HUBS2206 Human Biochemistry and Cell Biology Lecture 26: Principles of Signal Transduction
Learning Targets
Signal Transduction Cascade: Understanding what it is and how it works.
Extracellular Receptors: Identifying different types.
Protein Kinases and Phosphatases: Understanding their role, especially Tyr and Ser/Thr (de)phosphorylation of proteins.
Phosphorylation Cascade: Example: MAP kinase cascade is essential knowledge.
Death Receptor Signalling: Understanding its role in apoptosis.
Termination of Signalling: Importance of understanding why and how signalling is terminated.
Key Concept: Altered signalling cascades are associated with miscommunication and disease.
Signal Transduction
A signal molecule binds to a receptor protein, activating intracellular signal molecules, which alter target proteins, creating a response.
Signal Transduction via Cell-Surface Receptors
Most signalling molecules are hydrophilic and cannot enter the cell.
They must act through cell-surface receptors.
Cellular Response to External Signals
Cells must possess a receptor for a particular signalling molecule to respond.
The actual response depends on the intracellular machinery that integrates and interprets the signal.
The same signalling molecule can elicit different responses in different cells (e.g., acetylcholine).
Overview: Flow of Information
Hormone / Ligand: First messenger.
Receptor: Reception of signal.
Signal Transduction: Involves second messengers, relay, and signalling molecules.
Response: Involves changes in effectors.
Cell-Surface Receptors: Three Types
Enzyme-linked receptors:
G-protein coupled receptors (GPCRs):
Ion channel-coupled receptors:
Complexity of Signal Transduction Pathways
Extracellular ligand binding to a receptor is converted into complex intracellular signals.
The process involves primary transduction, relay, transduction and amplification, integration, spreading, anchoring, and modulation within the cytosol and nucleus.
Complexity of Signal Integration
Cells have multiple cell-surface receptors and often integrate multiple signals.
Different ways signals can be integrated:
One receptor activates multiple pathways.
Different receptors activate the same pathway.
Different receptors activate different pathways; one pathway affects the other.
Cytostasis and Differentiation circuits (anti-growth factors).
Viability circuits (survival factors, cytokines, death factors).
Why Study Cellular Signalling?
To understand molecular mechanisms of disease, researchers comparatively study signalling pathways in “normal” cells versus diseased cells.
Most cancer-associated modifications of cell signalling are yet to be fully elucidated.
Therapeutic Strategies
Understanding signalling can help develop new therapeutic strategies.
Many current drugs target ligands, receptors, and major key signal transduction molecules.
Examples:
Antibodies can be used as drugs to bind ligands or receptors to prevent receptor activation.
Drugs can mimic ligands and bind to receptors to enhance signalling.
Many drugs inhibit protein kinase activity.
Critical Role of Protein Phosphorylation
Protein phosphorylation plays a crucial role in signal transduction.
Regulation of Protein by Phosphorylation
Approximately 1/3 of proteins are regulated by phosphorylation/dephosphorylation, primarily on Ser, Thr, or Tyr amino acids.
Mediated by protein kinases and phosphatases.
An active protein kinase transfers a phosphate group from ATP onto a protein substrate.
An active protein phosphatase dephosphorylates the protein (takes off the phosphate group).
Role of Protein Phosphorylation
Changes in the phosphorylation state of a substrate are associated with protein conformational (shape) changes.
Changes in the phosphorylation state can lead to changes in:
Protein activity (activation or inactivation).
Protein interactions (promote binding or detachment).
Distribution within the cell (e.g., translocation from cytosol to nucleus or plasma membrane).
Protein Kinases and Phosphatases - Numbers
Homo sapiens: 23,000 genes; ~2-4% of protein-coding genes = kinases or phosphatases.
Substrates of kinases or phosphatases can be other kinases or phosphatases, receptors, metabolic enzymes, cytoskeletal, scaffolding, nuclear proteins and transcription factors, and ion channels.
Many factors regulate the activity of protein kinases and phosphatases, including binding of activators/inhibitors, ions, signalling molecules, and post-translational modifications.
~200 PROTEIN PHOSPHATASES
Tyr phosphatases
Ser/Thr phosphatases
~518 PROTEIN KINASES
90 Tyr kinases
428 Ser/Thr kinases
Signalling by Phosphorylation
ON SWITCH: Typically kinase-mediated protein phosphorylation.
OFF SWITCH: Typically phosphatase-mediated dephosphorylation.
Note that there are some cases wherein phosphorylation can turn off signals and dephosphorylation can turn on signals.
Typical Phosphorylation Cascade
In quiescent cells, many protein kinases are in an inactivated state.
Upon cell stimulation, they typically become phosphorylated, resulting in their activation.
Activation of signalling cascades leads to altered balance between kinase and phosphatase activity.
MAP Kinase Cascade
Mitogen-activated protein kinases (MAP kinase or MAPK or ERK) integrate various extracellular signals.
Target cytoplasmic and nuclear (transcription factors) proteins.
MAPK cascade: series of 3 protein kinases.
Receptor activation leads to activated Raf phosphorylating and activating MEK.
MEK phosphorylates and activates ERK.
ERK phosphorylates other proteins.
Results in activation of pre-existing proteins and changes in gene expression.
Important for control of cell growth and survival.
Organisation of Signalling Pathways
Specific, appropriate response of cells to external stimuli requires integration of multiple signalling pathways.
Stimulation of cell surface receptors initiates cellular signals governed by post-translational modifications (e.g., phosphorylation).
To increase specificity, information transfer inside the cell is highly organised.
Proteins may need recruitment to specific subcellular locations like plasma membrane microdomains.
Adaptor proteins: small, contain protein-binding modules that link 2 proteins together, facilitating larger signalling complexes.
Protein scaffolds or anchor proteins help relay messages by serving as docking sites for multiple signalling proteins involved in the pathway; regulate protein activity in multi-protein complexes.
Docking proteins localize at the membrane next to an activating receptor, to which they bind in a phosphorylation-dependent manner.
Spatial Organisation of Signalling Pathways
The way information is transferred in cells is often highly organized.
Components include receptors (R) and signalling components (X, Y, and Z).
Information is transmitted through protein-protein interactions using signal transduction domains.
Scaffolds function to hold together individual components of signalling pathways, creating macromolecular signalling complexes.
These complexes can aggregate in specific locations within the cell, such as lipid rafts and caveolae.
Signalling by Death Receptors
Death receptors are members of the tumour necrosis factor receptor superfamily that can mediate caspase activation and apoptosis.
Assembly of DISC = Death-inducing signalling complex.
TNF ligand binds as a trimer and activates a transmembrane receptor (trimer).
Recruitment of adaptor proteins (TRADD, FADD) that interact with ‘death domains’ present in the receptor (cytoplasmic side).
Adaptor proteins recruit additional pathway-specific enzymes to the TNF-R1 complex: formation of DISC leads to activation of caspases and apoptosis.
Termination of Signalling
Many ways to “turn” off the signal at several levels:
Removal of ligand (hormone, growth factor, etc.). Most ligands rapidly fall off the receptor. Most ligands are short-lived, rapidly removed from circulation or degraded in the extracellular space.
Receptor-level inactivation by dephosphorylation, binding of inhibitory protein, internalization leading to receptor degradation (through lysosomal digestion), recycling, or sequestration; desensitisation: the receptor no longer responds to the signal (e.g., insulin resistance).
Intracellular signal transduction molecules: inactivation (often via dephosphorylation by protein phosphatases), binding of inhibitory protein, degradation/removal, changes in localisation, sequestration.
Critical to turn the signal “off” to restore an inactive state for homeostasis. Persistent activation of growth factor signalling leads to cancer.
Summary
Signal transduction can occur via activation of cell-surface receptors.
Activation of receptors leads to a signal transduction cascade that relays the message via signalling molecules.
Results in a change in effectors to induce a cellular response.
Signal cascades are highly complex, and multiple cascades/pathways often intersect (cross-talk).
Dysregulation of signalling cascades leads to disease.
Phosphorylation/dephosphorylation is a major mechanism of intracellular signal transduction.
Termination of the signal is just as important as the initiation.