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Overview: The Cellular Internet
• Cell-to-cell communication is essential for
organisms
• Biologists have discovered some universal
strategies and mechanisms of cellular
regulation
• Response is determined by combined effects
of multiple signals
Yeast Mating-a
classic system
1
Exchange
of mating
factors
 factor
Receptor

a
a factor
Yeast cell,
mating type a
2
Mating
Yeast cell,
mating type 

a
An example of
cell signalling
3
New a/
cell
a/
• Yeast Mating System Exemplifies Cell
Communication
• Aka Signal Transduction
• Simple but all the key features are present
• Reception, Transduction, Response
• A signal
transduction
pathway is a series
of steps by which a
signal on a cell’s
surface is converted
into a specific cellular
response
• Pathway similarities suggest that ancestral
signaling molecules evolved in prokaryotes and
were modified later in eukaryotes
• The concentration of signaling molecules
allows bacteria to detect population density
• Quorum-sensing in Vibrio fischerii
When cells are
touching
signalling
is easy
Plasma membranes
Gap junctions
between animal cells
(a) Cell junctions
(b) Cell-cell recognition
Plasmodesmata
between plant cells
• In many other cases, cells must share
information with cells they are not touching
• They use (1) local regulators, messenger
molecules that travel only short distances
• And (2) chemicals called hormones for longdistance signaling
Local signaling in animals is either paracrine or synaptic
Local signaling
Electrical signal
along nerve cell
triggers release of
neurotransmitter
Target cell
Secreting
cell
Local regulator
diffuses through
extracellular fluid
(a) Paracrine signaling
Neurotransmitter
diffuses across
synapse
Secretory
vesicle
Target cell
is stimulated
(b) Synaptic signaling
Both are short-range but synaptic signaling occurs only in the
nervous system
Long-distance signaling usually involves different organs communicating
Endocrine cell
Blood
vessel
Hormone travels
in bloodstream
to target cells
Target
cell
(2) Hormonal signaling
The messenger molecules are called hormones
There are three stages in cell signaling
CYTOPLASM
EXTRACELLULAR
FLUID
Plasma membrane
1 Reception
2 Transduction
3 Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction pathway
Signaling
molecule
Reception
Transduction
Response
RECEPTION
TRANSDUCTION
RESPONSE
• The binding between a signal molecule
(ligand) and receptor is highly specific
• A shape change in a receptor is often the initial
transduction of the signal
• Most signal receptors are plasma membrane
proteins
• 3 examples from many possible
Reception- Example I
• A G protein-coupled receptor is a plasma
membrane receptor that works with the help of
a G protein
• The G protein acts as an on/off switch: If GDP
is bound to the G protein, the G protein is
inactive
• GTP/GDP are chemically very similar to
ATP/ADP but contain Guanine not Adenine
Mechanism of G protein coupled reception
Plasma
membrane
G protein-coupled
receptor
Activated
receptor
Signaling molecule
GDP
CYTOPLASM
GDP
Enzyme
G protein
(inactive)
GTP
2
1
Activated
enzyme
GTP
GDP
Pi
Cellular response
3
4
The relay protein is called a “G Protein”
Inactive
enzyme
Reception- Example II
• Receptor tyrosine kinases are membrane
receptors that attach phosphates to tyrosines
• Has intra- and extracellular domains, also
membrane domain
• A receptor tyrosine kinase can trigger multiple
signal transduction pathways at once
• Kinase is an enzyme that attaches a phosphate
to a substrate
Ligand-binding site
Signaling
molecule (ligand)
Signaling
molecule
 Helix
Tyrosines
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosine
kinase proteins
CYTOPLASM
Dimer
1
2
Activated relay
proteins
Tyr
Tyr
Tyr
Tyr
P Tyr
P Tyr
Tyr
Tyr
P
6 ATP
Activated tyrosine
kinase regions
6 ADP
Tyr
Tyr
P Tyr
Tyr
P
Tyr
P Tyr
P Tyr
Tyr
P
P
P
P
Tyr P
Tyr
Fully activated receptor
tyrosine kinase
Inactive
relay proteins
3
4
Cellular
response 1
Cellular
response 2
Reception- Example III
• A ligand-gated ion channel receptor acts as a
gate when the receptor changes shape
• When a signal molecule binds as a ligand to
the receptor, the gate allows specific ions, such
as Na+ or Ca2+, through a channel in the
receptor
Ligandgated
ion
channel
1 Signaling
molecule
(ligand)
Gate
closed
Ligand-gated
ion channel receptor
2
Ions
Plasma
membrane
Gate open
Cellular
response
3
Gate closed
Reception
Signal
Binding of epinephrine to G protein-coupled receptor (1 molecule)
Transduction:
Transduction
an example
of a key
type of
transduction
pathway
and
response
An example
of a
response
(Do not
memorize this
cascade)
Inactive G protein
Active G protein (102 molecules)
Inactive adenylyl cyclase
Active adenylyl cyclase (102)
This type of
signal
transduction
pathway is
called a
cascade
ATP
Cyclic AMP (104)
Inactive protein kinase A
Active protein kinase A (104)
Inactive phosphorylase kinase
Active phosphorylase kinase (105)
Inactive glycogen phosphorylase
Active glycogen phosphorylase (106)
Response
Glycogen
Glucose-1-phosphate
(108 molecules)
Cyclic AMP
is the key
inside the
cell.
Adenylyl cyclase
Phosphodiesterase
Pyrophosphate
P
ATP
Pi
cAMP
Cyclic AMP is the “second messenger”
AMP
Ethylene Response in plants
Identify: Reception, Transduction, Response