Transcript Signal - My CCSD

Cell Communication
Signal-transduction pathway
 Cell signaling is the way that cells have to
respond to external stimuli
• Increase in temperature
• Fight of flight response
• Increased aerobic activity
 In each case, signal transduction pathways
are a key component in cell
communication
Signal-transduction pathway
Signal on a cell’s surface is converted into a
specific cellular response (binds to a receptor)
 Transduced = changed (protein changes shape)
 Cell junctions connect cell to cell

• Animal – gap junctions
• Plant – plasmodesmata

Direct Contact communication
• Cell –cell recognition, important in development and immune
system
Local signaling (short distance):
Paracrine (Local regulators - growth factors)
Synaptic (neurotransmitters – electrical to chemical
signal)
Long distance: hormones
Hormones
Endocrine System
 Travel through the
circulatory system
 Plant hormones =
growth regulators

Signal transduction
Stages of cell signaling

3 steps:
1. Reception: target cell detection
2. Transduction: single-step or series of changes
3. Response: triggering of a specific cellular response
Cell signaling
Step 1: Reception



Only Target Cells have the
receptor to bind the signal
molecule
Signal molecule is a ligand –
only binds to specific
molecules
Binding Usually changes
shape of receptor
• Activates receptor
• Causes aggregation
(clumping)
Step 2: Transduction
Usually multistep
 Amplifies response
 Mostly involves
proteins
 Relay molecules are
activated or
deactivated by
phosphorylation

Protein phosphorylation
Protein activity regulation (ON/OFF switch)
 Adding phosphate from ATP to a protein
activates proteins
 Enzyme: protein kinases (1% of all our genes)
 Reversal enzyme: protein phosphatases

• Removes the phosphate group
• Phosphorylation (activation) is only temporary
• Do not want the protein to be continually “ON”
Second messengers



Non-protein signaling
pathway
Ex: cyclic AMP (cAMP)
Ex: Glycogen breakdown
with epinephrine
• Epinephrine activates
receptor in membrane


Enzyme: adenylyl cyclase
G-protein-linked receptor
in membrane (guanosine
di- or tri- phosphate)
Cellular responses to signals
Cytoplasmic activity
regulation
 Cell metabolism
regulation
 Nuclear transcription
regulation

Plant and Animal Hormones
G-Protein-Linked Receptors
One example of secondary messenger is the Gprotein-linked receptors
 We will discuss the steps as an example
• Step 1: There is a “loosely” bound protein
bound to the cytosol side of the plasma
membrane (G-protein)
– Inactive if bound with GDP
– Active if bound with GTP (like ATP, but with
guanine)

G-Protein-Linked Receptors

Step 2: Another
protein acts as the
receptor protein
• When a chemical
signal binds to the
receptor protein,
changes shape and
allows the G-protein to
bind
G-Protein-Linked Receptors
Step 3: When the Gprotein binds to the
receptor protein, the
G-protein changes
shape
 Step 4: The new
conformation of the Gprotein causes GTP to
displace GDP (Gprotein is now active)

G-Protein-Linked Receptors
Step 5: Activate Gprotein now moves
free of the receptor
protein
 Step 6: The G-protein
moves along the
plasma membrane
until it binds to
another membrane
protein (enzyme)

G-Protein-Linked Receptors
Step 7: The G-protein
binds to the
membrane-bound
enzyme and changes
its shape (activates
the enzyme)
 Step 8: Allows the
enzyme to perform its
function (ie. Convert
ATP to cAMP)

G-Protein-Linked Receptors

Note: The G-protein is
also an enzyme
• It acts as a GTPase
enzyme (catalyzes the
breakdown of GTP to
GDP)
• This takes a little bit of
time so the G-protein
can be active for a
limited time
• This allows the G-protein
to “regenerate”