5.2.1 cell communication and cell recognition
A Cell communication
Three ways commmunication
a Secret chemical signal
b Contact-depend signal( 接触性依赖的通讯）
c Gap junction signal
signal transduction systems
hormones ion channels hormones
Seven-spanning G protein-linked receptors
adenylate cyclase G proteins
regulate cell survival.
regulate cell behavior.
An absence of signals
results in cell death
Endocrine signaling –
signaling molecules act on
target cells distant from
their site of synthesis by
cells of endocrine organs
Paracrine signaling –
signaling molecules released
by a cell only affect target
cells in close proximity
Autocrine signaling – cells
respond to substances that
they themselves release
Secret chemical signal
• B cell recognition
– Impinge from the outside
and interact with the cell
– Depend on the stimuli
– Signal trasduction
cascade for activation of
– Genetic programs
Signaling is about
groups of cells and
group of cells informs
another group of cells
what to do.
Signal transduction refers to how the presence of an
extracellular signal can produce a change in the
intracellular state of the cell without the initial signal
crossing the membrane.
Communication by extracellular signals usually involves
1) synthesis and
2) release of signaling molecules by the signaling cell
3) transport of the signal to the target cell
4) detection of the signal by a specific receptor protein
5) and change in cellular metabolism or gene expression
triggered by the receptor-signaling molecule complex
6) removal of the signal, often terminating the cellular
C Cell singnal
• Cell signal
• 气体信号分子 NO
Cells constantly communicate with one another,
through direct contact and by chemical
messengers, such as hormones.
This communication is cell signaling.
In response to a signal from nerve cells,
endothelial cells that line blood vessels make
and release nitric oxide.
Nitric oxide enters muscle cells in the vessel
wall causing them to relax and dilate.
Blood flow increases and more oxygen can reach
organs such as the heart.
The 1998 Nobel prize was awarded to two U.S.
scientists for discovering this mechanism.
Nitric oxide binds directly to an
intracellular enzyme causing a rapid
Signal molecules move between cells
Growth factors promote survival and
stimulate cell proliferation.
Acetylcholine is a neurotransmitter.
Thyroid hormone stimulates metabolism.
Nitric oxide relaxes muscle cells in
Adrenaline increases heart contraction.
Testosterone induces secondary male
Types of signals
– Receptors which have N
terminal face outwards
and C terminal inside the
– When bound to a signal
molecule, changes its
– Signal molecules are
specific to their receptors
Types of signals contd.
• Intercellular Endocrine – Travel through
– Mostly triggered by the Paracrine – In the visinity
Autocrine – Same cell type
extracellular signal Juxtacrine – Along cell
– Which converts the membranes
extracellular into an
– Eg. - G protein,
GTPase, cAMP, Ca++,
and many more
– Also called as second
Fast and slow responses
Post induction events
– life span of the
receptor – ligand
• Intracellular signal
– Ca++ activation
– NO (Nitric Oxide)
D Cell receptor
Cell receptor:intracellular receptor
cell surface receptor
Types of receptors
– That span the thickness
of the plasma membrane
– Intracellular domain and
– Signal to concentration
molecules (Ca++) – Ion
– Cell potential
Types of receptors
• Nuclear receptors
– Soluble proteins localised within the
cytoplasm or the nuceloplasm
– Ligand activated trascription activators
– Hormone regulation
– Steroid receptors (located within cytosol)
• Apocomplex formation in the absence of
the steroid (also contains chaperone
– RXR and Orphan receptors
In general, signaling by cell-surface receptors
initiates a cascade of biochemical events
that result changes in the metabolism of
How are signals transmitted into the cell?
Hydrophobic signal molecules such as
testosterone or gaseous signal
molecules such as nitric oxide can
cross the membrane.
Most signal molecules bind to a cell-
surface receptor protein where the
information is converted from one
form to another in a process called
Three classes of
The Receptor cell-surface
Ion-channel-linked receptors open an
ion channel in response to the signal
G-protein-linked receptors activate an
intracellular G-protein that in turn
activates intracellular enzymes.
Enzyme-linked receptors directly
activate a membrane bound enzyme.
Signal molecules that
do not enter the cell
bind to cell-surface
Signal molecules that
enter the cell bind to
Depending on type of cell and stimulus, the
response might involve:
1. Change in gene expression.
2. Alteration of activity of metabolic enzymes.
3. Reconfiguration of cytoskeleton.
4. Change in ion permeability.
5. Secretion of a hormone or protein.
6. Activation of DNA synthesis.
7. Death of cell.
The same signal molecule can induce
different responses in different cell types
Examples of Signal
B 酶偶联的受体（enzyme-linked receptor)
C G蛋白欧联的受体(G protein -linked receptor)
Cell-surface receptor categories
Can be separated into four classes:
1) G protein-linked receptors
2) Ion-channel receptors
3) Receptors lacking intrinsic catalytic activity
but directly associated with cytosolic protein
4) Receptors with intrinsic enzymatic activity
B 酶偶联的受体（enzyme-linked receptor）
Receptor tyrosine kinases (RTKs), which bind to peptide/protein
hormones, may exist as dimers or dimerize during binding to ligands.
Ligand binding leads to activation of the kinase activity of the
receptor and autophosphorylation of tyrosine residues in its cytosolic
domain (see Figure 20-31). The activated receptor also can
phosphorylate other protein substrates.
Ras is an intracellular GTPase switch protein that acts downstream
from most RTKs. Like Gsa , Ras cycles between an inactive GDP-
bound form and active GTP-bound form. Ras cycling requires the
assistance of two proteins, GEF and GAP, (see Figure 20-22),
whereas Gsa cycling does not.
Unlike GPCRs, which interact directly with an associated G protein,
RTKs are linked indirectly to Ras via two proteins, GRB2 and Sos
(see Figure 20-23).
• The SH2 domain in GRB2, an adapter protein, binds to
specific phosphotyrosines in activated RTKs. The two
SH3 domains in GRB2 then bind Sos, a
guaninenucleotide exchange factor, thereby bringing Sos
close to membrane-bound Ras · GDP and activating its
Binding of Sos to inactive Ras causes a large
conformational change that permits release of GDP and
binding of GTP.
Normally, Ras activation and the subsequent cellular
response is induced by ligand binding to an RTK.
However, in cells that contain a constitutively active Ras,
the cellular response occurs in the absence of ligand
C G蛋白欧联的受体(G protein -linked
C-1 CAMP signal pathway
C-2 磷脂酰肌醇信号通路(Double messenger
signal pathway G proteins
• 1994 Nobel prize. Discovery of G-protein coupled
receptors and their role in signal transduction. Gilman,
Alfred G. and Rodbell, Martin.
• 43,409 articles in Medline that mention G proteins.
• 1. Basic facts about G proteins
– Each G-protein consists of 3 sub-units: Ga, Gb, and Gg.
– These sub-units can be together or separated.
– Each G-protein has a binding site that can be occupied by
either GDP or GTP.
– A G-protein is said to be inactive when it is bound to GDP.
– A G-protein is said to be active when it is bound to GTP.
Small GTP-binding proteins include (roles indicated):
initiation & elongation factors (protein synthesis).
Ras (growth factor signal cascades).
Rab (vesicle targeting and fusion).
ARF (forming vesicle coatomer coats).
Ran (transport of proteins into & out of the nucleus).
Rho (regulation of actin cytoskeleton)
All GTP-binding proteins differ in conformation depending
on whether GDP or GTP is present at their nucleotide
Generally, GTP binding induces the active state.
Adenylate Cyclase (Adenylyl Cyclase) catalyzes:
ATP cAMP + PPi
The reaction is driven forward by the cleavage of
PPi, catalyzed by Pyrophosphatase: PPi 2 Pi
Cyclic-AMP is suited to be a cAMP NH2
Synthesis & degradation of N
cAMP are both spontaneous,
but enzymes are required to
synthesize these reactions. H2 O
5' C 4'
H H 1'
Enzymes that synthesize O
H 3' 2' H
and degrade cAMP are P O OH
cyclicAMP (cAMP) pathway
• Up regulation: When a ligand binds to a
receptor in a cell membrane the effect is that
– A. the receptor becomes activated
– B. the nucleotide binding site on the G-protein is
– C. GTP replaces GDP,
– D. GDP is released,
– E. and Ga-GTP gets disassociated from GbGg.
– F. Ga being active triggers the binding of Ga-GTP to a
membrane bound adenylate cyclase molecule,
• activating it for production of cyclicAMP (cAMP).
Cyclic AMP (cAMP) pathway
• Down regulation – when the ligand dissociates
from the receptor the effects are
– A. GTP is hydrolyzed by a GTPase activity on Ga
– B. Ga-GTP becomes Ga-GDP,
– C. and disassociates from adenylate cyclase
molecule, making the later inactive.
– D. Ga then reassociates with GbGg
– E. CyclicAMP (in the cytoplasm) is then inactivated
by the enzyme phosphodiesterase, which hydrolyzes
it to AMP.
G protein activation/inactivation cycle
a g g a cytosol
GDP b b GTP
GTP GDP ATP cAMP + PPi
2. Hormone binding to a 7-helix receptor (GPCR) causes
a conformational change in the receptor that is
transmitted to the G protein.
The nucleotide-binding site on Ga becomes more
accessible to the cytosol, where [GTP] > [GDP].
Ga releases GDP & binds GTP (GDP-GTP exchange).
Turn off of the signal:
1. Ga hydrolyzes GTP to GDP + Pi. (GTPase).
The presence of GDP on Ga causes it to rebind
to the inhibitory bg complex.
Adenylate Cyclase is no longer activated.
2. Phosphodiesterase catalyzes hydrolysis of
Turn off of the signal (cont.):
3. Hormone receptor desensitization occurs.
This process varies with the hormone.
Some receptors are phosphorylated via G-protein-coupled
The phosphorylated receptor may then bind to a protein
arrestin that blocks receptor-G-protein activation and
promotes removal of the receptor from the membrane by
4. Protein Phosphatase catalyzes removal by hydrolysis of
phosphates that were attached to proteins via Protein Kinase
Signal amplification is an important feature of
One hormone molecule can lead to formation
of many cAMP molecules.
Each catalytic subunit of Protein Kinase A
catalyzes phosphorylation of many proteins
during the life-time of the cAMP.
View an animation of a G-protein signal cascade.
• 5.2.4 细胞表面整联蛋白介导的信号传递
• 5.2.5 细胞信号传递的基本特征与蛋白激酶
across the plasma
membrane can cause a
cascade of events
that amplify the signal
and distribute it to
influence several cell
processes in parallel.
Why do cells communicate?
During development, cells differentiate
to adopt specialized roles.
Cells need to know whether to live, die,
Regulation of metabolism.
Secondary sexual characteristics.
Why do cells communicate?
How are signals transmitted between
How are signals transmitted across cell
membranes into the cell interior?
How are signals transmitted within a
How do signals affect cell function?
What is the relationship between cell
signaling and cancer?
Cell signaling and cancer
RAS is an intracellular GTP binding protein that
is activated by enzyme linked receptors
Activated RAS proteins create a cascade of
events that can change protein activity and
RAS proteins control cell proliferation, cell
survival, and cell differentiation
30% of human cancers have mutations in RAS
genes which are then called oncogenes
1. Compare the structures, locations and functions of
preproinsulin and insulin.
2. Outline the events from translation of the preproinsulin
gene to exocytosis of insulin.
3. Describe the mechanisms by which glucose promotes insulin
4. Identify the components of insulin secretion that are
defective in type II diabetes mellitus.