1.3. Signal transduction: receptors, G proteins, second messengers. Flashcards
- How do cells communicate to each other?
Cells communicate by releasing extracellular signal molecules like hormones and neurotransmitters that bind to receptor proteins located in the plasma membrane, cytoplasm or nucleus.
- What are the 4 types of extracellular signing molecules?
- Peptides and proteins (insulin)
- Amines (epinephrine & norepinephrine)
- Steroid hormones (aldosterone, estrogen)
- Small molecules (a.a., nucleotides, ions, gases)
How are intracellular signals amplified?
- The signal is transduced into the activation or inactivation of intracellular messengers.
- Intracellular signaling proteins (e.g., kinases, phosphatases etc) interact with and regulate the activity of target proteins, thereby modulate the cellular function
- Path:
Signaling molecule
-> Receptor protein
-> Intracellular signaling
-> Target proteins
- What are the 6 basic mechanisms of intercellular (cell-to-cell) communication?
- Contact-dependent
- Paracrine
- Autocrine
- Synaptic
- Endocrine
- Gap junction
- 6 basic mechanisms of intercellular (cell-to-cell) communication
a/ Mechanism of contact-dependent?
signaling molecule of one cell binds directly to a plasma membrane receptor of another cell
- 6 basic mechanisms of intercellular (cell-to-cell) communication
b/ Mechanism of paracrine?
molecules are released and act locally on another type of cell
- 6 basic mechanisms of intercellular (cell-to-cell) communication
c/ Mechanism of autocrine?
release of a molecule affects the same cell or the other cells of the same type:
- 6 basic mechanisms of intercellular (cell-to-cell) communication
d/ Mechanism of Synaptic?
Synaptic: release neurotransmitters at synapse which affects the function of other neurons or cells (fast; millisecond)
- 6 basic mechanisms of intercellular (cell-to-cell) communication
e/ Mechanism of Endocrine?
hormones are secreted into blood, which are widely dispersed in the body (slow; sec to min)
- 6 basic mechanisms of intercellular (cell-to-cell) communication
f/ Mechanism of Gap junction?
Gap junction: junctions that allow intracellular signal molecules to diffuse from the cytoplasm of a cell to an adjacent cell.
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4a. The role of receptors in cell communication
Receptors receive signals from first messengers (e.g., epinepherine or neurotransmitters) and these can lead to a cascade of complexed reactions which results in yield of second messengers like cAMP that affect cellular function.
4b. 5 Requirements of a receptor in cell communication
1/ ‘’bifunctional’’ – they both recognize the ligand and produce a biological response in the target cell
2/ High affinity (recognize ligands in low concentration) (10-9 mM)
3/ Reversible binding to their mediators
4/ Receptor-ligand binding is specific, but not always
5/ Receptor-ligand binding is able to be saturated (reach a point where all receptors already are bound)
- What are first messengers and second messengers?
First messengers are external mediators (insulin,..)
Second messengers
Examples
Ca2+
IP3
DAG
cGMP
Arachidonic acid
- ligands
a/ classification of ligands based on how the receptors bind to ligand
1/ Agonist
- Full agonist: has 100% efficacy -> provokes biological response
- Partial agonist: less than 100% efficacy
2/ Antagonist: has 0% efficacy, but occupies a receptor site (thus, blocks both agonists and inverse agonist)
- Ligands
a/ classification of ligands based on locations
1/ Lipophobic
- Bind to plasma membrane receptors
2/ Lipopholic
- Must depend on carrier proteins
- Bind to plasma membrane receptors
- Bind to intracellular receptors including cytosol and nucleus receptors
- What are the 2 types of intracellular signaling that act as molecular switch?
- Signaling by phosphorylation
- Signaling by GTP-binding protein
- 2 types of intracellular signaling proteins that act as molecular switch
a/ Mechanism and example of Signaling by phosphorylation
1/ Mechanism: ATP phosphorylates signaling proteins, activating them when they have a bound phosphate
2/ For example, glycogen phosphorylase is activated by phosphorylation
- 2 types of intracellular signaling proteins that act as molecular switch
b/ Mechanism of Signaling by GTP-binding proteins
1/ GTP can become bound to the G-proteins (hence their name, G-protein is short for guanine nucleotide binding protein), and binding GTP makes them active
2/ GEF (guanine exchange factor) removes GDP from inactive G-proteins, which allows binding of GTP/reactivation
3/ To deactivate, GAP (GTPase activating protein) stimulates the G-protein’s intrinsic GTPase activity, helping it to cleave the phosphate from its bound GTP, reforming GDP and leaving the G-protein inactive
- Definition of signal transduction
Signal transduction is a Process by which an extracellular (EC) signal is converted to an intracellular signal, which is transmitted through a cell as a series of molecular events, eventually resulting in a cellular response.
- List 4 classes of plasma membrane receptor
1/ G-protein-coupled receptors (GPCRs or 7TM)
2/ Ligand-gated ion channels
3/ Receptors which possess catalytic activity
4/ Receptors associated with an effector with catalytic activity
- Classification of all receptors
1/ G-protein-coupled receptors (GPCRs or 7TM)
- E.G, muscarinic ACh receptor (mAChR)
2/ Ion channel receptors
- E.g, nicotinic ACh receptor (nAChR)
3/ Receptors with enzymatic activity
- Guanylyl cycle (ex: ANP receptor)
- Ser/Thr-kinase (ex: TGFα receptors)
- Tyrosine-kinase (ex: growth factor/insulin receptors)
4/ Enzyme-linked receptors (no intrinsic enzymatic activity)
5/ Regulated intramembrane proteolysis (RIP)
- G-Protein-coupled receptors (GPCRs)
a/ Structure of G-Protein-coupled receptors (GPCRs)
1/ GPCRs have 7 transmembrane domains
2/ GPCRs are heterometric
- They have α (alpha), β (beta) and γ (gamma) subunits
- G-Protein-coupled receptors (GPCRs)
b/ Mechanism of GPCR activation and effector activation
- Agonist ligand binds -> receptor activates
- G proteins interact with receptor leads to…
a/ Receptor conformational change
b/ GDP to GTP exchange
c/ G-protein activated - Activated G-protein (GTP-bound) dissociates from receptor
- α-GTP dissociates from βγ-subunit
- α-GTP and βγ-subunits interact with their appropriate effectors
- α-GTP hydrolyses into GDP -> inactivates α + promotes reassembly of trimer
- G-Protein-coupled receptors (GPCRs)
b/ The role of GPCR
1/ Heterotrimeric G proteins function as transducers to activate many signaling pathways.
2/ One ligand can be bound to distinct receptors
- G-Protein-coupled receptors (GPCRs)
c/ The 5 types of GPCR
1/ Gs proteins (s = stimulatory)
2/ Gi proteins (Inhibitory)
3/ Golf (olfactory)
4/ Gq (G11) proteins
5/ G12/13 proteins
- G-Protein-coupled receptors (GPCRs)
d1/ The role of Gs proteins
Regulates adenylyl cyclase, resulting in increased cAMP
- G-Protein-coupled receptors (GPCRs)
d2/ The examples of Gs proteins
1/ β-adrenergic receptor
2/ ACTH receptor
3/Glucagon receptor
*Note: the pituitary hormone adrenocorticotropin (ACTH)
- G-Protein-coupled receptors (GPCRs)
d3/ The mechanism of Gs proteins
1/ α-GTP increases the activity of adenylyl cyclase (AC)
2/ AC converts ATP to cAMP
3/ cAMP activates protein kinase A (PKA)
4/ phosphodiesterase (PDE) helps break down cAMP
- G-Protein-coupled receptors (GPCRs)
e1/ The mechanism and function of Gi/Go G-protein
Gi/o inhibits AC activity -> decreases the cAMP production
- G-Protein-coupled receptors (GPCRs)
e2/ The 4 functions of Gi/Go G-protein
1/ Inhibition of adenylyl cyclase (cAMP↓)
2/ Activation of K+-channels (leads to hyperpolarization of cells)
3/ Inhibition of Ca2+-channels (can be activated by PKA from Gs)
4/ Phospholipase A2 activation -> arachidonic acid release
- G-Protein-coupled receptors (GPCRs)
e3/ The 3 examples of Gi/Go G-protein
(1) α2-AR (Alpha-2 adrenergic receptor)
(2) M2 + M4 ACh receptor (Muscarinic acetylcholine receptor M4 and M2)
(3) opiate receptors
- G-Protein-coupled receptors (GPCRs)
f/ The mechanism of Golf G-protein
- Odorant molecules will reach the odorant receptor
- G⍺-olf is activated
- Adenylyl cyclase activity increases
- cAMP concentration increases
- G-Protein-coupled receptors (GPCRs)
g/ The mechanism of G⍺t (transducin) subunit
1/ Actions of G⍺t (transducin) subunit is mediated by cGMP phosphodiesterase
2/ Light molecules come to the receptor
-> G⍺t (transducin) subunit leaves the complex and then bind to phosphodiesterase
-> activates phosphodiesterase
-> the breakdown of cGMP leads to the closure of cGMP-dependent channels
- G-Protein-coupled receptors (GPCRs)
h1/ The role of Gq (G11) proteins
leads to increase in intracellular calcium concentration
- G-Protein-coupled receptors (GPCRs)
h2/ The mechanism of Gq (G11) proteins
1/ Stimulates phospholipase C (PLC), which converts the membrane-bound PIP2 (phosphatidylinositol 4,5-bisphosphate) into 2/ IP3 (inositol-1,4,5 triphosphate) and DAG (diacylglycerol)
3/ DAG activates the enzyme protein kinase C (PKC) which in turn causes phosphorylation of effector proteins
-> DAG stays in the membrane
4/ IP3 causes sarcoplasmic reticulum and ER to release calcium stores
5/ Calcium is used as a signal to regulate effector mechanism (can be a hormone or being bound to calmodulin (CaM)
- G-Protein-coupled receptors (GPCRs) - Gq (G11) proteins
h3/ The role of Calcium
Calcium can be used as a signal to regulate effector mechanisms, including:
1/ Hormones: ADH, Angiotensin II, GnRH
2/ Binding to calmodulin (CaM), which:
- Binds to CaM dependent kinases, phosphorylating myosin light chain kinase (MLCK), which initiates smooth muscle contraction
- Activates cAMP phosphodiesterase, which degrades cAMP to AMP (reducing the effect of the Gs G-protein)
*Note: ADH = Human vasopressin, also called antidiuretic hormone
GnRH = Gonadotropin-releasing hormone
- G-Protein-coupled receptors (GPCRs) -
i1/ The role of G12/13 proteins
Role: regulates guanine nucleotide exchange factors - GEF
- G-Protein-coupled receptors (GPCRs) -
i2/ The mechanism of G12/13 proteins
1/ Activate Rho small G-proteins
2/ Cause activation of Rho kinase which leads to smooth muscle contraction
3/ Both G12 and G13 regulate the actin cytoskeleton, G13 helps with tyrosine kinase pathway
4/ Rho-GTP: signals for sensory receptors (vision = transducin, taste = gustducin, olfactory)
- G-Protein-coupled receptors (GPCRs)
j1/ Location of adrenergic receptors
can be found in target tissues of the sympathetic nervous system
- G-Protein-coupled receptors (GPCRs)
j2/ Characteristics of adrenergic receptors
Activated by epinephrine or norepinephrine
- Adrenal medulla can produce both norepinephrine and epinephrine
- Sympathetic nerve only produce norepinephrine
- G-Protein-coupled receptors (GPCRs) - adrenergic receptors
j3/ Receptor: alpha 1
Affinity: ???
G-protein: ???
Signal transduction: ???
Location: ???
- G-Protein-coupled receptors (GPCRs) - adrenergic receptors
j4/ Receptor: alpha 2
Affinity: ???
G-protein: ???
Signal transduction: ???
Location: ???
- G-Protein-coupled receptors (GPCRs) - adrenergic receptors
j5/ Receptor: beta 1
Affinity: ???
G-protein: ???
Signal transduction: ???
Location: ???
- G-Protein-coupled receptors (GPCRs) - adrenergic receptors
j6/ Receptor: beta 2
Affinity: ???
G-protein: ???
Signal transduction: ???
Location: ???
- G-Protein-coupled receptors (GPCRs) - adrenergic receptors
j7/ Receptor: beta 3
Affinity: ???
G-protein: ???
Signal transduction: ???
Location: ???
- G-Protein-coupled receptors (GPCRs)
k/ Termination of GPCR signaling
1/ Ligand (agonist) dissociation
2/ Receptor desensitization (β-arrestin binding)
- GPCRK (G-protein coupled receptor kinase) phosphorylates activated receptor once the receptor is activated by the binding of the agonist
3/ Receptor binds to β-arrestin molecules
4/ Βeta-arrestin uncouples the receptor from the G-proteins (desensitization)
- G-Protein-coupled receptors (GPCRs)
l/ 5 steps of endocytosis of GPCR
- Activation & signaling
- Phosphorylation and desentization (GRK, arrestin)
- Targeting to CCP & internalization
- Sorting (or signaling)
- Degradation or recycling
- Ion channel receptors
a/ What are the 5 characteristics of ion channels
1/ extremely fast (act on a scale of milli seconds!)
2/ ligand gated (ACh)
3/ appropriate for neurotransmitters
4/ uses paracrine signal transmission
5/ comparatively works faster than the GPCRs, but most of the ion channel receptor ligands also trigger GPCR activity
- Ion channel receptors
b/ What are the 2 types of ligand-gated ion channel receptors based on function?
1/ Excitatory receptor
2/ Inhibitory receptor
- Ion channel receptors
b2/ List the 5 ligands of excitatory receptor
1/ Acetylcholine (nicotinergic)
2/ Glutamate (NMDA receptor)†
3/ Glutamate (AMPA, Kainate)
4/ Serotonin (5HT3 receptor)
5/ ATP (P2X)
- Ion channel receptors
b3/ Excitatory receptor
Ligand: Acetylcholine (nicotinergic)
Specificity: ???
Na+/K+
- Ion channel receptors
b4/ Excitatory receptor
Ligand: Glutamate (NMDA receptor)†
Specificity: ???
Na+/K+ and Ca2+
- Ion channel receptors
b5/ Excitatory receptor
Ligand: Glutamate (AMPA, Kainate)
Specificity: ???
Na+/K+
- Ion channel receptors
b6/ Excitatory receptor
Ligand: Serotonin (5HT3 receptor)
Specificity: ???
Na+/K+
- Ion channel receptors
b7/ Excitatory receptor
Ligand: ATP (P2X)
Specificity: ???
Na+/K+ and Ca2+
- Ion channel receptors
b8/ List 2 ligands of inhibitory receptor
1/ γ-Aminobutyric acid type A (GABAA)
2/ Glycine
- Receptors which possess catalytic activity
a/ What are the 3 examples?
- Tyrosine receptor
- Guanylyl cyclase
- Serine-threonine kinase receptor
- Receptors which possess catalytic activity
b1/ Characteristics of receptor tyrosine kinases (RTKs)
1/ Has growth factor (GF) receptors, insulin receptors, epidermal GF (EGF)
2/ Has extracellular binding site, but an intracellular tyrosine-kinase (T-K) domain
- Receptors which possess catalytic activity
b3/ What is the General mechanism of receptor tyrosine kinases (RTKs)?
1/ Ligand binding to two GF receptors facilitates their dimerization
2/ Activated dimers phosphorylate each other (autophosphorylation)
3/ The phosphorylated tyrosine domains n are then be recognized by GBR2, an SH2-containing protein
- Receptors which possess catalytic activity
b5/ Receptor tyrosine kinases (RTKs) - What is the 10-step process of RAS pathway?
1/ Ligand binding causes receptor dimerization
2/ The activated RTK phosphorylates itself
3/ GRB2, an SH2-containing protein, recognizes the phosphotyrosine residues
4/ The binding of GRB2 recruits SOS
5/ SOS activates Ras by causing GTP to replace GDP on Ras
6/ The activated GTP-Ras recruits Raf-1 and activates it
7/ Raf-1 phosphorylates and activates MEK
8/MEK phosphorylates and activates MAPK
9/ MAPK works as an important effector molecule by phosphorylating many cellular proteins
10/ MAPK translocate the nucleus where it phosphorylates a transcription factor
- Receptors which possess catalytic activity
c1/ What is the role and example of PIP3 kinase (phosphatidylinositol 3,4,5 – triphosphate kinase)?
Role: inhibition of apoptosis
E.g, oncogene is the gene that has the potential to cause cancer
- Receptors which possess catalytic activity
c2/ What is the mechanism of PIP3 kinase (phosphatidylinositol 3,4,5 – triphosphate kinase)?
1/ PIP3 kinase SH2 domain bind to the activated phosphorylated tyrosine receptor
2/ PIP2 is phosphorylated into PIP3
3/ PIP3 gets recognized by proteins with PH domains (PDK1 and PKB)
4/ PIP3 – PDK1 activates PIP3 – PKB
5/ Active PKB phosphorylates the protein ‘’BAD’’, which releases the active death- inhibitory protein
Inhibition of apoptosis (makes cells immortal)
- Receptors which possess catalytic activity
d1/ What is the role of guanylyl cyclase receptor?
synthesizes cGMP from GTP (just like AC does with ATP)
- Receptors which possess catalytic activity
d2/ What is the Mechanism of guanylyl cyclase receptor?
cGMP -> protein kinase G (PKG) -> cause various cellular responses, such as vasodilation
- Receptors which possess catalytic activity
d3/ What is the characteristics of guanylyl cyclase receptor?
Belong to ANP-receptor family
- Receptors which possess catalytic activity
d4/ How does guanylyl cyclase work?
- cytosolic Ca2+ interacts with calmodulin, it activates NO synthase
- NO synthase makes NO (can spontaneously decompose)
-> Can bind to soluble guanylyl cyclase -> makes cGMP
- Receptors associated with an effector with catalytic activity
a/ What are the characteristics of Receptors associated with an effector with catalytic activity?
1/ They are in GF hormone receptor, but has no intrinsic tyrosine kinase activity
2/ Signaling pathway mediated by Janus-kinase (JAK), leads to activated STAT protein that goes to the nucleus and regulates gene expression
- Receptors associated with an effector with catalytic activity
b/ What are the 3 Examples of ligands of Receptors associated with an effector with catalytic activity?
Examples of ligands: cytokines, prolactin, growth hormone (GH)
- Receptors associated with an effector with catalytic activity
c/ What is the Mechanism of Receptors associated with an effector with catalytic activity? ( 5 steps)
1/ Receptors are associated with JAK
2/ Once the ligand binds to the receptor, JAK gets phosphorylatedreceptor dimerizes
3/ Phosphorylated JAK then phosphorylates the dimerized receptors
4/ Phosphorylated receptors then phosphorylate the STAT (=signal transducer and activator of transcription) protein -> STAT dimerizes
5/ Dimerized STAT proteins enter the nucleus and regulates gene expression
- intracellular receptors
a/ What are Characteristics of intracellular receptors?
Typically, a receptor for hydrophobic molecules.
- intracellular receptors
b/ What is the mechanism of intracellular receptors?
1/ Ligand Binds to cytosolic Hsp90
2/ NLS (nuclear localizing sequence) revealed
3/ The receptor goes to nucleus
4/ Transcription regulation
- intracellular receptors
c/ What are the examples of intracellular receptors?
1/ Peptide hormone synthesis
+) ER-> Golgi -> exocytosis (hormone – insulin- is excreted)
2/ Insulin-regulated GLUT4 exocytosis
3/ Receptor endocytosis
+) Clathrin
4/ Nutrient receptor internalization
+) LDL (low-density lipoproteins) ‘’bad cholesterol’’
+) Transferrin (blood plasma glycoprotein – ferric-ion delivery)
