Intracellular signaling molecules perform several crucial functions:
Relay: Pass the signal onward.
Amplify: Turn one message into many through second messengers.
This involves a single signal leading to a cascade of events, greatly increasing the magnitude of the response.
Integrate: Detect signals from multiple pathways before relaying a message.
This function allows the cell to consider various inputs to coordinate an appropriate response.
Distribute: Regulate one or more effector proteins.
Signaling molecules can activate different effector proteins, leading to diverse cellular outcomes.
Diagram of Intracellular Signaling Pathways Components:
The diagram (Figure 15-55 from Molecular Biology of the Cell 6e) illustrates various components and their interactions within intracellular signaling pathways:
Signal Molecule: The initial trigger that binds to a receptor.
GPCR (G Protein-Coupled Receptor): A common type of receptor that activates G proteins.
RTK (Receptor Tyrosine Kinase): Another type of receptor that phosphorylates tyrosine residues on target proteins.
G protein: A protein that relays signals from GPCRs.
Phospholipase C: An enzyme activated by G proteins, leading to the production of second messengers.
Grb2: An adaptor protein that binds to activated RTKs.
PI 3-kinase: Phosphoinositide 3-kinase, which phosphorylates lipids to create docking sites for signaling proteins.
Diacylglycerol (DAG): A second messenger produced by phospholipase C that activates protein kinase C (PKC).
IP3 (Inositol Trisphosphate): A second messenger that triggers the release of Ca2+ from the endoplasmic reticulum.
Adenylyl Cyclase: An enzyme that produces cyclic AMP (cAMP).
Ras-GEF (Sos): A guanine nucleotide exchange factor that activates Ras.
PI(3,4,5)P3: A phosphoinositide that acts as a docking site for signaling proteins.
Ras: A small GTPase involved in cell growth and differentiation.
MAP kinase kinase kinase: A kinase that phosphorylates and activates MAP kinase kinase.
Cyclic AMP (cAMP): A second messenger that activates protein kinase A (PKA).
Calmodulin: A calcium-binding protein that activates CaM-kinase.
PKA: Protein kinase A, which phosphorylates target proteins.
CaM-kinase: Calmodulin-dependent protein kinase, which phosphorylates target proteins.
PKC: Protein kinase C, which phosphorylates target proteins.
MAP kinase kinase: A kinase that phosphorylates and activates MAP kinase.
PDK1: Phosphoinositide-dependent kinase 1, which phosphorylates and activates Akt kinase.
MAP kinase: Mitogen-activated protein kinase, which phosphorylates transcription regulators and other target proteins.
Akt kinase: A kinase involved in cell survival and growth.
Transcription Regulators: Proteins that control gene expression.
Many Target Proteins: The ultimate effectors of the signaling pathway.
Integration of Multiple Extracellular Signals
Cells integrate multiple extracellular signals to produce a coordinated response.
Different signals (A, B, C, D) bind to receptors and activate different intracellular kinases.
These kinases (kinase 1, kinase 2) phosphorylate a target protein.
The combined effect of these phosphorylations determines the cell response.
Modulation of Signaling Pathways by Feedback
Signaling pathways are often modulated by feedback loops.
Positive Feedback: Pathway 1 stimulates the production of Y, which further activates pathway 1 (+).
This can lead to a strong, sustained response.
Negative Feedback: Pathway 2 stimulates the production of Y, which inhibits pathway 2 (T).
This provides a means for self-regulation and prevents over-activation.
GPCR Negative Feedback: Phosphorylation by Activated Kinases
Activated GPCRs can be desensitized through phosphorylation by GPCR kinases (GRKs).
Steps:
Activated GPCR stimulates GRK to phosphorylate it.
Arrestin binds to the phosphorylated GPCR at multiple sites.
This binding prevents further signaling by the GPCR.
ATP is converted to ADP during the phosphorylation by GRK.
Transcription Regulator Negative Feedback: NF-κB
NF-κB is a transcription regulator inhibited by IκBα.
Extracellular signals inhibit IκBα, leading to NF-κB activation.
Activated NF-κB induces increased transcription of IκBα, creating a negative feedback loop.
This loop helps to fine-tune the inflammatory response regulated by NF-κB.
Testing a Signaling Pathway Initiated by a New Drug
Hypothesis: Drug C protects against neurodegeneration by activating Nrf2.
Experiment: (The transcript does not provide details of the experiment or results.)
Result: (The transcript does not provide results.)
Conclusion: (A conclusion would be drawn based on the experimental results to support or refute the hypothesis.)