cell communication chapter 7. pathway similarities suggest that ancestral signaling molecules...
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Cell Communication
Chapter 7
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• Pathway similarities suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes
• Local signaling can be cell to cell using gap and plasmodesmata
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Fig. 11-4Plasma membranes
Gap junctionsbetween animal cells
(a) Cell junctions
Plasmodesmatabetween plant cells
(b) Cell-cell recognition
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EXTRACELLULARFLUID
Plasma membrane
CYTOPLASM
Receptor
Signalingmolecule
Relay molecules in a signal transduction pathway
Activationof cellularresponse
Transduction Response2 3Reception1
Cell communication is in 3 stages.
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Receptors in the Plasma Membrane• Most water-soluble signal molecules bind to
specific sites on receptor proteins in the plasma membrane
• There are three main types of membrane receptors:– G protein-coupled receptors– Receptor tyrosine kinases– Ion channel receptors
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• A G protein-coupled receptor 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
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Signaling-molecule binding site
Segment thatinteracts withG proteins
G protein-coupled receptor
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G protein-coupledreceptor
Plasmamembrane
EnzymeG protein(inactive)
GDP
CYTOPLASM
Activatedenzyme
GTP
Cellular response
GDP
P i
Activatedreceptor
GDP GTP
Signaling moleculeInactiveenzyme
1 2
3 4
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• Receptor tyrosine kinases are membrane receptors that attach phosphates to tyrosines
• A receptor tyrosine kinase can trigger multiple signal transduction pathways at once
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Signalingmolecule (ligand)
Ligand-binding site
Helix
TyrosinesTyr
Tyr
Tyr
Tyr
Tyr
Tyr
Receptor tyrosinekinase proteins
CYTOPLASM
Signalingmolecule
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Dimer
Activated relayproteins
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
PPP
P
PP
Cellularresponse 1
Cellularresponse 2
Inactiverelay proteins
Activated tyrosinekinase regions
Fully activated receptortyrosine kinase
6 6 ADPATP
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
Tyr
PPP
PPP
1 2
3 4
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• 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
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Signalingmolecule(ligand)
Gateclosed Ions
Ligand-gatedion channel receptor
Plasmamembrane
Gate open
Cellularresponse
Gate closed3
2
1
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Intracellular Receptors
• Some receptor proteins are intracellular, 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
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Small Molecules and Ions as Second Messengers
• The extracellular signal molecule that binds to the receptor is a pathway’s “first messenger”
• Second messengers are small, nonprotein, water-soluble molecules or ions that spread throughout a cell by diffusion
• Second messengers participate in pathways initiated by G protein-coupled receptors and receptor tyrosine kinases
• Cyclic AMP and calcium ions are common second messengers
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First messengerFig. 11-11
G proteinAdenylylcyclase
GTP
ATPcAMP
Secondmessenger
Proteinkinase A
G protein-coupledreceptor
Cellular responses
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Fig. 11-16b
CONCLUSION
Matingfactor G protein-coupled
receptor
GDP GTP
Phosphory- lation cascade
Shmoo projectionforming
Fus3
Fus3 Fus3
Formin Formin
P
P
P
ForminP
Actinsubunit
Microfilament
1
2
3
4
5
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Signal Amplification
• Enzyme cascades amplify the cell’s response• At each step, the number of activated products
is much greater than in the preceding step
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Fig. 11-15
Reception
Transduction
Response
Binding of epinephrine to G protein-coupled receptor (1 molecule)
Inactive G protein
Active G protein (102 molecules)
Inactive adenylyl cyclaseActive adenylyl cyclase (102)
ATPCyclic AMP (104)
Inactive protein kinase AActive protein kinase A (104)
Inactive phosphorylase kinaseActive phosphorylase kinase (105)
Inactive glycogen phosphorylase
Active glycogen phosphorylase (106)
GlycogenGlucose-1-phosphate
(108 molecules)
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Termination of the Signal
• Inactivation mechanisms are an essential aspect of cell signaling
• When signal molecules leave the receptor, the receptor reverts to its inactive state
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Apoptosis (programmed cell death) integrates multiple cell-signaling pathways
• Apoptosis is programmed or controlled cell suicide
• A cell is chopped and packaged into vesicles that are digested by scavenger cells
• Apoptosis prevents enzymes from leaking out of a dying cell and damaging neighboring cells
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Fig. 11-20a
Ced-9protein (active)inhibits Ced-4activity
Mitochondrion
Ced-4 Ced-3Receptorfor death-signalingmolecule
Inactive proteins
(a) No death signal
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Fig. 11-20b
(b) Death signal
Death-signalingmolecule
Ced-9(inactive)
Cellformsblebs
ActiveCed-4
ActiveCed-3
Activationcascade
Otherproteases
Nucleases
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Apoptotic Pathways and the Signals That Trigger Them
• Caspases are the main proteases (enzymes that cut up proteins) that carry out apoptosis
• Apoptosis can be triggered by:– An extracellular death-signaling ligand – DNA damage in the nucleus– Protein misfolding in the endoplasmic reticulum
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• Apoptosis evolved early in animal evolution and is essential for the development and maintenance of all animals
• Apoptosis may be involved in some diseases (for example, Parkinson’s and Alzheimer’s); interference with apoptosis may contribute to some cancers
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Fig. 11-21
Interdigital tissue 1 mm
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You should now be able to:
1. Describe the nature of a ligand-receptor interaction and state how such interactions initiate a signal-transduction system
2. Compare and contrast G protein-coupled receptors, tyrosine kinase receptors, and ligand-gated ion channels
3. List two advantages of a multistep pathway in the transduction stage of cell signaling
4. Explain how an original signal molecule can produce a cellular response when it may not even enter the target cell
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5. Define the term second messenger; briefly describe the role of these molecules in signaling pathways
6. Explain why different types of cells may respond differently to the same signal molecule
7. Describe the role of apoptosis in normal development and degenerative disease in vertebrates
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