1. Cell Communication – Signal Transduction
External signal is received and converted
to another form to elicit a response

2. Signal Transduction & G Protein-coupled Receptors
Topics
Signal Trans.: From Extracellular Signal
to Cellular Response
Cell-Surface Receptors & Signal
Transduction Proteins
G Protein-coupled Receptors (GPCRs):
Structure and Mechanism
GPCRs That Regulate Ion Channels
GPCRs That Regulate Adenylyl Cyclase
GPCRs That Regulate Cytosolic Calcium

3. Learn the G protein cycle of reactions involved in GPCR signaling.
Learning Objectives
Learn the general properties of signaling molecules (ligands), cell-surface receptors, & intracellular signal transduction components.
Learn the G protein cycle of reactions involved in GPCR signaling.
Learn the epinephrine receptor signal trans pathway used for control of glycogen degradation.
Learn about the GPCR-stimulated IP3/DAG signaling pathway.

4. General Principles of Signal Transduction
refers to the overall process
of converting extracellular
signals into intracellular
responses .
Key players in signal transduction are :
signaling molecules, receptors,
signal transduction proteins
second messengers, and
effector proteins.

5. Cells respond to signals by changing the activity of existing enzymes (fast) and/or the levels of expression of enzymes and cell components (slower) by gene regulation (Steps 7a & 7b).
Receptors and signal transduction systems have evolved to detect and respond to hormones, growth factors, drugs & neurotransmitters.

6. Intercellular Communication

7. Intercellular Communication
Communication between cells requires:
ligand: the signaling molecule
receptor protein: the molecule to which the receptor binds
-may be on the plasma membrane or within the cell

9. Structure and function of receptors
Globular proteins acting as a cell’s ‘letter boxes’
Located mostly in the cell membrane
Receive messages from chemical messengers coming from other cells
Transmit a message into the cell leading to a cellular effect
Different receptors specific for different chemical messengers
Each cell has a range of receptors in the cell membrane making it responsive to different chemical messengers

10. Mechanism
Receptors contain a binding site (hollow or cleft in the receptor surface) that is recognised by the chemical messenger
Binding of the messenger involves intermolecular bonds
Binding results in an induced fit of the receptor protein
Change in receptor shape results in a ‘domino’ effect
Domino effect is known as Signal Transduction, leading to a chemical signal being received inside the cell
Chemical messenger does not enter the cell. It departs the receptor unchanged and is not permanently bound

11. Overall process of receptor/messenger interaction
Signal transduction
Binding interactions must be:
- strong enough to hold the messenger sufficiently long
for signal transduction to take place
- weak enough to allow the messenger to depart
Implies a fine balance
Drug design - designing molecules with stronger binding interactions results in drugs that block the binding site - antagonists

12. Messenger binding Bonding forces Ionic H-bonding van der Waals
Example:
vdw
interaction
Phe
H-bond
Binding site
Ser O H
ionic
bond
Asp
CO2
Receptor

13. How does the Binding Site Change Shape? Substrate binding
Bonding forces
Induced fit - Binding site alters shape to maximise intermolecular bonding
Phe
Ser O H
Asp
CO2
Phe
Ser O H
Asp
CO2
Induced
Fit
Intermolecular bonds not optimum length for maximum binding strength
Intermolecular bond lengths optimised

14. External signals are converted to
Internal Responses
Cells sense and respond to the environment
Prokaryotes: chemicals
Humans:
light - rods & cones of the eye
sound – hair cells of inner ear
chemicals in food – nose & tongue
Cells communicate with each other
Direct contact
Chemical signals

15. General principles 1. Signals act over different ranges.
2. Signals have different chemical natures.
3. The same signal can induce a
different response in different cells.
4. Cells respond to sets of signals.
5. Receptors relay signals via
intracellular signaling cascades.

16. Cells detect signal & respond
1º messenger
Cells
detect
signal & respond
Effector
Enzymes
Target
Enzymes
2º messengers
Signal transduction: ability of cell to translate
receptor-ligand interaction into a change in behavior
or gene expression

17. Relay molecules in a signal transduction pathway
Primary
Messenger
Secondary
Messengers
Target
Enzymes
EXTRACELLULAR
FLUID
Receptor
Signal
molecule
Relay molecules in a signal transduction pathway
Plasma membrane
CYTOPLASM
Activation
of cellular
response
Reception 1
Transduction 2
Response 3
Cascade Effect

18. activates 100 control factors
Each protein in a signaling pathway
Amplifies the signal by activating multiple copies of the next component in the pathway
- activates an enzyme activity, processes 100 substrates /second
primary signal
Primary enzyme activates 100 target enzymes
Each of the 100 enzymes activates an
additional 100 downstream target enzymes
Each of the 10,000 downstream targets
activates 100 control factors
so rapidly have
1,000,000 active control factors

19. 1 102 104 105 106 108 A Signal Cascade amplification
Glucose-1-phosphate (108 molecules)
Glycogen
Active glycogen phosphorylase (106)
Inactive glycogen phosphorylase
Active phosphorylase kinase (105)
Inactive phosphorylase kinase
Inactive protein kinase A
Active protein kinase A (104)
ATP
Cyclic AMP (104)
Active adenylyl cyclase (102)
Inactive adenylyl cyclase
Inactive G protein
Active G protein (102 molecules)
Binding of epinephrine
to G-protein-linked receptor
(1 molecule)
Transduction
Response
Reception
A Signal
Cascade
amplification 1
102
104
105
106
108

20. Receptors relay signals via intracellular SIGNALING CASCADES
amplification

21. Main Types of Receptors
ION CHANNEL RECEPTORS
G-PROTEIN-COUPLED RECEPTORS
KINASE-LINKED RECEPTORS
INTRACELLULAR RECEPTORS

22. ion-channel-linked Trimeric G-protein-linked enzyme-linked
Cell-surface receptors -large &/or hydrophilic ligands
ion-channel-linked
Trimeric
G-protein-linked
enzyme-linked
(tyrosine kinase)

23. Receptor protein is part of an ion channel protein complex
Signal transduction
Control of ion channels
Receptor protein is part of an ion channel protein complex
Receptor binds a messenger leading to an induced fit
Ion channel is opened or closed
Ion channels are specific for specific ions (Na+, Ca2+, Cl- , K+)
Ions flow across cell membrane down concentration gradient
Polarises or depolarises nerve membranes
Activates or deactivates enzyme catalysed reactions within a cell

24. Nerve Cell communication
Ion channel receptors
Cellular response
Gate open
Gate close
Ligand-gated
ion channel receptor
Plasma Membrane
Signal molecule (ligand)
Gate closed
Ions
Examples:
Muscle Contraction
Nerve Cell communication

25. Review: Na+ Cl- - + Remember the Na+/K+ ATPase (Na+/K+ pump)?
[Na+] inside ~10mM; outside ~150mM
[K+] inside ~150mM; outside ~5mM
cell has membrane potential ~ -60mV
-60mV K+ A-
Na+
Cl- - +

26. Intercellular Communication
How ?

27. Intercellular Communication
There are four basic mechanisms for cellular communication:
1. direct contact
2. paracrine signaling
3. endocrine signaling
4. synaptic signaling

28. Intercellular Communication
Direct contact – molecules on the surface of one cell are recognized by receptors on the adjacent cell

29. Intercellular Communication
Paracrine signaling – signal released from a cell has an effect on neighboring cells
local
ex. nitric oxide,
histamines,
prostaglandins

30. Intercellular Communication
Endocrine signaling – hormones released from a cell affect other cells throughout the body
long distance
ex. Estrogen, Thyroxine, GH
Epinephrine ….

31. Intercellular Communication
Synaptic signaling – nerve cells release the signal (neurotransmitter) which binds to receptors on nearby cells

32. Intercellular Communication
When a ligand binds to a receptor protein, the cell has a response.
signal transduction: the events within the cell that occur in response to a signal
Different cell types can respond differently to the same signal.

33. Receptor Types There are 3 subclasses of membrane receptors:
1. channel linked receptors – ion channel that opens in response to a ligand
2. enzymatic receptors – receptor is an enzyme that is activated by the ligand
3. G protein-coupled receptor – a G-protein (bound to GTP) assists in transmitting the signal

35. Intercellular Communication
A cell’s response to a signal often involves activating or inactivating proteins.
Phosphorylation is a common way to change the activity of a protein.
protein kinase – an enzyme that adds a phosphate to a protein
phosphatase – an enzyme that removes a phosphate from a protein

37. Signal Transduction Components: Kinases/Phosphatases
Proteins that participate in intracellular signal transduction fall into two main classes--protein kinases/phosphatases and GTPase switch proteins. Kinases use ATP to phosphorylate amino acid side-chains in target proteins. Kinases typically are specific for tyrosine or serine/threonine sites. Phosphatases hydrolyze phosphates off of these residues. Kinases and phosphatases act together to switch the function of a target protein on or off .

38. Kinases - Phosphorylation
Phosphatase - Dephosphorylation
Tyrosine-OH Tyr-Kinases
Serine-OH Ser/Thr-Kinases
Threonine-OH
„dual specificity“ Kinases

40. There are about 600 kinases and 100 phosphatases encoded in the human genome. Activation of many cell-surface receptors leads directly or indirectly to changes in kinase or phosphatase activity. Note that some receptors are themselves kinases (e.g., the insulin receptor).

42. Growth hormone receptor
Tetrameric complex constructed in presence of growth hormone GH
GH binding
&
dimerisation OH
Binding
of kinases HO OP PO
ATP
ADP
Activation and
phosphorylation
GH receptors
(no kinase activity) OH HO
kinases
Kinase active site
opened by induced fit
Growth hormone binding site
Kinase active site

43. Intracellular receptors
Chemical messengers must cross cell membrane
Chemical messengers must be hydrophobic
Example-steroids and
steroid receptors
CO2H
H2N
Steroid
binding region
Zinc
DNA binding region
(‘zinc fingers’)
Zinc fingers contain Cys residues (SH)
Allow S-Zn interactions

44. Intracellular Receptors
steroid hormones
-have a nonpolar, lipid-soluble structure
-can cross the plasma membrane to a steroid receptor
-usually affect regulation of gene expression
An inhibitor blocks the receptor from binding to DNA until the hormone is present.

45. Intracellular receptor Mechanism
Co-activator
protein
Receptor
Cell
membrane
DNA
Receptor-ligand
complex
Dimerisation
Messenger
1. Messenger crosses membrane
2. Binds to receptor
3. Receptor dimerisation
5. Complex binds to DNA
6. Transcription switched on or off
7. Protein synthesis activated or inhibited
4. Binds co-activator protein

46. Intracellular Receptors
A steroid receptor has 3 functional domains:
1. hormone-binding domain
2. DNA binding domain
3. domain that interacts with coactivators to affect gene expression

48. END PART I