Topic 4.3: Signal Transduction

Introduction to Signal Transduction

Signal transduction is the process by which cells respond to external signals through a series of molecular events, often leading to a cellular response. This mechanism allows cells to sense and adapt to changes in their environment, ensuring proper function and survival.

• Signal Transduction: The conversion of a signal from outside the cell to a functional change within the cell.

• Receptor: A protein on the cell surface or within the cell that binds to signaling molecules (ligands).

• Transduction Cascade: A series of molecular interactions that amplify and transmit the signal.

• Cellular Response: The final effect, such as gene expression, enzyme activation, or apoptosis.

Example: Growth factor binding to its receptor triggers a phosphorylation cascade, activating transcription factors and leading to gene expression.

Environmental Response

Cells constantly monitor their environment and adjust their behavior accordingly. Environmental changes can initiate or limit signal transduction pathways, affecting cellular activity.

• Environmental Signals: Include changes in nutrient concentration, presence/absence of signaling molecules, and physical conditions.

• Phosphorylation: The addition of a phosphate group to a protein, often regulating its activity in signal transduction.

• Transcription Factors: Proteins that regulate gene expression in response to signals.

Example: Growth factor binding leads to phosphorylation of transcription factors, which then activate gene expression in the nucleus.

Cell Responses from Signal Transduction

Epinephrine Stimulates Glycogen Breakdown

Epinephrine (adrenaline) is a hormone that triggers the breakdown of glycogen to glucose, providing rapid energy for the fight-or-flight response in mammals.

• Process: Epinephrine binds to cell surface receptors → Activates G-protein pathway → Activates adenylyl cyclase → Increases cAMP → Activates protein kinase → Breaks down glycogen into glucose.

• Significance: Provides immediate energy for muscle activity during stress.

Example: Muscle cells rapidly mobilize glucose in response to epinephrine.

Mating Pheromones in Yeast

Yeast cells use chemical signals called pheromones to communicate and initiate mating.

• Process: Yeast cells release pheromones → Pheromones bind to receptors on other yeast cells → Activates gene expression for mating.

• Significance: Ensures reproduction and genetic diversity in yeast populations.

Example: MATa and MATα yeast cells respond to each other's pheromones to initiate mating.

Growth Factor/Cytokines

Growth factors and cytokines are signaling molecules that regulate cell growth, division, and immune responses.

• Growth Factors: Stimulate cell proliferation and differentiation.

• Cytokines: Regulate immune responses and inflammation.

• Interleukins: A type of cytokine that signals immune cells to proliferate during an immune response.

Example: Interleukin signaling leads to the proliferation of white blood cells during infection.

Additional info: Signal transduction pathways are highly conserved across species and play a central role in development, disease, and cellular homeostasis.