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Cell Communication
Lecture 4
Fall 2008
How do cells communicate?
Fig. 11.3
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How do cells communicate?
Signal Transduction
Pathways
A mechanism linking a
mechanical or
chemical stimulus to a
specific cellular
response
Fig. 11.2
Local and Long Distance Signaling:
An Overview
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Local signaling – direct contact
• Cell junctions connect cytoplasm
• Cell-cell recognition
– Interactions between cell surface molecules (e.g.
glycoproteins, glycolipids)
Fig. 11.4
Local and Long Distance Signaling:
An Overview
Local signaling – messenger molecules
• Local regulators
– Messenger molecules that travel only a
short distance
• Paracrine signaling
– Discharge of local regulators by one cell
(secreting cell)
– Acts on cells nearby
• Synaptic signaling
– Release of neurotransmitter molecules
from a nerve cell to a target cell
Fig. 6.21
Fig. 11.5
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Local and Long Distance Signaling:
An Overview
Long distance signaling
• Endocrine signaling
– Hormones released into
bloodstream
– Travel to target cell
Fig. 11.5
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Overview of Cell Signaling
Three stages
• Reception
• Transduction
• Response
• Reception
– Target cell detects signaling
molecule
– Signaling molecule binds to
receptor protein
• Cell surface
• Inside cell
Fig. 11.6
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Overview of Cell Signaling
• Transduction
– Conversion of signal to a form that can bring about a
cellular response
– May be several steps with intermediaries: signal
transduction pathway
• relay molecules
• Response
Fig. 11.6
Reception
High specificity
• Receptor proteins
– On plasma membrane
• G protein coupled receptors
• Receptor tyrosine kinases
• Ion channel receptors
– Inside cell
• Ligand
– A molecule that binds specifically with another
molecule
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Transduction
Signal Transduction Pathways
• A mechanism linking a mechanical or
chemical stimulus to a specific cellular
response
• Reception of signal by receptor protein
on plasma membrane
Signal Transduction Pathways
Phosphorylation
activates a protein
• Phosphate group is
transferred from ATP
(adenosine
triphosphate) to the
protein
• Protein changes
conformation
• Relay molecules often
protein kinases
– Protein that transfers
phosphate groups from
ATP to another protein
Fig. 11.9
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Signal Transduction Pathways
• Phosphorylation
cascade
– a series of different
molecules in a pathway
are phosphorylated in
turn, and add phosphate
group to next molecule in
pathway
• Dephosphorylation
returns protein to
inactive form
– Protein phosphatases
• enzymes that rapidly
remove phosphate groups
from a protein
– Turns off / resets the
pathway
Fig. 11.9
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Response
• Occurs in nucleus or
cytoplasm
• Response in nucleus
– Often protein synthesis
– Activates transcription
factor to turn gene “on”
(or off)
• Start/stop transcription of
RNA from DNA
Fig. 11.14
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Response
• Response in cytoplasm
– E.g., breakdown of
glycogen into glucose
Fig. 11.15
Signal Amplification
• At each step in the cascade,
the number of activated
products is greater than the
proceeding step
• Proteins remain in activated
form long enough to
process many other
molecules before becoming
inactive
Fig. 11.15
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Transduction when using Intracellular
Receptors
• Intracellular receptor proteins
– Cytoplasm
– Nucleus
• Signal molecule must cross plasma membrane
– Hydrophobic or small
• E.g., steroids hormones, thyroid hormones
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Transduction when using Intracellular
Receptors
• Signal molecule binds with
receptor protein in cell
– Becomes active
• Receptor protein often a
transcription factor, or
activates a transcription factor
– Start/stop transcription of RNA
from DNA
– Results in protein synthesis
Fig. 11.8
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Specificity of Cell Signaling
• Response of a cell to a signal depends on the
type of proteins found in the target cell
Fig. 11.17
Signal Transduction in Yeast
• How do yeast grow towards each other?
• Read Fig. 11.16 Inquiry: How do signals
induce directional cell growth in yeast. Focus
on methodology.
Fig. 11.16
Fig. 11.2
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