4.3 - Signal Transduction

Signal Transduction is the process through which cells respond to external stimuli by converting signals received at cellular receptors into specific cellular responses. This complex process influences a variety of biological functions, including gene expression, cell division, and organismal behavior.

Examples of Signal Transduction

1. Quorum Sensing in Bacteria:

   • Mechanism: Bacteria use quorum sensing to monitor their population density through the production and release of chemical signals known as autoinducers. These molecules can freely pass through cell membranes.

   • Function: As the concentration of autoinducers increases within the bacterial community, they accumulate inside the bacterial cells. When the internal concentration of these signals exceeds a certain threshold, it triggers a signal transduction pathway that can alter gene transcription and affect behaviors like bioluminescence, virulence factor production, and biofilm formation.

   • Regulatory Outcome: This mechanism ensures that certain activities are only initiated when sufficient numbers of bacteria are present, optimizing resource use and coordinating group behavior.

2. Epinephrine Signaling via GPCR Pathway:

   • Role of Epinephrine: In mammals, epinephrine (adrenaline) plays a crucial role in the "fight or flight" response. It binds to G-protein-coupled receptors (GPCRs) on target cells.

   • Signal Amplification: The binding of epinephrine to GPCRs activates G proteins, leading to the activation of enzymes like adenylate cyclase, which increases the production of cAMP. This serves as a second messenger to activate further signaling pathways that prepare the body for rapid action.

3. Influence on Gene Expression and Cell Function:

   • Mating Pheromones: In yeast, pheromones can initiate mating processes by binding to receptors that activate a signal transduction cascade, culminating in changes in gene expression needed for mating.

   • SRY Protein: This is a sex-determining factor that, when activated, can initiate the transcription of genes involved in male development.

   • Ethylene in Plant Ripening: Ethylene acts as a hormone in plants, binding to receptors that activate signal transduction pathways leading to the ripening of fruits. This is a critical process for seed dispersal and fruit consumption.

Epinephrine Signaling Pathway: Mechanism and Cellular Responses

• Role of Epinephrine: Commonly known as adrenaline, epinephrine is a hormone crucial for the body's fight or flight response to perceived threats. It enhances physiological changes that prepare the body for rapid action, such as increased heart rate, enhanced blood flow to muscles, reduced digestion, and elevated glucose levels for energy.

Detailed Pathway of Epinephrine Signaling

1. Receptor Binding:

   • Initiation: Epinephrine binds to a specific transmembrane receptor known as a G-protein-coupled receptor (GPCR).

   • Receptor Activation: The binding of epinephrine causes the GPCR to change shape, initiating the signal transduction pathway.

2. Activation of G-Protein:

   • GTP Binding: The change in shape of the GPCR causes the G-protein associated with the receptor to exchange GDP for GTP on its alpha subunit, activating the G-protein.

3. Activation of Adenylyl Cyclase:

   • Enzyme Action: The activated G-protein alpha subunit then activates adenylyl cyclase, an enzyme located on the inner side of the cell membrane.

   • cAMP Production: Adenylyl cyclase converts ATP into cyclic AMP (cAMP), a secondary messenger that plays a critical role in transmitting the adrenaline signal within the cell.

4. Phosphorylation Cascade:

   • Protein Kinase A Activation: cAMP activates Protein Kinase A (PKA), which is a critical step in the phosphorylation cascade.

   • Target of PKA: PKA then phosphorylates various proteins within the cell, altering their function to respond to the epinephrine signal.

5. Glycogen Breakdown:

   • Activation of Glycogen Phosphorylase: One significant target of PKA is glycogen phosphorylase, an enzyme responsible for breaking down glycogen into glucose.

   • Energy Mobilization: The hydrolysis of glycogen by glycogen phosphorylase provides quick energy (glucose), which is essential for the fight or flight response, facilitating either rapid movement or combat readiness.

Yeast Pheromones and Gene Expression

• Mechanism of Action:

  • Pheromone Reception: Yeast cells have G-protein-coupled receptors (GPCRs) that detect mating pheromones.

  • Signal Transduction: When these pheromones bind to GPCRs, the receptors activate via GTP-binding, initiating a phosphorylation cascade.

  • Gene Expression and Cell Division: This cascade ultimately leads to the activation of transcription factors that regulate genes responsible for yeast growth and division.

SRY Protein and Male Development

• Genetic Basis:

  • Y Chromosome: The Y chromosome contains the SRY gene, which is critical for male sex determination.

  • Protein Function: Expression of the SRY gene produces the SRY protein. This protein triggers a biochemical pathway that leads to the production of testosterone and Anti-Müllerian hormone, crucial for male development.

• Developmental Impact:

  • Embryonic Expression: Initially, embryos express the X chromosome; the activation of the Y chromosome, specifically the SRY gene, redirects the developmental pathway towards male differentiation. This explains why the default developmental pathway is female unless altered by the presence of SRY.

Ethylene and Fruit Ripening

• Role of Ethylene:

  • Trigger for Ripening: Ethylene is a gaseous hormone produced by ripening fruits, which acts as a signal to accelerate the ripening process.

  • Feedback Mechanism: The release of ethylene by fruits like apples creates a positive feedback loop. As ethylene levels increase, they trigger the expression of enzymes that further promote ripening, leading to more ethylene production.

• Community Effect:

  • Spread of Ripening: Ethylene released by one fruit can affect surrounding fruits, leading to a synchronized ripening process across multiple fruits. This is advantageous for plants in natural settings, as synchronized ripening can enhance the chances of seed dispersal by attracting more fruit-eating animals at once.