Download Cell Communication (Chapter 11)

Cell Communication
Chapter 11
Biology
Campbell • Reece
Overview

We can look at simple organisms like
yeast and bacteria as examples for
all cells
• Ex. Yeast communicate to identify their
mate

Signal-transduction pathway The process by which a signal on a
cell’s surface is converted into a
specific cellular response
Communication Between Yeast
Direct Contact


Direct contact –
through cell
junctions that
directly connect the
cytoplasm of one cell
to the cytoplasm of
an adjacent cell
Or through receptors
on the surface of
cells
Local vs. Long Distance Signaling


Local – cells secrete a local regulator, a
substance that influences cells in the
vicinity
Greater distances – cells secrete
hormones which can travel far
3 stages of cell signaling
1.
2.
Reception – the target cell’s
detection of a signal coming from
outside the cell
Transduction – converts the signal
to a form that can bring about a
specific cellular response
 Sometimes occurs as a single step
 Sometimes requires a series of
different molecules – signaltransduction pathway
3 stages of cell signaling
3.
Response – may be almost any
imaginable cellular activity
 Ex. Enzyme catalysis, rearrangement
of the cytoskeleton, activation of
specific genes
Signal Receptors

A signal molecule binds to a receptor
protein in the cell membrane (ligand
binding), causing the protein to
change shape
• Ligand – a small molecule that
specifically binds to a larger one

The change of shape may directly
activate the receptor or may cause
the aggregation of two or more
receptor molecules
G-Protein-Coupled Receptors



A plasma membrane receptor that works
with the help of a G protein
G proteins are important in embryonic
development, sensory reception (visual &
smell)
G protein functions as an on/off switch
• When GDP (guanosine diphosphate) is
attached the G protein is inactive
• When GTP (guanosine triphosphate) is
attached the G protein is active
Example G-Protein Receptor
1.
2.
3.
4.
A signal molecule binds to the Gprotein-linked receptor
The receptor changes shape &
binds to an inactive G protein
A GTP displaces the GDP, activating
the G protein
G protein binds to another protein,
usually an enzyme, & alters its
activity, triggering the next step
G Protein-Coupled Receptors
Receptor Tyrosine Kinases
A kinase is an enzyme that catalyzes
the transfer of phosphate groups
 Specialized for triggering more than
one signal-transduction pathway at
once
 Before a signal molecule binds, the
receptors exist as individual
polypeptides

Activation of Tyrosine-Kinase
1.
2.
3.
The ligand binding causes two
receptor polypeptides to aggregate,
forming a dimer (a protein with two
polypeptides)
This activates the tyrosine-kinase,
which then adds phosphates to the
tyrosines on the tail
It is now recognized by specific
relay proteins inside the cell
Activation of Tyrosine-Kinase
4.
5.
6.
Each protein binds to a specific
phosphorylated tyrosine, and
changes shape
One tyrosine-kinase receptor dimer
may activate ten or more different
proteins simultaneously
Triggering as many different
transduction pathways & cellular
responses
Receptor Tyrosine Kinases
Ion-Channel Receptors


Ligand-gated ion
channels – protein
pores that open or close
in response to a
chemical signal,
allowing or blocking the
flow of specific ions
Very important in the
nervous system
Intracellular Receptors
Not all signal receptors are
membrane proteins
 Some are proteins in the cytosol or
nucleus of target cells
 To reach these receptors, the
chemical messenger must be able to
pass through the membrane

• Steroid hormones, nitric oxide
Intracellular Receptors
Signal-Transduction Pathways
When signal receptors are membrane
proteins, the transduction stage is
usually a multistep pathway
 This allows the signal to be greatly
amplified
 Relay molecules, usually proteins,
relay the signal from receptor to
response

Protein Phosphorylation
The activation of a protein by adding
one or more phosphate groups to it
 Protein kinase – an enzyme that
transfers phosphate groups from ATP
to a protein
 Many pathways contain protein
kinases that create a “phosphorylation
cascade”
 Protein phosphatases – enzymes
that remove phosphate groups from
proteins

Protein Phosphorylation
Second Messengers
Not all components of signaltransduction pathways are proteins
 Second messengers - Small,
nonprotein, water-soluble molecules
or ions

• Can readily spread throughout a cell by
diffusion
• Two most widely used are cyclic AMP
and calcium ions
Cyclic AMP
The use of cyclic AMP as a second
messenger was discovered by Earl
Sutherland
 He found that when epinephrine
binds to the plasma membrane of a
liver cell, the concentration of cAMP
in the cytosol increased

• Adenylyl cyclase converts ATP to cAMP
in response to the epinephrine
cAMP as a Second Messenger
Calcium Ions
Neurotransmitters, growth factors,
and some hormones induce
responses that increase Ca2+
concentration
 More widely used as a second
messenger than cAMP
 Causes many responses including
muscle contraction, cell division and
secretion of certain substances

DAG and IP3
Diacylglycerol (DAG) and inositol
triphosphate (IP3) are other
second messengers used to release
calcium from cell’s endoplasmic
reticulum
 Calcium ions may activate a signaltransduction protein directly or by
means of calmodulin

• Calmodulin – a Ca2+ binding protein
Calcium in signaling pathways
Cellular Responses

In response to a signal, a cell may
regulate activities in the cytoplasm
or transcription in the nucleus
• An ion channel in the plasma membrane
may open or close
• Cell metabolism may change
• Synthesis of proteins may be regulated
by turning on or off the genes
Cellular Responses
Signal Amplification


Elaborate enzyme
cascades amplify
the cell’s response
to a signal
At each step, the
number of
activated molecules
is much greater
than the preceding
step
The Specificity of Cell Signaling


Both the liver and heart respond to
epinephrine, but in different ways
Different kinds of cells have different
collections of proteins
Branching and “Cross-talk”
Branching of pathways and “crosstalk” (interaction) between pathways
are important in regulating and
coordinating the cellular response
 The use of some of the same
proteins in more than one pathway
allows the cell to economize on the
number of different proteins it must
make

Scaffolding Proteins


Proteins are too large
to diffuse quickly
through the cytosol
Scaffolding proteins –
large relay proteins to
which several other
relay proteins are
simultaneously
attached
• Enhances the speed and
accuracy of signal
transfer
Turning “Off” the Signal

The signal molecule leaves the
receptor…
• The GTP bound to the G protein is
hydrolyzed to GDP
• Phosphodiesterase converts cAMP to
AMP
• Protein phosphatases inactivate
phosphorylated kinases

The cell is now ready to respond to a
fresh signal