Signaling molecules &
Cell surface receptors

Cell Communication: An
Overview
 Cells communicate with one another through

Direct channels of communication

Specific contact between cells

Intercellular chemical messengers

Receptor animation
 Cell surface receptors

Cell Communication
 To survive, cells must

Communicate with their neighbors

Monitor environmental conditions

Respond appropriately

Cell Signaling

Apoptosis
Fig. 7-1, p. 140

Signals relayed between cells
1.
2.
3.


Direct intercellular signaling
Cell junctions allow signaling molecules to pass from one cell to another
Contact-dependent signaling
Some molecules are bound to the surface of cells and serve as signals to cell coming in contact with them

Autocrine signaling
Cells secrete signaling molecules that bind to their own cell surface or neighboring cells of the same type
8

Signals relayed between cells
3.

Paracrine signaling
Signal does not affect cell secreting the signal but does influence cells in close proximity (synaptic signaling)
4.

Endocrine signaling
Signals (hormones) travel long distances and are usually longer lasting
9

10

Cell Signaling

Signaling Molecules
 Small molecules
 Peptides
 Proteins
 LIGANDS

Receptor affinity
 High affinity

Low concentration of ligand; most receptors are occupied
 Low affinity

High concentration of ligand for most rectors to be occupied

Receptor affinity
 Dissociation constant K d

Measures the affinity of the receptor-ligand complex

The concentration of ligand at which half the receptors are occupied

Example
 Erythroid progenitor cell ~1000 surface receptors for erythropoietin (Epo)
 Only 100 receptors need to bind Epo to induce cell division
 Max cellular response less than K d

Vasoconstriction occurs when epinephrine (adrenaline) binds to the a
-adrenergic receptor on vascular smooth muscle cells. One approach to treating high blood pressures is to administer competitive inhibitors that bind to the a
-adrenergic receptor. The K d for binding of epinephrine to this receptor is ~0.6 m
M. Which of the following compounds might be good candidate drugs for high blood pressure? K d for binding to the a
-adrenergic
33% 33% 33% receptor are shown
.
1.
Compound A:
K d
= 1pM
2.
3.
Compound B:
K d
= 0.6 m
M
Compound C:
K d
= 60 m
M
1 2 3

Intercellular Chemical
Messengers
 Controlling cell

Releases signal molecule that causes response of target cells
 Target cell processes signal in 3 steps:

Reception, transduction, response
 Signal transduction

Series of events from reception to response

3 stages of cell signaling
1.
2.
3.


Receptor activation
Signaling molecule binds to receptor

Signal transduction
Activated receptor stimulates sequence of changes- signal transduction pathway
Cellular response
Several different responses
• Alter activity of 1 or more enzymes
• Alter structural protein function
• Change gene expression – transcription factor
21

Signal Transduction
Fig. 7-2, p. 142

23

 animation
Amazing cells

Which of the following best describes a signal transduction pathway?
1.
2.
3.
Binding of a signal molecule to a cell protein
Catalysis mediated by an enzyme
Series of changes in a series of molecules resulting in a response
33% 33% 33%
1 2 3

a. Reception by a cell-surface receptor
Polar (hydrophilic) signal molecule
Activation
Receptor embedded in plasma membrane
Target cell
Plasma membrane
Polar signal molecules cannot pass through the plasma membrane. In this case they bind to a receptor on the surface.
Fig. 7-3a, p. 142

b. Reception by a receptor within cell
Nonpolar
(hydrophobic) signal molecule
Activation
Receptor within cell
Nonpolar signal molecules pass through the plasma membrane and bind to their receptors in the cell.
Fig. 7-3b, p. 142

Intracellular receptors
 Some receptors are inside the cell
 Estrogen example


Passes through membrane and binds to receptor in nucleus
Dimer of estrogen•receptor complexes binds to DNA
• Transcription factors regulate transcription of specific genes
28

29

Cell Communication Systems with Surface Receptors
 Peptide hormones and neurotransmitters

Primary extracellular signal molecules recognized by surface receptors in animals
 Surface receptors

Integral membrane glycoproteins
 Signaling molecule

Bound by a surface receptor

Triggers response pathways within the cell

Surface Receptors
 Cell communication systems based on surface receptors have 3 components:
(1) Extracellular signal molecules
(2) Surface receptors that receive signals
(3) Internal response pathways triggered when receptors bind a signal

Peptide Hormones
 Peptide hormones

Small proteins
 Growth factors

Special class of peptide hormones

Affect cell growth, division, differentiation

Neurotransmitters
 Neurotransmitters include

Small peptides

Individual amino acids or their derivatives

Chemical substances

Surface Receptors
 Surface receptors

Integral membrane proteins

Extend entirely through the plasma membrane
 Binding of a signal molecule

Induces molecular change in the receptor that activates its cytoplasmic end

Ligand
 Signaling molecule
 Binds noncovalently to receptor with high degree of specificity
 Binding and release between receptor and ligand relatively rapid
 Ligands alter receptor structureconformational change
 Once a ligand is released, the receptor is no longer activated
35

Response of Surface Receptor
Fig. 7-4, p. 143

37

Cellular Response Pathways
 Cellular response pathways

Operate by activating protein kinases
 Protein kinases add phosphate groups

Stimulate or inhibit activities of target proteins, producing cellular response

39

Cellular Response Pathways
 Protein phosphatases

Reverse response

Remove phosphate groups from target proteins
 Receptors are removed by endocytosis

When signal transduction is finished

Phosphorylation
Fig. 7-5, p. 144

Amplification
 Each step of a response pathway catalyzed by an enzyme is amplified

Each enzyme activates hundreds or thousands of proteins that enter next step in pathway
 Amplification

Allows full cellular response when few signal molecules bind to receptors

Amplification
Fig. 7-6, p. 145

44

1.
Which of the following steps in an intracellular signaling pathway amplifies the signal?
Synthesis of a secondary messenger
25% 25% 25% 25%
2.
Activation of a protein kinase
3.
4.
Binding of ligand to receptor
1 & 2
1 2 3 4

In reactions mediated by protein kinases, what does phosphorylation of successive proteins do to drive the reaction?
1.
2.
3.
Make functional
ATP
Change a protein from its inactive to active form
Change a protein from its active to inactive form
33% 33% 33%
1 2 3

1.
2.
3.
Which of the following is an example of signal amplification?
catalysis of many cAMP molecules by several simultaneously binding signal molecules activation of 100 molecules by a single signal binding event activation of a specific gene by a growth factor
33% 33% 33%
1 2 3

Cell surface receptors
1.




Enzyme-linked receptors
Found in all living species
Extracellular domain binds signal
Causes intracellular domain to become functional catalyst
Most are protein kinases
49

Receptor Tyrosine Kinases
 Receptor tyrosine kinases bind signal molecule

Protein kinase site becomes active

Adds phosphate groups to tyrosines in the receptor itself, and to target proteins
 Phosphate groups added to cytoplasmic end of receptor are recognition sites for proteins activated by binding to the receptor

Protein Kinase Activity
Fig. 7-7, p. 146

G-Protein –Coupled Receptors
 G proteins: Key molecular switches in second-messenger pathways
 Two major G-protein –coupled receptor response pathways involve different second messengers

G-Protein-Coupled Receptors
 G-protein-coupled receptors activate pathways

Binding of the extracellular signal molecule
(first messenger) activates a site on the cytoplasmic end of the receptor

G-protein-coupled receptors
 Signals binding to cell surface are first messenger
 Many signal transduction pathways lead to production of second messengers


Relay signals inside cells
Examples
• cAMP
• Ca 2+
• Diacylglycerol and inositol triphosphate
54

G-Protein-Coupled Receptors
Fig. 7-8, p. 147

Now what?
 How does binding a signaling molecule induce a cellular response?

57

7-pass transmembrane receptor
+ G protein

G proteins are trimeric = 3 subunits

Inactive State
Receptor binds ligand
G-protein associates with receptor
GTP is exchanged for
GDP a subunit and
 subunit activated

a

What next?

What are some target proteins?

G-Protein Activation
 Activated receptor turns on a G protein , which acts as a molecular switch
 G protein

Active when bound to GTP

Inactive when bound to GDP

Active G Protein
 Active G protein

Switches on the effector of the pathway
(enzyme that generates second messengers )
 Second messengers

Small internal signal molecules

Activate the protein kinases of the pathway

Response Pathways
Fig. 7-9, p. 147

Second Messengers: cAMP
 1st of two major pathways triggered by Gprotein-coupled receptors
 Effector (adenylyl cyclase) generates cAMP as second messenger
 cAMP activates specific protein kinases

cAMP Receptor-Response
Pathways
Fig. 7-10, p. 148

cAMP
Fig. 7-11, p. 148

Adenylyl cyclase
ATP
Pyrophosphate cAMP
(second messenger)
Phosphodiesterase
AMP
Fig. 7-11, p. 148

70

 One effect of cAMP is to activate protein kinase A (PKA)
 Activated catalytic PKA subunits phosphorylates specific cellular proteins
 When signaling molecules no longer produced, eventually effects of PKA reversed
71

72

cAMP has 2 advantages
1.
Signal amplification

Binding of signal to single receptor can cause the synthesis of many cAMP that activate
PKA, each PKA can phosphorylate many proteins
2.
Speed

In one experiment a substantial amount of cAMP was made within 20 seconds after addition of signal
73

Now what?
 How does binding a signaling molecule induce a cellular response?

Membrane-bound Enzymes
Second messengers

Adenylate cyclase

a
Adenylate Cyclase
 cAMP

Adenylyl cyclase
Always “on” so cAMP is quickly broken down

cAMP activates cAMP-dependent protein kinase
(A-kinase)

Regulates the activity of the target protein

Activated A-kinase can modulate gene regulation

Example of cell regulation by an increase in cAMP levels
 Fight or flight response- muscle cells

When an animal is frightened or stressed the adrenal gland releases epinephrine into the bloodstream

Epinephrine example
 Fight-or-flight hormone
 Different effects throughout body
 Stimulates heart muscle cells to beat faster
 Caffeine inhibits phosphodiesterase


Enzyme removes cAMP once a signaling molecule is no longer present
Inhibition causes cAMP to persist for longer so heart beats faster
84

85


-adrenergic receptors
 Circulating epinephrine binds

-adrenergic receptors on muscle and liver cells
 Liberates glucose and fatty acids
 animation

A-kinase phosphoryates an enzyme to break down glycogen to release glucose

88

β- adrenergic receptors
 β- adrenergic receptors are GPCR
 Different types are coupled to different G proteins
 Gs (stimulatory) G proteins activate adenylyl clyclase
 Gi (inhibitory) G proteins inhibit adenylyl cyclase ( α1 and α2)

Phophoprotein phosphatase (PP)

Pathway Controls
 cAMP pathways are balanced by reactions that eliminate second messengers

Stopped by protein phosphatases that continually remove phosphate groups from target proteins

Stopped by endocytosis of receptors and their bound extracellular signals

92

Grb2-
SH2 adaptor protein
Sos -
Guanine nucleotide exchange factor -
Ras-GEF

1. Receptor binds ligand
2. Tyrosines phosphorylated
3. Grb2/Sos bind pY
4. Sos exchanges
GTP for
GDP

Activated Ras recruits
Raf to the plasma membrane -
Raf - protein kinase that initiates the MAP kinase cascade

Ras and G α (trimeric G proteins)
 Similar structure and function and ubiquity in eukaryotic cells suggest a single type of
GTPase originated early in evolution
 Gene encoding this ancestral protein duplicated and evolved > 100 different intracellular switch proteins

Active GTP
 Ras-GTP active conformation

Ras-mitogen-activated protein kinase (MAPK)
 Similar to cAMP signaling cascade - both provide pathways by which extracellular signals can influence gene expression

Cascade of Protein Kinases
 Active Ras activates Raf (ser/thr kinase)
 Raf activates MEK
 MEK activates MAPK
 MAPK activates other proteins
(transcription factors)

Gene Regulation: Ras
 Some pathways in gene regulation link certain receptor tyrosine kinases to a specific G protein (Ras)
 When the receptor binds a signal molecule, it phosphorylates itself

Adapter proteins then bind, bridging to and activating Ras

Activated Ras
 Activated Ras turns on the MAP kinase cascade
 Last MAP kinase in cascade phosphorylates target proteins in the nucleus

Activates them to turn on specific genes
 Many of these genes control cell division

Mutations
 Mutated systems can turn on the pathways permanently, contributing to progression of some forms of cancer

The importance of Ras
 Early 1980’s, several human tumors were found to contain a mutant of Ras
 Ras mutations are found in over 30% of all human tumors
 Mutation in the Ras gene that lead to tumor formation prevent the protein from hydrolzying the bound GTP back to GDP

Ras always “on”

The importance of Ras
 Ras is a small G protein held to the inner surface of the plasma membrane by a lipid group that is embedded in the inner leaflet of the bilayer
 Ras is a single subunit G protein
 Cycles between an active [GTP-bound] form and an inactive [GDP-bound] form
 Activates a kinase cascade (MAP
Kinase)

Gene Regulation
Fig. 7-13, p. 151

Gene Activation:
Steroid Hormone Receptors
Fig. 7-14, p. 152

Cell Response
 Cell response to a steroid hormone

Depends on whether it has an internal receptor for the hormone
 Type of response within the cell

Depends on the genes that are recognized and turned on by an activated receptor

Cross-Talk
 Cell signaling pathways communicate with one another to integrate responses to cellular signals
 May result in a complex network of interactions between cell communication pathways

Cross-Talk
Fig. 7-15, p. 153

Core signaling
Pathways
Many target proteins