1
Q
Major Scientists: Gilman
A
Discovered heterotrimeric G proteins, central to GPCR signaling.
2
Q
Major Protein: Src (Sarc) kinase
A
Early cytosolic tyrosine kinase that defined SH2 and SH3 domains, crucial for protein–protein interactions.
3
Q
Major Protein: Ras
A
Small GTPase, acts as a molecular switch; mutations (e.g., RasQ61K) cause constant activation → cancer.
4
Q
Major Protein: Herceptin / HER2
A
Example of receptor tyrosine kinase (RTK) targeted in breast cancer therapy.
5
Q
Major Protein: CaM-kinase II
A
Calcium/calmodulin-dependent kinase involved in memory and learning; activated by autophosphorylation.
6
Q
Major Protein: Cholera toxin
A
Locks Gsα in GTP-bound form → constant cAMP production → diarrhea.
7
Q
G-Protein–Coupled Receptors (GPCRs): Inactive state
A
GDP-bound
Inactivation: Intrinsic GTPase activity hydrolyzes GTP → GDP (aided by GAP).
Desensitization: via phosphorylation and arrestin binding → internalization by endocytosis.
8
Q
G-Protein–Coupled Receptors (GPCRs): Active
A
GTP-bound
Activation trigger: Ligand binding → conformational change → receptor acts as GEF, exchanges GDP for GTP.
Activated form: Gα–GTP + Gβγ both regulate effectors (ion channels or enzymes)
9
Q
G-Protein–Coupled Receptors (GPCRs): Function
A
Stimulates or inhibits adenylyl cyclase, PLC, etc.
10
Q
G-Protein–Coupled Receptors (GPCRs):
A
Origin: Plasma membrane
Destination: Cytoplasmic signaling cascades
Trigger: Ligand binding (e.g., hormones, neurotransmitters)
11
Q
Protein: Arrestin; function
A
Terminates signaling, can mediate new intracellular signals.
12
Q
Protein: Arrestin; Active
A
Bound to phosphorylated receptor
13
Q
Protein: Arrestin; Inactive
A
Not bound
14
Q
Enzyme-Coupled Receptors (RTKs): Inactive
A
Monomeric
15
Q
Enzyme-Coupled Receptors (RTKs): Active
A
Activation trigger: Ligand-induced dimerization → autophosphorylation on tyrosines.
16
Q
Enzyme-Coupled Receptors (RTKs): Downstream signaling
A
Downstream signaling: Creates docking sites for SH2/PTB domain proteins.
17
Q
Enzyme-Coupled Receptors (RTKs): Examples
A
HER2: Dimerizes → activates growth signaling; targeted by Herceptin.
Insulin receptor: Dimeric RTK recruits IRS-1 via PTB domain → downstream signaling.
18
Q
Enzyme-Coupled Receptors (RTKs): Regulation
A
c-Cbl adds ubiquitin → endocytosis & degradation (prevents over-signaling).
Adaptor proteins (Grb2, Shc) link RTKs to Ras → MAPK cascade → transcriptional activation.
19
Q
Protein Bound State: Ras
A
GTP-bound (active)
20
Q
Protein Function: Ras
A
Activates MAPK cascade
21
Q
Protein Bound State: Ras-GAP
A
Stimulates GTP hydrolysis
22
Q
Protein Function: Ras-GAP
A
Inactivates Ras
23
Q
Protein Bound State: Ras-GEF
A
Promotes GDP→GTP exchange
24
Q
Protein Function: Ras-GEF
A
Activates Ras
25
Enzyme-Coupled Receptors (RTKs): Proteins
Ras
Ras-GAP
Ras-GEF
26
Enzyme-Coupled Receptors (RTKs)
Origin: Plasma membrane
Destination: Nucleus (via MAPK signaling)
Trigger: Growth factor binding, dimerization
27
Second Messengers and Amplification
Messengers:
cAMP
IP₃
DAG
Ca²⁺
28
cAMP Source
Adenylyl cyclase (activated by Gsα)
29
cAMP Effect
Activates PKA, phosphorylates targets
30
IP₃ Source
Phospholipase C (PLCβ or PLCγ)
31
IP₃ Effect
Releases Ca²⁺ from ER
32
DAG Source
Phospholipase C (PLCβ or PLCγ)
33
DAG Effect
Activates PKC
34
Ca²⁺ Source
Released via IP₃
35
Ca²⁺ Effect
Activates calmodulin, CaM-kinases
36
Second Messengers and Amplification: Feedback control
Positive feedback (e.g., glycogen breakdown amplifies cAMP signaling)
Negative feedback (phosphatases deactivate kinases).
37
Calcium and Calmodulin Pathway
Calmodulin = universal Ca²⁺-binding protein (changes shape upon binding).
Target activation: Wraps around target proteins (e.g., CaM-kinase).
CaM-kinase II: Autophosphorylates → remains active after Ca²⁺ falls.
Function: Converts short Ca²⁺ spikes into long-term cellular effects (memory trace).
38
Cytoskeleton and Structural Proteins
Actin Filaments
Intermediate Filaments
Microtubules
39
Cytoskeleton and Structural Protein Actin Filaments Monomer:
Monomer: Actin (binds ATP/ADP).
40
Cytoskeleton and Structural Protein Actin Filaments: Dynamics
Polymerization at + end; depolymerization at − end = treadmilling
41
Cytoskeleton and Structural Protein Actin Filaments:
ATP hydrolysis powers turnover.
Accessory proteins:
Thymosin & Profilin: Control monomer pool.
Capping proteins: Block + end elongation.
Drugs affecting actin: Cytochalasins, Latrunculin (depolymerize); Phalloidin (stabilizes).
42
Cytoskeleton and Structural Protein Intermediate Filaments:
Examples: Keratin (skin), Lamins (nucleus).
Function: Mechanical strength; resist stress.
Defects: Keratin mutations → fragile skin; Lamin defects → muscular dystrophy & premature aging.
42
Cytoskeleton and Structural Protein: Microtubules
Building block: α/β-tubulin dimers.
Dynamic instability: Continuous polymerization/depolymerization.
Labeling: GFP–tubulin, EB1–GFP used to track growth in live imaging.
43
GPCR activation: Triggers and Results
Trigger: Ligand binding
Results: GTP loading on Gα
44
Triggers and Regulation Mechanisms: Processes
GPCR activation
RTK activation
CaM-kinase activation
Cytoskeleton remodeling
PKA activation
45
RTK activation: Triggers and Results
Trigger: Dimerization
Result: Autophosphorylation
46
CaM-kinase activation: Triggers and Results
Trigger: Ca²⁺/calmodulin binding
Result: Autophosphorylation
47
Cytoskeleton remodeling: Triggers and Results
Trigger: ATP/GTP hydrolysis
Result: Polymerization dynamics
48
PKA activation: Triggers and Results
Trigger: cAMP binding
Result: Catalytic subunit release
49
What happens to a GPCR when it binds its ligand?
It changes conformation, activates its G protein by acting as a GEF, and may be internalized after phosphorylation.
50
How do RTKs activate Ras?
RTK autophosphorylation recruits Grb2, which binds Sos (a Ras-GEF) → exchanges GDP for GTP on Ras → activates MAPK cascade.
51
What is the difference between Gα and Ras?
Both are GTPases; Gα is heterotrimeric, Ras is monomeric; both switch between active (GTP-bound) and inactive (GDP-bound) states.
52
How does cholera toxin affect signaling?
Locks Gsα in GTP-bound form → continuous activation of adenylyl cyclase → ↑cAMP → persistent PKA activation → chloride efflux & dehydration.
53
What happens if actin depolymerization is inhibited?
Cells cannot recycle actin monomers → cell division and movement halt.
54
Why does a mutation like RasQ61K cause cancer?
Prevents GTP hydrolysis → Ras locked “on” → continuous cell proliferation signals.
55
What is the role of Ca²⁺/calmodulin?
Binds Ca²⁺ → activates target enzymes (e.g., CaM-kinases) → regulates transcription and metabolism.
56
How do c-Cbl and arrestin terminate signaling?
c-Cbl ubiquitinates RTKs → degradation; arrestin binds GPCRs → blocks G-protein binding, promotes endocytosis.
57
Cell signaling
communication system that allows cells to respond to internal/external changes.
Found in both unicellular (yeast, bacteria) and multicellular organisms.
58
Cell signaling Purpose:
Coordinate behavior — e.g., growth, survival, differentiation, apoptosis (programmed cell death).
59
Cell signaling Basic path
Signal → Receptor → Intracellular signaling → Effector proteins → Cellular response.
60
Cell signaling Examples:
Cortisol cream → steroid hormone enters skin cells → gene regulation → reduces inflammation.
Platelet-derived growth factor (PDGF) → stimulates cell proliferation for wound healing.
IgE + histamine → allergic reaction; antihistamines block histamine receptors.
61
Signals and Receptors: Types of signaling
Contact-dependent: Cells touch (e.g., immune cell recognition).
Paracrine: Local signaling (short range; e.g., PDGF).
Synaptic: Neurons → neurotransmitters (fast, specific).
Endocrine: Hormones via bloodstream (long-distance; e.g., insulin, cortisol).
62
Signal molecules include:
Hydrophilic signals (can’t cross membrane): peptides, proteins → bind to cell-surface receptors.
Hydrophobic signals (can cross): steroids, gases (NO, ethylene in plants) → bind intracellular receptors.
63
Receptors:
Specific proteins that recognize particular ligands (signals).
Receptor binding → conformational change → activation.
64
Intracellular Signaling Molecules & Second Messengers: Function
Relay, amplify, and integrate the signal inside the cell.
Second messengers: small molecules that spread and amplify signals.
65
Second Messengers
cAMP – activates PKA.
IP₃ – releases Ca²⁺ from ER.
DAG – activates PKC.
Ca²⁺ – activates calmodulin → CaM-kinase.
66
Intracellular Signaling Molecules & Second Messengers: Switching mechanisms
Phosphorylation:
-Protein kinases add phosphate (turn ON).
-Protein phosphatases remove phosphate (turn OFF).
GTP-binding proteins:
-Active: GTP-bound
-Inactive: GDP-bound
-Regulated by GEFs (activate) and GAPs (inactivate).
67
Intracellular Signaling Molecules & Second Messengers: Examples
Yeast mating uses G-protein signaling to detect opposite pheromones → stop dividing → fuse.
68
Features of Cell Signaling
Specificity: Correct signal binds correct receptor.
Sensitivity: Even small signal concentrations can trigger big responses (amplification).
Integration: Cells often need multiple signals to act (e.g., survival + division).
Adaptation: Repeated exposure → decreased response (desensitization).
69
Features of Cell Signaling: Feedback regulation
Positive = amplify (e.g., clotting).
Negative = shut off (e.g., receptor desensitization).
70
Features of Cell Signaling: Response speed
Fast = modify existing proteins (sec–min).
Slow = change gene expression (hrs).
71
Intro to Signaling: Why does cortisol work when applied as a cream?
It’s a steroid hormone (hydrophobic), diffuses through membranes, binds intracellular receptor → regulates gene expression.
72
Intro to Signaling: Why can the same neurotransmitter (acetylcholine) cause different effects in different tissues?
Different receptors and intracellular signaling proteins in each cell type determine response.
73
Intro to Signaling: What’s the difference between paracrine and endocrine signaling?
Paracrine = local diffusion; Endocrine = hormones travel through bloodstream to distant targets.
74
G-Proteins: Molecular switches
GTP-binding proteins toggle between active (GTP) and inactive (GDP).
75
G-Proteins: Two Main Types
Heterotrimeric G-proteins (α, β, γ) — linked to GPCRs.
Monomeric GTPases (like Ras) — linked to enzyme-coupled receptors (RTKs).
76
G-Proteins: Regulators
GEF (Guanine Exchange Factor): swaps GDP → GTP (activates).
GAP (GTPase Activating Protein): enhances GTP hydrolysis (inactivates).
77
G-Proteins: Cycle
Ligand binds receptor → receptor activates GEF → G-protein binds GTP → effector activated → GTP hydrolyzed → inactive again.
78
G-Proteins: Examples
Yeast pheromone response uses G-protein activation → cytoskeleton rearrangement (“shmoo” formation).
79
G-Proteins: Diseases
Cholera toxin locks Gsα “on” (constant cAMP) → dehydration.
Pertussis toxin locks Giα “off” → prevents inhibition of cAMP.
80
G-Proteins: What are the two states of a G-protein?
Active: GTP-bound; Inactive: GDP-bound.
81
G-Proteins: What proteins regulate G-proteins?
GEFs (activate by exchanging GDP for GTP); GAPs (inactivate by stimulating GTP hydrolysis).
82
G-Protein-Coupled Receptors (GPCRs): Mechanism
Ligand binds receptor.
Conformational change → activates G-protein (via GEF activity).
Gα–GTP and Gβγ both signal downstream.
Intrinsic GTPase → Gα hydrolyzes GTP → reassociates with βγ → OFF.
83
G-Protein-Coupled Receptors (GPCRs): Desensitization
GPCR kinase (GRK) phosphorylates receptor → arrestin binds → prevents further activation.
83
G-Protein-Coupled Receptors (GPCRs): Effector proteins
Adenylyl cyclase → makes cAMP.
Phospholipase C (PLC) → makes IP₃ + DAG.
84
G-Protein-Coupled Receptors (GPCRs): Examples
Adrenaline (fight-or-flight) → β-adrenergic receptor → ↑cAMP.
Acetylcholine in heart → GPCR → slows heartbeat.
Smell and vision also use GPCRs (rhodopsin = GPCR).
85
GPCRs: What enzyme produces cAMP in GPCR signaling?
Adenylyl cyclase (activated by Gsα).
86
GPCRs: How does arrestin stop GPCR signaling?
Binds phosphorylated receptor → blocks G-protein interaction → promotes receptor endocytosis.
86
Protein Kinase (Enzyme)-Coupled Receptors:
Most common: Receptor Tyrosine Kinases (RTKs)
Structure: Single transmembrane helix + cytosolic kinase domain.
87
Protein Kinase (Enzyme)-Coupled Receptors: Activation
Ligand binding → receptor dimerizes.
Each monomer autophosphorylates tyrosine residues.
Phosphotyrosines = docking sites for SH2/PTB-domain proteins.
88
Protein Kinase (Enzyme)-Coupled Receptors: Adaptor proteins
Grb2 binds RTK → recruits Sos (GEF) → activates Ras.
Ras–GTP triggers MAP kinase cascade → gene expression changes.
89
Protein Kinase (Enzyme)-Coupled Receptors: Inactivation
c-Cbl adds ubiquitin → RTK degradation.
90
Protein Kinase (Enzyme)-Coupled Receptors: Examples
PDGF receptor → wound healing.
HER2 (target of Herceptin) → overactive in breast cancer.
91
Enzyme-Coupled Receptors: What is the result of RTK dimerization?
Cross-phosphorylation → activation of kinase activity and downstream signaling.
92
Enzyme-Coupled Receptors: Which adaptor connects RTK to Ras?
Grb2–Sos complex.
93
MAP Kinase Cascades:
Purpose: Amplify and distribute signals.
Sequence:
RTK → Ras–GTP → Raf (MAPKKK) → MEK (MAPKK) → ERK (MAPK).
94
MAP Kinase Cascades: Results
Gene expression changes → growth, proliferation, differentiation.
Fast amplification due to sequential phosphorylation.
95
MAP Kinase Cascades: Termination
MAPK phosphatases remove phosphates.
96
MAP Kinase Cascades: Examples
Growth factors like EGF and PDGF activate MAPK → cell division.
97
MAPK: What activates Ras?
Sos (GEF) recruited by Grb2 to RTK.
98
MAPK: What’s the final step in a MAPK cascade?
ERK phosphorylates transcription factors → gene activation.
99
Steroid Hormones and Receptors
Steroid hormones: Cortisol, estrogen, testosterone.
Hydrophobic → pass through membrane.
Bind to intracellular receptors (cytosolic or nuclear).
Receptor = transcription factor (binds DNA at hormone-response elements).
100
Steroid Hormones and Receptors: Examples
Cortisol binds receptor → enters nucleus → alters gene expression → anti-inflammatory response.
101
Steroid Signaling: Why do steroid hormones not need membrane receptors?
They’re hydrophobic and can diffuse through lipid bilayer.
102
Steroid Signaling: What happens when a steroid binds its receptor?
The receptor–hormone complex enters the nucleus and modifies gene transcription.
103
Selective Signaling Pathways in Animals: Diversity of responses
Same signal = different effects depending on cell type and receptor.
104
Selective Signaling Pathways in Animals: Acetylcholine example
Heart → slows beat (GPCR).
Salivary glands → secretion (GPCR).
Skeletal muscle → contraction (ion-channel receptor).
105
Selective Signaling Pathways in Animals:
Survival factors: Prevent apoptosis.
Growth factors: Stimulate division.
Differentiation factors: Promote specialization.
106
Selective Signaling Pathways in Plants: Plant hormones (phytohormones)
Auxin: Cell elongation, phototropism.
Cytokinin: Promotes cell division.
Ethylene: Gas hormone → fruit ripening.
107
Selective Signaling Pathways in Plants:
Mechanisms: Often receptor kinases and small signaling proteins.
Lecture example: Ethylene (gas) triggers gene expression changes → ripening.
108
Selective Pathways: Why do identical hormones cause different responses in different cells?
Each cell expresses different receptors and signaling proteins.
109
Selective Pathways: What gas hormone controls fruit ripening?
Ethylene.
110
The Cytoskeleton
Cytoskeleton = dynamic framework providing shape, support, and movement.
111
The Cytoskeleton: Three major types
Intermediate filaments (mechanical strength)
Actin filaments (microfilaments) (cell shape, movement)
Microtubules (organization, transport, mitosis)
112
The Cytoskeleton: Function
Cell structure, intracellular transport, division, motility.
113
Intermediate Filaments: Function
Structure: Rope-like, stable fibers (~10 nm).
Function: Resist mechanical stress.
114
Intermediate Filaments: Types
Keratin: Epithelial cells (skin, hair).
Lamins: Nuclear lamina (support nucleus).
Neurofilaments: Nerve axons.
115
Intermediate Filaments: Defects
Keratin mutation → fragile skin (blistering).
Lamin mutation → premature aging (progeria).
116
Intermediate Filaments: What’s the main function of intermediate filaments?
Provide mechanical stability and stress resistance.
117
Intermediate Filaments: Where are lamins found?
In the nuclear envelope, forming the nuclear lamina.
118
Actin Filaments (Microfilaments):
Structure: Thin (7 nm) filaments of actin monomers (ATP-bound).
Polarity: + (fast-growing) and − (slow-growing) ends.
119
Actin Filaments (Microfilaments): Dynamics
Treadmilling: Addition at + end, loss at − end.
ATP hydrolysis regulates stability.
120
Actin Filaments (Microfilaments): Associated proteins
Profilin, thymosin: Regulate monomer pool.
Capping proteins: Block elongation.
Arp2/3 complex: Nucleates branched actin (lamellipodia).
121
Actin Filaments (Microfilaments): Drugs
Cytochalasin & Latrunculin (depolymerize).
Phalloidin (stabilizes).
122
Actin Filaments (Microfilaments): Functions
Cell movement, muscle contraction, cytokinesis, shape changes.
123
Actin: What is treadmilling?
Actin adds at + end and depolymerizes at − end simultaneously.
124
Actin: What protein nucleates branched actin networks?
Arp2/3 complex.
125
Microtubules:
Structure: Hollow tubes (25 nm) of α/β-tubulin dimers.
Polarity: + end (grows), − end (anchored at centrosome).
Dynamic instability: Alternates between growth and shrinkage.
126
Microtubules: Drugs
Colchicine & nocodazole: Depolymerize.
Taxol: Stabilizes.
127
Microtubules: Functions
Organelle positioning.
Mitotic spindle formation.
Vesicle transport (motor proteins).
128
Microtubules: What causes microtubule dynamic instability?
GTP hydrolysis on β-tubulin
129
Microtubules: What drug stabilizes microtubules?
Taxol.
130
Motors on Cytoskeletal Tracks: Actin-based
Myosin family (muscle contraction, transport).
131
Motors on Cytoskeletal Tracks: Microtubule-based
Kinesin: Moves toward + end (outward transport).
Dynein: Moves toward − end (inward transport).
132
Motors on Cytoskeletal Tracks:
Motor proteins convert ATP → movement.
Functions:
Organelle movement, cilia/flagella beating, vesicle transport.
133
Motors: Which motor moves cargo toward the cell periphery?
Kinesin.
134
Motors: Which motor is responsible for cilia and flagella movement?
Dynein
135
Signal → Receptor → Effector → Response Logic Across Systems
General Principle
Every signaling pathway follows this core sequence:
Signal (ligand) → Receptor (sensor) → Effector (enzyme or channel) → Response (cellular change)
This logic is conserved in GPCRs, RTKs, and steroid receptors, though their mechanisms differ in speed, location, and signal type.
136
(GPCRs) Signal:
Small molecules or peptides (e.g., acetylcholine, epinephrine, odorants).
137
(GPCRs) Receptor:
7-transmembrane GPCR on plasma membrane.
138
(GPCRs) Effector:
Membrane-bound enzyme (adenylyl cyclase or phospholipase C).
139
(GPCRs) Response:
Changes in second messengers (cAMP, IP₃, DAG) → altered enzyme activity, ion flux, or gene expression.
140
(GPCRs) Step-by-step (from lecture & book):
Signal binds receptor → conformational change in GPCR.
G-protein activation: GPCR acts as a GEF, swapping GDP for GTP on Gα.
Effector activation:
-Gα–GTP activates adenylyl cyclase → ↑ cAMP → PKA activation.
-Or activates PLC → IP₃ + DAG → Ca²⁺ release + PKC activation.
Response: Rapid change (e.g., smooth muscle relaxation, heart rate slowing).
Termination: GTP hydrolysis → inactivation → receptor desensitized via arrestin.
141
(GPCRs) Example (lecture):
Acetylcholine in the heart: binds GPCR → opens K⁺ channels via βγ subunit → slows heartbeat.
Cholera toxin: locks Gsα “on” → continuous adenylyl cyclase activity → excessive cAMP → ion loss → diarrhea.
142
Receptor Tyrosine Kinases (RTKs): Signal
Growth factors or hormones (e.g., PDGF, EGF, insulin).
143
(RTKs): Effector
Small GTPase (Ras) or kinase cascades (MAPK pathway).
144
(RTKs): Receptor
RTK (single-pass transmembrane receptor with kinase domain).
145
(RTKs): Response
Long-term changes in gene expression (growth, division, survival).
146
(RTKs): Step-by-step
Signal binds → receptor dimerizes.
Autophosphorylation: Each receptor phosphorylates the other’s tyrosines.
Docking sites form: SH2-domain proteins (Grb2, PI3K, PLCγ) bind.
Ras activation: Grb2 recruits Sos (GEF) → activates Ras-GTP.
MAP kinase cascade:
Ras → Raf (MAPKKK) → MEK (MAPKK) → ERK (MAPK) → gene regulation.
Response: Transcription of genes for cell proliferation, survival, or differentiation.
Termination: Dephosphorylation by phosphatases or ubiquitination (c-Cbl) → degradation.
147
(RTKs): Example (lecture)
PDGF: binds RTK → activates MAPK → fibroblast proliferation in wound healing.
HER2: RTK overexpressed in breast cancer → uncontrolled signaling → treated with Herceptin (blocks dimerization).
Ras mutation (e.g., RasQ61K): prevents GTP hydrolysis → Ras always “on” → constant growth signal → cancer.
148
Steroid Hormone Receptors: Signal
Hydrophobic molecules (e.g., cortisol, estrogen, testosterone).
149
Steroid Hormone Receptors: Receptor
Intracellular (cytoplasmic or nuclear).
150
Steroid Hormone Receptors: Effector
The receptor itself (acts as a transcription factor).
151
Steroid Hormone Receptors: Response
Changes in gene transcription → slow, long-lasting effects.
152
Steroid Hormone Receptors: Step-by-step
Signal diffuses through membrane (hydrophobic).
Binds receptor → receptor changes shape → enters nucleus.
Effector = receptor–hormone complex binds DNA at hormone-response elements (HREs).
Response: Alters transcription of specific genes → new proteins made.
153
Steroid Hormone Receptors: Example (lecture)
Cortisol: crosses membrane → binds intracellular receptor → reduces inflammatory gene expression → used in creams for skin inflammation.
154
GPCR:
Signal type: Hydrophilic (peptides, neurotransmitters)
Receptor location: Plasma membrane (7TM)
Effector: G-protein → enzymes
Response speed: Fast (sec–min)
Example: Acetylcholine, adrenaline
155
RTK
Signal type: Growth factors, insulin
Receptor location: Plasma membrane (1TM)
Effector: Kinase cascade (Ras→MAPK)
Response speed: Intermediate (min–hr)
Example: PDGF, HER2
156
Steroid Receptor
Signal type: Hydrophobic hormones
Receptor location: Cytosol or nucleus
Effector: Transcriptional regulation
Response speed: Slow (hrs–days)
Example: Cortisol
157
Cytoskeletal Filaments: Structure, Dynamics, and Function
All three filament systems maintain cell shape, motility, and internal organization, but differ in composition and behavior.
158
Intermediate Filaments
Structure: Rope-like fibers of various proteins (10 nm diameter).
Subunits: Tissue-specific (keratin, lamins, vimentin).
Dynamics: Relatively stable (no polarity, no motor activity).
Function: Mechanical strength, especially under stress.
Example:
-Keratin: Resists stretching in skin; mutations → blistering.
-Lamins: Line nuclear envelope; mutations → progeria (premature aging).
159
Actin Filaments (Microfilaments)
Structure: Two-stranded helix of actin (7 nm).
Dynamics: Rapid turnover via ATP hydrolysis → treadmilling.
Polarity: + end (grows faster), − end (shrinks).
Associated Proteins:
Arp2/3 complex: nucleates branches.
Profilin, thymosin: control monomers.
Capping proteins: stop elongation.
Functions:
Cell motility (lamellipodia, filopodia).
Cytokinesis (contractile ring).
Muscle contraction (with myosin motors).
Lecture note: Yeast “shmoo” shape during mating relies on actin polymerization guided by signaling.
160
Microtubules
Structure: Hollow tubes of α/β-tubulin dimers (25 nm).
Polarity: + end grows, − end anchored at centrosome.
Dynamics: Dynamic instability due to GTP hydrolysis.
Functions:
Organelle positioning.
Mitotic spindle formation (chromosome movement).
Vesicle transport (tracks for kinesin and dynein).
Lecture examples: Labeled tubulin (EB1–GFP) used to visualize growing microtubule ends.
161
Motor Proteins
Myosin
Kinesin
Dynein
162
Myosin
Track: Actin
Direction: + end
Function: Contraction, transport
163
Kinesin
Track: Microtubule
Direction: + end
Function: Organelle/vesicle transport outward
164
Dynein
Track: Microtubule
Direction: − end
Function: Inward transport, cilia/flagella beating
165
Intermediate Filaments
Subunit: Varies (keratin, lamins)
Structure: Rope-like
Diameter: 10 nm
Polarity: None
Dynamics: Stable
Function: Mechanical strength
166
Actin Filaments
Subunit: Actin monomers
Structure: Two-stranded helix
Diameter: 7 nm
Polarity: Yes
Dynamics: Treadmilling (ATP)
Function: Movement, shape
167
Microtubules
Subunit: α/β-tubulin dimers
Structure: Hollow tube
Diameter: 25 nm
Polarity: Yes
Dynamics: Dynamic instability (GTP)
Function: Transport, mitosis
168
Cortisol
Type: Steroid hormone
Receptor: Intracellular receptor
Effector: Receptor–DNA binding
Cellular Response: Gene regulation → anti-inflammatory
169
PDGF
Type: Growth factor
Receptor: RTK
Effector: Ras → MAPK cascade
Cellular Response: Cell division → wound healing
170
Yeast pheromones
Type: Peptide
Receptor: GPCR
Effector: G-protein → actin remodeling
Cellular Response: Cell fusion (“shmoo” formation)
171
Acetylcholine
Type: Neurotransmitter
Receptor: GPCR or ion channel
Effector: K⁺ channels / G-proteins
Cellular Response: Heart rate ↓ or muscle contraction
172
HER2
Type: RTK
Receptor: Dimerization & autophosphorylation
Effector: MAPK signaling
Cellular Response: Growth → cancer if overactive
173
Ras
Type: Small GTPase
Receptor: Downstream of RTK
Effector: MAPK cascade
Cellular Response: Growth control; oncogenic when locked on
174
Ethylene (plants)
Type: Gas hormone
Receptor: Intracellular receptor (ER membrane)
Effector: Kinase cascade
Cellular Response: Fruit ripening, senescence
175
MAP Kinase Cascade: Roles of MAPKKK, MAPKK, and MAPK
Overview
The MAP kinase cascade (Mitogen-Activated Protein Kinase cascade) is a highly conserved signaling module used by Receptor Tyrosine Kinases (RTKs) — such as PDGF and EGF receptors — to transmit growth and differentiation signals from the cell surface to the nucleus.
It’s a three-tiered phosphorylation relay that amplifies and specifies signals.
176
MAPKKK (MAP Kinase Kinase Kinase) — Top Level
Full name: Mitogen-Activated Protein Kinase Kinase Kinase.
Typical example: Raf (in animal cells).
RTK → Ras–GTP → MAPKKK (Raf) → phosphorylates MAPKK.
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MAPKKK: Function
MAPKKK (Raf) phosphorylates and activates the next kinase in the pathway: MAPKK (MEK).
Uses ATP to add phosphate groups to serine/threonine residues on MAPKK.
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MAPKKK Activated by:
Ras–GTP, a small GTPase that’s activated by RTKs through the Grb2–Sos complex.
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MAPKK Function:
Dual-specificity kinase — it phosphorylates both threonine and tyrosine residues on the next kinase: MAPK (ERK).
Bridges cytoplasmic signals to nuclear targets.
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MMAPKK (MAP Kinase Kinase) — Middle Level
Full name: Mitogen-Activated Protein Kinase Kinase.
Typical example: MEK.
MAPKKK (Raf) → MAPKK (MEK) → phosphorylates MAPK (ERK).
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MAPKK Activated By:
MAPKKK (Raf).
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MAPK (MAP Kinase) — Final Level
Full name: Mitogen-Activated Protein Kinase.
Typical example: ERK (Extracellular signal-Regulated Kinase).
MAPKK (MEK) → MAPK (ERK) → gene expression changes.
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MAPK Activated by:
MAPKK (MEK) through dual phosphorylation.
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MAPK Function:
Once activated, MAPK translocates into the nucleus.
Phosphorylates various transcription factors (e.g., Elk-1, c-Fos) → activates gene expression.
Drives cellular outcomes such as proliferation, differentiation, or survival depending on context.
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MAP Cellular Effects
Short-term: Cytoplasmic enzyme activation (e.g., changes in metabolism).
Long-term: Nuclear transcription factor activation → gene expression → cell cycle progression.
Outcome depends on duration/intensity:
Transient MAPK activation → cell division.
Sustained MAPK activation → differentiation.
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MAP Regulation:
Activation: Sequential phosphorylation cascade (amplifies signal).
Termination:
MAPK phosphatases (MKPs) deactivate MAPK.
Ras–GTP hydrolyzed to Ras–GDP by GAP (turns pathway off).
Receptor internalization or degradation (e.g., c-Cbl ubiquitination).
BIS 104
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