Initiation of Signal Transduction Pathways by Receptors
Definition
Signal transduction
Signal transduction is the process by which cells respond to external signals, converting them into a cascade of chemical responses inside the cell.
- Signal transduction is the process by which a cell converts an external signal into a specific response.
- It begins when a signaling molecule (ligand) binds to a receptor on the cell surface or inside the cell.
- This binding triggers a series of molecular events, often involving multiple steps, to produce a cellular response.
- Signal transduction pathways are essential for coordinating cellular activities and ensuring that cells respond appropriately to their environment.
- Imagine you’re in a crowded room, and someone whispers your name.
- Instantly, your attention shifts.
- This is similar to how cells respond to signals in their environment.
- But how does a cell "hear" these signals and respond appropriately?
- The answer lies in signal transduction pathways.
Signal transduction pathways are highly specific and efficient, ensuring that cells respond accurately to their environment.
The Three Stages of Signal Transduction
- Reception: The signaling molecule binds to a receptor.
- Transduction: The signal is relayed through a series of molecules inside the cell.
- Response: The cell performs a specific action, such as activating a gene or releasing a substance.
- Think of signal transduction as a relay race.
- The ligand starts the race by binding to the receptor, which then passes the "baton" (the signal) through various molecules until the final runner (the effector) completes the task.
Types of Receptors
- Receptors are proteins that recognize and bind specific signaling molecules. They can be classified into two main types:
- Transmembrane Receptors: Located in the plasma membrane, these receptors bind signaling molecules that cannot cross the membrane, such as proteins and peptides.
- Intracellular Receptors: Found inside the cell, these receptors bind signaling molecules that can diffuse through the membrane, such as steroid hormones.
Transmembrane Receptors: Relaying Signals Across the Membrane
- Transmembrane receptors span the plasma membrane, with one part exposed to the extracellular environment and another part inside the cell.
- When a signaling molecule binds to the extracellular domain, it causes a structural change in the receptor, activating its intracellular domain.
- Think of the transmembrane receptor as a doorbell.
- When you press it (ligand binding), it sends a signal to the house (cell), triggering a response (e.g., someone answering the door).
G-Protein-Coupled Receptors (GPCRs)
GPCRs are one of the most common types of transmembrane receptors. They work in partnership with a G protein, which is attached to the inner side of the plasma membrane.
- Ligand Binding: A signaling molecule binds to the GPCR, causing a conformational change in the receptor.
- G Protein Activation: The activated receptor causes the G protein to exchange GDP for GTP, activating the G protein.
- Signal Relay: The activated G protein dissociates into subunits, which interact with other proteins in the cell, such as enzymes or ion channels, to propagate the signal.
- When epinephrine (adrenaline) binds to its GPCR on liver cells, it activates a G protein.
- This, in turn, activates the enzyme adenylyl cyclase, which converts ATP into cyclic AMP (cAMP), a secondary messenger.
- cAMP amplifies the signal, leading to the breakdown of glycogen into glucose.
Receptor Tyrosine Kinases (RTKs)
RTKs are another type of transmembrane receptor. They have an enzymatic domain that adds phosphate groups to specific proteins, a process called phosphorylation.
- Ligand Binding: A signaling molecule binds to the RTK, causing two receptor molecules to dimerize (pair up).
- Activation: The dimerized receptors phosphorylate each other, activating their kinase domains.
- Signal Relay: The phosphorylated receptor recruits and activates other proteins in the cell, triggering a cascade of events.
- The insulin receptor is an RTK.
- When insulin binds, it activates the receptor, leading to the insertion of glucose transporters into the plasma membrane, allowing glucose to enter the cell.
Intracellular Receptors: Directly Influencing Gene Expression
- Some signaling molecules, like steroid hormones, can cross the plasma membrane and bind to receptors inside the cell.
- These receptors often act as transcription factors, directly influencing gene expression.
- Ligand Binding: The signaling molecule enters the cell and binds to its receptor in the cytoplasm or nucleus.
- Complex Formation: The ligand-receptor complex binds to specific DNA sequences, promoting or inhibiting the transcription of target genes.
- Response: The cell produces specific proteins in response to the signal.
- Testosterone binds to its receptor in the cytoplasm.
- The hormone-receptor complex then enters the nucleus and activates genes involved in muscle growth and development.
- How do signal transduction pathways illustrate the balance between specificity and amplification in biological systems?
- Can you think of other systems where a small input leads to a large output?
What happens when a signaling molecule binds to a receptor on a cell?
