Transcript MB207_14 - MB207Jan2010

MB207 Molecular Cell Biology
Cell Communication
General principles of cell communication
• All cells have some ability to sense and respond to specific chemical signals.
→ Intercellular communication.
• Signal is transmitted by regulatory chemical messengers and receptors that
are located on the receptors located on the surfaces of the cells that may be
quite distant from the secreting cells.
• Three general categories of chemical signaling:
i) Cytoplasmic connections between cells
ii) Cell-to-cell contact-mediated signaling
iii) Free diffusion between cells
- Distants (hormones)
- Adjacent cells (within interstitial space)
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• Autocrine signals → Signals that act on the same cell that produces them.
•Paracrine signals → Signals that are released locally, where they diffuse to act at
short range on nearby tissues.
• Endocrine signals → Signals that are produced at great distances from their
target tissues and are carried by the circulatory system to
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various sites in the body.
Local diffusion
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Long-distance diffusion
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Three stages of signal transduction
1) Reception of extracelluar signal by cell.
2) Transduction of signal from outside of cell to inside of cell.
→ Often includes multiple steps.
3) Celluar response.
→ Response is initiated and/or occurs entirely within receiving cell.
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1. Reception
2a. Transduction
2b. Transduction
2c. Transduction
2d. Transduction
3. Response
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1. Reception
2a. Transduction
2b. Transduction
3. Response
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More than one response can result form the reception of a single
ligand
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Cell Communication - reception
• Communication between cells requires:
i) ligand: the signaling molecule
ii) receptor protein: the molecule to which
the receptor binds
-may be on the plasma membrane or
within the cell
•
Receptors are defined by location:
i) Cell surface receptors
→ located on the plasma membrane.
ii) Intracellular receptors
→ located within the cell.
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Intracellular signaling
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Protein and Polypeptide Hormones:
Synthesis and Release
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Figure 7-3: Peptide hormone synthesis, packaging, and release
DNA-binding domain in the receptor
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Three classes of cell-surface receptors
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Signaling pathway from a cellsurface receptor to the
nucleus generates various
kinds of intracellular signaling.
• A series of signaling proteins and
small intracellular mediators relay the
extracellular signal into the cell,
causing a change in gene expression.
• The signal is amplified , altered
(transduced), and distributed en
route.
• The signaling pathway activates (or
inactivates) target proteins that alter
cell behaviour.
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Intracellular signaling protein ac as molecular switches
• A signaling protein is activated by the addition of phosphate group and
inactivated by the removal of phosphate.
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Signal integration
(A) Extracellular signals A and B both activate a different series of protein
phosphosrylation, each of which leads to the phosphorylation of protein Y but at
different sites on the protein. Protein Y is activated only when both of these sites are
phosphorylated, and therefore it becomes active only when signals A and B are
simulataneously present.
(B) Extracellular signals A and B lead to the phosphorylation of two proteins, a and b,
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which then binds to each other to create the active protein.
Signaling through G-protein linked cell-surface
receptors
G-protein linked-receptors:
• Largest family of cell-surface receptors and found in all eukaryotes.
• Mediate the responses to an enormous diversity of signal molecules including
hormones, neurotransmitters and local mediators.
• Signals molecule that activated them are varied in structure which includes proteins,
peptides, derivatives of amino acids and fatty acids etc.
• Ligand binding causes a change in receptor comformation that activates a particular
G-protein (abbrv. guanine-nucleotide binding protein).
• The structure consists of a single polypeptide chain that threads back and forth across
the lipid bilayer seven times (aka. serpentine receptors).
• It is thought that G-protein linked-receptors that mediate cell-cell signaling in
multicellular organisms evolved from sensory receptors that were possessed by their
unicellular eucaryotic ancestors.
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Structure of G-protein linked-receptors
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Regulation of G-protein linked-receptors
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The disassembly of an activated G-protein into two
signaling components
(A)
In the unstimulated state, the
receptor and G-protein are both
inactive.
(B)
Binding of an extracellular signal to
the receptor changes the
conformation of the receptor, which
in turn alters the conformation of the
G-protein that is bound to the
receptor.
(C)
The alteration of the α subunit of the
G-protein allows it to exchange its
GDP for GTP. This causes the G
protein to break up into two active
components – an α subunit and a βγ
complex, both can regulated the
activity of target proteins in the
plasma membrane. The receptor
stays active while the external signal
molecule is bound to it, and can
therefore catalyze the activation of
many molecules of G protein.
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The switching off of the G-protein α subunit by the
hydrolysis of its bound GTP
• After a G-protein α subunit activates its target
protein, it shuts itself off by hydrolyzing its
bound GTP to GDP.
• This inactivates the α subunit, which
dissociates from the target protein and
reassociates with a βγ complex to re-form an
inactive G protein.
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Cyclic AMP is a second messenger for stimulatory G
protein (Gs)
• In a reaction catalyzed by the enzyme
adenylyl cyclase, cyclic AMP (cAMP) is
synthesized from ATP through a
cyclization that removes two phosphate
groups as pyrophosphate.
• Cyclic AMP is unstable in the cell
because it is itself hydrolyzed by a specific
phosphodiesterase to form 5’-AMP.
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• The binding of an extracellular signal
molecule to its G-protein linkedreceptor leads to the activation of
adenylyl cyclase and a rise in cyclic
AMP concentration.
• The increase in cyclic AMP activates
PKA in the cytosol and released
catalytic subunits then move into the
nucleus, where they phosphorylates
cyclic AMP response element (CREB)
gene regulatory element.
• Once phosphorylated, CREB recruits
the coactivator CBP which stimulates
gene transcription.
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Inositol triphosphate and diacylglycrol as second
messenger for G protein
• The activated receptor stimulates the plasma membrane-bound enzyme phospholipase C-β via G
protein. It can be activated by the α subunit of Gq or by βγ complex of another G protein, or by both.
•Two intracellular messenger molecules are produced when PI(4,5)P2 is hydrolyzed by phospholipase
C-β.
• Inositiol triphosphates diffuses through the cytosol and release Ca2+ from the endoplasmic reticulum
by binidng to and opening IP3-gated Ca2+ release channels in the endoplasmic reticulum membrane.
• Diacylglycerol remains in the plasma membrane and together with phospatidylserine and Ca2+, helps
activate the enzyme protein kinase C which is recruited from the cytosol to the cytosolic face of the 27
plasma membrane.
Three major families of trimetric G proteins
Family Family
members
I
II
Action
mediated by
Gs
α
Activates adenylyl cyclase; activates Ca2+
channels
Golf
α
Activates adenylyl cyclase in olfactory sensory
neurons
Gi
α
Inhibits adenylyl cyclase
Go
βγ
Activates K+ channels
βγ
Activates K+ channels; inactivates Ca2+ channels
α and βγ
III
Functions
Activates phospholipase C-β
Gt
α
Activates cyclic GMP phosphoesterase in
vertebrate photoreceptors
Gq
α
Activates phospholipase C-β
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Signaling through enzyme-linked cell-surface receptors
• Recognized through their role in responses to extracellular signal proteins
that promote growth, proliferation, differentiation and survival of cells in
animal tissues.
• Signal proteins are known as growth factors and usually act as loal
mediators at every low conentration (about 10-9 to 10-11 M).
• Enzyme-linked receptors are transmembrane proteins (each subunit
usually has only one) with their ligand-binding domain on the outer surface
of the plasma membrane.
• Their cytosolic domain either has an intrinsic enzyme activity or associates
directly with an enzyme.
• Six classes of enzyme-linked receptors:
i) receptor tyrosine kinases
ii) tyrosine-kinase-associated receptors
iii) receptor-like tyrosine phosphatases
iv) receptor serine/threonine kinases
v) receptor guanylyl cyclases
vi) histidine-kinase-associated receptors
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Structure of receptor tyrosine kinase
• Consist of a single polypeptide chain with only one transmembrane segment.
• The extracellular portion of the receptor contains ligand-binding domain. The
cytosolic portion of the receptor contains tyrosine residues that are in fact
themselves targets for tyrosine kinase portion of the receptor.
→ receptor + protein kinase
• In some cases, the receptor and tyrosine kinase are two separate proteins. It
can bind to the receptor and be activated when the receptor binds its ligand.
→ nonreceptor tyrosine kinase
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Activation of receptor tyrosine kinase
• Signal transduction is initiated when ligand binds, causing the receptor tyrosine kinase
to aggregate.
→ Two receptor molecules luster together within the plasma membrane when they bind
ligand (dimerization)
• Tyrosine kinase associated with each receptor phosphorylates the tyrosine kinase of
neighbouring receptors (autophosphorylation).
• Subsequently, relay proteins bind to the phosphorylated receptors and ativate
subsequent protein to trigger a response.
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RTK initiate a signal transduction cascade involving Ras
and MAP kinase
• Upon ligand binding, RTK aggregate and undergo autophosphorylation.
• Once a receptor is phosphorylated at tyrosine residues in its cytosolic tails, proteins
with SH2 domain such as GRB2 bind to the receptor, recruiting GEF for Ras.
• Ras-GEF causes the activation of Ras by helping it to release GDP and acquire GTP.
• Activated Ras then activates several downstream signaling pathways.
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• Activated Ras triggers the phosphorylation of protein kinase Raf.
• Activated Raf in turn phosphorylates serine and threonine residues in protein kinase
MEK.
• MEK then phosphorylates threonines and tyrosine residues of mitogen-activated protein
kinases (MAPKs).
•One of the funtion of MAPKs is to phosphorylate transcription factors that regulate gene
expression.
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kinase cascade
– a series of protein kinases that
phosphorylate each other in
succession
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Cross-talk of transduction pathway
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Specificity of cell signaling
•
The same ligand gives rise to different responses according to their
cell type.
→ Cells differ in terms of their protein contents.
→
The same ligand gives rise to different responses:
i) Cells differ in terms of their proteins
ii) Different proteins respond differently to the same environmental signals
(note, though, same receptors, different relay)
iii) Different cells behave differently because some, but not all proteins can 36
differ between cell types