AP Biology – Ms. Whipple
BCHS
The yeast,
Saccharomyces
cerevisiae, has two
mating types, a and 
 Cells of different
mating types locate
each other via
secreted factors
specific to each type

1. Exchange
of mating
factors

a
1
2. Mating
3. New a/ cell

a
a/
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
 Signal transduction pathways convert signals
on a cell’s surface into cellular responses
 Pathway similarities in plants & bacteria
suggest that ancestral signaling molecules
evolved in prokaryotes and were modified
later in eukaryotes
 The
concentration of signaling molecules
allows bacteria to sense local population
density – Quorum Sensing
 Quorum Sensing allows bacterial populations
to coordinate their behaviors so that they
can carry out activities that are only
productive when performed by a given
number of cells in synchrony.
 One example of this is a Biofilm, an
aggregation of bacterial cells that derive
nutrition from the surface they are on.
(a) Cell junctions
Gap junctions
between animal cells
(b) Cell-cell recognition
Plasmodesmata
between plant cells
 In
many cases, animal cells communicate
using local regulators, messenger molecules
that travel only short distances
 The ability of a cell to respond to a signal
depends on whether or not it has a receptor
specific to that signal. No Receptor = No
Response
Electrical signal
along nerve cell
triggers release of
neurotransmitter.
Local signaling
Target cell
Secreting
cell
Local regulator
diffuses through
extracellular fluid.
(a) Paracrine signaling
Secretory
vesicle
Neurotransmitter
diffuses across
synapse.
Target cell
is stimulated.
(b) Synaptic signaling
Endocrine cell
Hormone travels
in bloodstream.
In long-distance
signaling, plants
and animals use
chemicals called
hormones
Blood
vessel
Target cell
specifically
binds
hormone.
(c) Endocrine (hormonal) signaling
 Epinephrine
stimulates the breakdown of
Glycogen in liver and muscle cells.
 This is useful during a “fight or flight”
response because it gives immediate energy
to muscles for fighting or fleeing.
1.
2.
Epinephrine does not interact directly with
the enzyme responsible for glycogen
breakdown; an intermediate step or series
of steps must be occurring inside the cell.
The plasma membrane is somehow involved
in transmitting the signal.
EXTRACELLULAR
FLUID
1 Reception
Receptor
Signaling
molecule
CYTOPLASM
Plasma membrane
A
signaling molecule binds to a receptor
protein, causing it to change shape.
 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
EXTRACELLULAR
FLUID
1 Reception
CYTOPLASM
Plasma membrane
2 Transduction
Receptor
Relay molecules in a signal transduction
pathway
Signaling
molecule
 Cascades
of molecular interactions relay
signals from receptors to target molecules in
the cell.
 The molecules that relay a signal from
receptor to response are mostly proteins
 Like falling dominoes, the receptor activates
another protein, which activates another,
and so on, until the protein producing the
response is activated
 At each step, the signal is transduced into a
different form, usually a shape change in a
protein
 Signal
transduction usually involves multiple
steps
 Multistep pathways can amplify a signal: A
few molecules can produce a large cellular
response
 Multistep pathways provide more
opportunities for coordination and regulation
of the cellular response
EXTRACELLULAR
FLUID
1 Reception
CYTOPLASM
Plasma membrane
2 Transduction
3 Response
Receptor
Activation
of cellular
response
Relay molecules in a signal transduction
pathway
Signaling
molecule
Cell signaling leads to regulation of transcription
or cytoplasmic activities
 The cell’s response to an extracellular signal is
sometimes called the “output response”
 Ultimately, a signal transduction pathway leads
to regulation of one or more cellular activities
 The response may occur in the cytoplasm or in
the nucleus
 Many signaling pathways regulate the synthesis
of enzymes or other proteins, usually by turning
genes on or off in the nucleus
 The final activated molecule in the signaling
pathway may function as a transcription factor

Figure 11.15
Growth factor
Reception
Receptor
Phosphorylation
cascade
Transduction
CYTOPLASM
Inactive
transcription
factor
Active
transcription
factor
P
Response
DNA
Gene
NUCLEUS
mRNA
 Other
pathways regulate the activity of
enzymes rather than their synthesis
 Signaling
pathways can also affect the
overall behavior of a cell, for example,
changes in cell shape
Figure 11.17
RESULTS
formin
Fus3
Wild type (with shmoos)
CONCLUSION
1 Mating
factor
activates
receptor.
Mating
factor G protein-coupled
Shmoo projection
forming
receptor
Formin
P
Fus3
GDP
GTP
2 G protein binds GTP
and becomes activated.
Fus3
Actin
subunit
P
Phosphorylation
cascade
Fus3
Formin
Formin
P
4 Fus3 phosphorylates
formin,
activating it.
P
3 Phosphorylation cascade
activates Fus3, which moves
to plasma membrane.
Microfilament
5 Formin initiates growth of
microfilaments that form
the shmoo projections.
A
Receptor for that signaling factor!!
A
molecule that specifically binds to another
molecule. For example, a signaling factor
binding with a receptor causing a shape
change and cellular response.
 Cell
Surface proteins make up 30% of all
human proteins but only 1% have been
examined by x-ray crystallography.
G
protein-coupled receptors (GPCRs) are
the largest family of cell-surface receptors
 A GPCR 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
Figure 11.7b
G protein-coupled
receptor
Plasma
membrane
Activated
receptor
1
Inactive
enzyme
GTP
GDP
GDP
CYTOPLASM
Signaling
molecule
Enzyme
G protein
(inactive)
2
GDP
GTP
Activated
enzyme
GTP
GDP
Pi
3
Cellular response
4
 Receptor
tyrosine kinases (RTKs) are
membrane receptors that attach phosphates
to tyrosines
 A receptor tyrosine kinase can trigger
multiple signal transduction pathways at
once
 Abnormal functioning of RTKs is associated
with many types of cancers
Figure 11.7c
Signaling
molecule (ligand)
Ligand-binding site
 helix in the
membrane
Signaling
molecule
Tyrosines
CYTOPLASM
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosine
kinase proteins
(inactive monomers)
1
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Dimer
2
Activated relay
proteins
3
Tyr
Tyr
P Tyr
Tyr P
P Tyr
Tyr P
Tyr
Tyr
P Tyr
Tyr P
P Tyr
Tyr P
Tyr
Tyr
P Tyr
Tyr P
P Tyr
Tyr P
6
ATP
Activated tyrosine
kinase regions
(unphosphorylated
dimer)
6 ADP
Fully activated
receptor tyrosine
kinase
(phosphorylated
dimer)
4
Inactive
relay proteins
Cellular
response 1
Cellular
response 2
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
 Intracellular
receptor proteins are found in
the cytosol or nucleus of target cells
 Small
or hydrophobic chemical messengers
can readily cross the membrane and activate
receptors
 Examples
of hydrophobic messengers are the
steroid and thyroid hormones of animals
 An
activated hormone-receptor complex can
act as a transcription factor, turning on
specific genes
Hormone
(testosterone)
EXTRACELLULAR
FLUID
Plasma
membrane
Receptor
protein
DNA
NUCLEUS
CYTOPLASM
Hormone
(testosterone)
EXTRACELLULAR
FLUID
Plasma
membrane
Receptor
protein
Hormonereceptor
complex
DNA
NUCLEUS
CYTOPLASM
Hormone
(testosterone)
EXTRACELLULAR
FLUID
Plasma
membrane
Receptor
protein
Hormonereceptor
complex
DNA
NUCLEUS
CYTOPLASM
Hormone
(testosterone)
EXTRACELLULAR
FLUID
Plasma
membrane
Receptor
protein
Hormonereceptor
complex
DNA
mRNA
NUCLEUS
CYTOPLASM
Hormone
(testosterone)
EXTRACELLULAR
FLUID
Plasma
membrane
Receptor
protein
Hormonereceptor
complex
DNA
mRNA
NUCLEUS
CYTOPLASM
New protein
 Probably
on the Plasma Membrane because
hydrophilic (water soluble) molecules cannot
easily get through the phospholipid bilayer.
 Possibility
of greatly amplifying signal!!