotrimeric G protein, cyclic AMP will bind with protein kinases's binding site, resulting in the phosphorylation of cytosolic proteins.
47
What happens to IP3?
serves as a second messenger, binds to the calcium channel on the membrane of the endoplasmic reticulum, opens a ligand-gated channel
48
How are calcium ions released from the ER into the cytosol?
IP3 acts as a second messenger and binds to the ligand-gated channel, opening it.
49
Describe 3 possible ways enzyme-linked receptors can work.
On the plasma membrane, receptors are activated by the binding of a ligand to the receptor.
1. An inactive catalytic domain can become active after the dimerization and binding of a signal molecule to 2 different receptors simultaneously.
2. A single ligand could activate the whole receptor.
3. 2 individual ligands activate 2 receptors, which come together in the plasma membrane and are activated.
50
What kind of receptors (within signal transduction) are receptor tyrosine kinases?
enzyme-linked receptor
51
What is the mechanism tyrosine kinases use to transduce a signal?
1. A ligand binds to the receptors after dimerizing.
2. The receptors cluster in the plasma membrane.
3. Step 2 triggers the receptors to autophosphorylate their cytoplasm tails.
4. Step 3 changes the topology of the cytoplasm tail, which can now bind to a signaling complex (variety of proteins).
5. Ras (a monomeric G protein) is activated by the GEF.
6. It triggers a cascade of phosphorylation on threonines and serine residues, starting with the MEK protein kinase.
7. MEK phosphorylates threonines and tyrosine residues in MAP kinases (mitogen-activated protein kinases).
8. MAPKs phosphorylate transcription factors that regulate gene expression (including Jun and Ets).
9. The transcription factors then regulate the expression of genes whose protein products are needed for cells to grow and divide.
10. Ras is inactivated by hydrolysis via GAP.
52
What is useful about the adaptor protein in the receptor tyrosine kinase mechanism for transduction?
It can recognize phosphorylated amino acids (especially tyrosine kinases) and bind.
53
What activates MAPKs?
activated when cells receive stimulus to grow and divide
54
What do MAPKs do?
phosphorylate transcription factors that regulate gene expression (ex. Jun and Ets family); those proteins then regulate the expression of genes needed for growth and division
55
What do the domains of the adaptor protein within the receptor tyrosine kinase pathway recognize and bind to?
SH2 domain binds to the phosphorylated tyrosine.
SH3 domain binds to the polyproline residues in the GEF.
56
What does the SH2 domain of the adaptor protein do?
It recognizes phosphorylated tyrosine and binds to it within the receptor tyrosine kinase pathway.
57
What does the SH3 domain of the adaptor protein do?
It recognizes polyproline and binds to and activates the GEF within the receptor tyrosine kinase pathway.
58
What activates the GEF within the receptor tyrosine kinase pathway?
The SH3 domain of the adaptor protein
59
What is the Ras protein's function?
It is a small monomeric G protein associated with the plasma membrane through a lipid anchor. When activated, it initates a MAP kinase cascade.
60
What does cAMP do?
activates protein kinase A
61
Ligands form noncovalent bonds with receptor proteins. What is necessary for receptors to make numerous bonds with a ligand?
binding site that fits the messenger molecule closely; appropriate amino acid side chains must be positioned to interact w/ligand
62
What 3 changes can a receptor make in response to a ligand binding?
1. Can change conformation
2. Can cluster with other receptors
3. Can do both
63
What are examples of G-protein coupled receptors?
opioid receptors
olfactory receptors
Beta-adrenergic receptors
hormone receptors (thyroid and follicle-stimulating hormones)
64
How can G-protein coupled receptors be regulated?
Via phosphorylation of specific amino acids in the cytosolic domain; inhibits ability to associate with G proteins
65
How do the subunits of the heterotrimeric G protein relate to one another?
The alpha subunit binds to GDP or GTP and detaches from the beta-gamma complex.
In contrast, the beta-gamma subunits are permanently bound together.
66
How long do heterotrimeric G proteins remain activated?
Only active when the alpha subunit disassociates from the beta-gamma complex and binds GTP; when GTP is hydrolyzed to GDP, the alpha subunit reforms the heterotrimeric complex and the G protein becomes inactive again
67
What are regulators of G protein signaling proteins?
GAP proteins - improve the alpha subunit efficiency at catalyzing GTP hydrolysis, speeding up the deactivation of a heterotrimeric G protein
68
What's an example of G protein beta-gamma complex signaling?
When acetylcholine binds the muscarinic acetylcholine receptor, the beta-gamma subunit opens potassium channels in the plasma membrane; alpha and beta-gamma subunits reassociate when acetylcholine is no longer present, causing potassium channels to close
Also, when mating factor signaling in yeast; when beta-gamma subunit associates with mating factor receptors, the scaffolding complex is recruited
69
What is cyclic AMP (cAMP)? What forms it?
a second messenger formed by adenylyl cyclase from ATP
70
How is AMP formed?
1. An activated alpha-subunit of a G protein binds to adenylyl cyclase.
2. Activated adenylyl cyclase forms cAMP from ATP.
3. Phosphodiesterase hydrolyzes cAMP to AMP.
71
Where is adenylyl cyclase located?
anchored in the plasma membrane; catalytic portion protrudes into the cytosol
72
Where is phosphodiesterase located?
in the cytosol
73
How does protein kinase A (PKA) work when active?
targeted by cAMP, PKA phosphorylates many proteins by transferring a phosphate from ATP to serine or threonine within the target protein
74
How does cAMP activate PKA?
PKA has 2 catalytic and 2 regulatory subunits, which inhibit the catalytic subunits in the absence of cAMP.
1. cAMP binds to the regulatory subunits, forcing them to change their conformation.
2. The catalytic subunits disassociate from the regulatory subunits. They are activated and free to phosphorylate target proteins in the cell.
75
What happens if phosphodiesterase is inhibited?
cAMP will not be degraded, will continue signaling
76
What would happen if the G protein-adenylyl cyclase system could not be shut off? Use cholera as an example.
The cholera bacterium secretes a toxin, which are endocytosed by molecules in the intestinal cells. A toxin is introduced that can modify Gs so it cannot hydrolyze GTP.
The cAMP level remaisn high, causing prolonged activation of a transporter - the intestine secretes chloride and sodium ions to maintain charge balance. To maintain osmolarity, water is also secreted, leading to dehydration and, if untreated, death.
77
How can cholera be treated? Why does this solution work?
Drinking a solution high in chloride and sodium ions and glucose; this forces sodium ions back into intestinal cells via the Na+/glucose symporter
78
What does PIP2 turn into when phospholipase C is activated?
IP3
79
What enzyme converts PIP2 into IP3?
Phospholipase C cleaves PIP2 into IP3 and DAG (diacylglycerol).
80
How do second messengers release calcium ions into the cytosol?
1. A ligand binds to a membrane receptor, activating the heterotrimeric Gq protein.
2. G protein's GTP-alpha subunit complex activates phospholipase C, which generates IP3 and DAG.
3. IP3 diffuses through the cytosol and binds to the IP3 receptor, a ligand-gated calcium channel, in the ER.
4. Step 3 opens the channel, releasing calcium ions into the cytosol.
5. Calcium elicits the desired physiological response.
81
What is the IP3 receptor? Where is it?
a ligand-gated calcium channel in the ER; binds IP3
82
What activates PKC?
the formation of DAG by phospholipase C and intracellular calcium release (due to IP3 binding to the IP3 receptor in the cytosol)
83
What maintains intracellular calcium concentration?
Calcium pumps in the plasma membrane and ER
84
How can intracellular calcium levels be elevated? Give one specific example.
Opening calcium channels in the plasma membrane
Releasing calcium from intracellular compartments (ex. release of calcium from ER via the IP3 receptor channel)
85
What are growth factors?
signals that signal kinases (usually receptor tyrosine kinases) for division and growth
86
List examples of receptor tyrosine kinases.
Insulin receptor
Nerve growth factor receptor
Epidermal growth factor receptor
87
Fibroblast growth factors and fibroblast growth factor receptors mediate signaling events in adult embryos and human tissues. How does this normally happen?
Normal FGFRs undergo autophosphorylation upon ligand binding.
88
What is a dominant negative mutation?
a mutation in receptor tyrosine kinases that overrides the function of a normal receptor, blocking signal transduction; when a mutant receptor dimerizes with a normal receptor, the pair cannot function
89
What are constitutively active mutations?
mutations that make fibroblast growth factor receptors hyperactive in signaling, even when no ligand is present
90
What is the difference between phopholipase Cgamma and phospholipase Cbeta?
Cgamma is activated by RTKs and contains an SH2 domain. Cbeta is activated by G protein-coupled receptors and doesn't have an SH2 domain.
91
What are scaffolding complexes?
when signaling components are assembled into large multiprotein compelxes which make cascading reactions more efficient and confine signals to a small area in the cell
92
What is the Ksr?
kinase suppressor of Ras, scaffolding complex that holds the kinases involved in receptor tyrosine kinase signaling, Raf, Mek, and MAPK together to promote phosphorylation
93
Describe the mating pathway of budding yeast.
In times of stress, cells of mating type a secrete a factor (chemical signal), which binds to a specific G protein-coupled receptors on nearby alpha cells while simultaneously secreting an alpha factor, which binds to corresponding receptors on a cells.
94
How does mating factor signaling work?
results in widespread changes in the two cells, including polarized secretion, alterations in the cytoskeleton, and changes in gene expression. Ultimately, the two cells fuse to create a diploid a/alpha cell
95
What kind of hormones are there?
amino acid derivatives
peptides
proteins
lipid-like hormones
96
What kind of hormone is epinephrine?
amino acid derivative, derived from tyrosine
97
What kind of hormone is vasopressin?
peptide hormone
98
What kind of hormone is insulin?
protein
99
What kind of hormone is testosterone?
steroid hormone - derived from cholesterol synthesized in gonads or adrenal cortex
100
What are adrenergic hormones?
epinephrine and norepinephrine
101
Describe the hormone strategy of adrenergic hormone action.
to put many of the normal bodily functions on hold and to deliver vital resources to the heart and skeletal muscles instead, as well as to produce a heightened state of alertness. When secreted into the bloodstream, epinephrine and norepinephrine stimulate changes in many different tissues or organs, all aimed at preparing the body for dangerous or stressful situations (the so-called “fight-or-flight” response). Overall, adrenergic hormones trigger increased cardiac output, shunting blood from the visceral organs to the muscles and the heart, and cause dilation of arterioles to facilitate oxygenation of the blood. In addition, these hormones stimulate the breakdown of glycogen to supply glucose to the muscles.
102
Where do adrenergic hormones bind?
G protein-coupled receptors called adrenergic receptors
103
What's the difference between alpha-adrenergic receptors and beta-adrenergic receptors?
Alpha receptors bind epinephrine and norepinephrine and are located on smooth muscles regulating blood flow. Beta receptors bind epinephrine much better than norepinephrine and are located on smooth muscles associated with arterioles.
104
Which G proteins do alpha-adrenergic receptors act through?
Gq proteins (when activated, stimulate phospholipase C, leading to IP3 and DAG production, which leads to higher intracellular calcium levels)
105
Which G proteins do beta-adrenergic receptors stimulate?
Gs, which stimulates the cAMP signal transduction pathway, leading to relaxation of smooth muscles
106
Describe the sequence of events leading to enhanced glycogen degradation.
1. Epinephrine molecule binds to a Beta-adrenergic receptor on PM of a liver or muscle cell.
2. Step 1 activates a neighboring Gs protein, which stimulates adenylyl cyclase, which generates cAMP from ATP. The increased cAMP concentration activates PKA.
3. PKA activates another sequence of events, beginning with phosphorylation of phosphorylase kinase.
4. The converstion of glycogen phosphorylase from phosphorylase b to phosphorylase a (the more active form).
This results in an increased rate of glycogen breakdown.
107
What's the difference between how activation of alpha-adrenergic and beta-adrenergic receptors impacts smooth muscle cells?
Alpha receptors cause constriction and diminished blood flow. Beta receptors cause muscles to relax.
108
What peptide hormones regulate blood glucose levels during periods of normal activity?
glucagon and insulin
109
What does glucagon do?
raises blood glucose by breaking down glycogen when level is too low; acts via Gs protein
110
What does insulin do?
reduces blood glucose levels by stimulating uptake of glucose into muscle and adipose cells and by stimulating glycogen synthesis
111
