Local and LongDistance Signaling · molecules evolved in prokaryotes and were modified later in...
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Ch11 Cell Signaling 525 Copy.notebook 1 October 21, 2011 Oct 2112:20 PM Evolution of Cell Signaling • 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 suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes Oct 2112:24 PM Local and LongDistance Signaling • Cells in a multicellular organism communicate by chemical messengers • Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells • In local signaling, animal cells may communicate by direct contact, or cellcell recognition
Transcript of Local and LongDistance Signaling · molecules evolved in prokaryotes and were modified later in...
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
Oct 2112:20 PM
Evolution of Cell Signaling
• 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 suggest that ancestral signaling molecules evolved in prokaryotes and were modified later in eukaryotes
Oct 2112:24 PM
Local and LongDistance Signaling
• Cells in a multicellular organism communicate by chemical messengers
• Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells
• In local signaling, animal cells may communicate by direct contact, or cellcell recognition
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
Oct 2112:25 PM
Figure 11.4 Communication by direct contact between cells
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• In many other cases, animal cells communicate using local regulators, messenger molecules that travel only short distances
• In longdistance signaling, plants and animals use chemicals called hormones
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
Oct 2112:27 PM
Figure 11.5 Local and longdistance cell communication in animals
Oct 2112:28 PM
The Three Stages of Cell Signaling: A Preview
• Earl W. Sutherland discovered how the hormone epinephrine acts on cells
• Sutherland suggested that cells receiving signals went through three processes:
1) Reception2) Transduction3) Response
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
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Figure 11.6 Overview of cell signaling
Oct 2112:29 PM
Figure 11.6 Overview of cell signaling
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
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Figure 11.6 Overview of cell signaling
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Concept 11.2: Reception: A signal 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
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
Oct 2112:33 PM
Receptors in the Plasma Membrane
• Most watersoluble signal molecules bind to specific sites on receptor proteins in the plasma membrane
• There are three main types of membrane receptors:
1) G proteincoupled receptors2) Receptor tyrosine kinases3) Ion channel receptors
Oct 2112:34 PM
• A G proteincoupled 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
Figure 11.7 Membrane receptors—G proteincoupled receptors, part 1
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
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Figure 11.7 Membrane receptors—G proteincoupled receptors, part 2
When GDP bound to G protein, the G protein is inactive. When signaling molecule binds to receptor, the receptor is activated and changes shape. It binds an inactive G protein, causing a GTP to displace the GDP. This activates the G protein.
Activated G protein dissociates from receptor, diffuses along the membrane, and binds to an enzyme, altering the enzyme's shape and activity. When enzyme is activated, it triggers the next step in a pathway leading to a cellular response.
Changes in the enzyme and G protein are only temporary, because the G protein also functions as a GTPase enzyme it hydrolyzes its bound GTP to GDP. Now inactive again, the G protein leaves the enzyme, which returns to its original state. The G protein is now available for reuse. The GTPase function of the G protein allows the pathway to shut down rapidly when the signaling molecule is no longer present.
Oct 2112:36 PM
• Receptor tyrosine kinases are membrane receptors that attach phosphates to tyrosines
• A receptor tyrosine kinase can trigger multiple signal transduction pathways at once
Figure 11.7 Membrane receptors—receptor tyrosine kinases
Before the signaling molecule binds, the receptors exist as individual polypeptides. Each has an extracellular ligandbinding site, an α helix spanning the membrane, and an intracellular tail containing multiple tyrosines.
Binding of a signaling molecule (such as a growth factor) causes two receptor polypeptides to associate closely with each other forming a dimer (dimerization).
Dimerization activates the tyrosine kinase region of each polypeptide; each tyrosine kinase adds a phosphate from an ATP molecule to a tyrosine of the tail of the other polypeptide.
Fully activated receptor protein is recognized by specific relay proteins inside the cell. Each such protein binds to a specific phosphorylated tyrosine, undergoing a resulting structural change that activates the bound protein. Each activated protein triggers a transduction pathway, leading to a cellular response.
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
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• A ligandgated 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
Figure 11.7 Membrane receptors—ion channel receptors
1) Gate remains closed until a ligand binds to the receptor.
2) When ligand binds to receptor and gate opens, specific ions can flow through the channel and rapidly change the concentration of that particular ion inside the cell. This change may directly affect the activity of the cell in some way.
3) When ligand dissociates from receptor, the gate closes and ions no longer enter the cell.
Oct 2112:40 PM
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 hormonereceptor complex can act as a transcription factor, turning on specific genes
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October 21, 2011
Oct 2112:41 PM
Figure 11.8 Steroid hormone interacting with an intracellular receptor
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Figure 11.8 Steroid hormone interacting with an intracellular receptor
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
Oct 2112:41 PM
Figure 11.8 Steroid hormone interacting with an intracellular receptor
Oct 2112:41 PM
Figure 11.8 Steroid hormone interacting with an intracellular receptor
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
Oct 2112:41 PM
Figure 11.8 Steroid hormone interacting with an intracellular receptor
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Concept 11.3: Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell
• 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
Ch11 Cell Signaling 525 Copy.notebook
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October 21, 2011
Oct 2411:06 AM
Signal Transduction Pathways
• 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
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