Signal Transduction and Cell Signaling, Part 1
Lecture # 17: Signal Transduction and Cell Signaling, Part 1
Major Topics Covered in Lecture
15.1 Signal Transduction Pathways: From Extracellular Signal to Cellular Response
15.2 Studying Cell-Surface Receptors and Signal Transduction Proteins
15.3 Structure and Mechanism of G Protein-Coupled Receptors
15.4 Regulating Metabolism of Many Cells: G Protein-Coupled Receptors That Activate or Inhibit Adenylyl Cyclase
Learning Objectives
Describe the general principles of cell signaling.
Describe the different types of molecules that act as signaling molecules.
Describe a "generic" signaling pathway.
Describe signaling through a trimeric G protein linked cell surface receptor.
Explain the difference between monomeric and trimeric G proteins.
Describe the different G protein accessory proteins.
Describe the different secondary messengers.
Understand the basic mechanisms of activation and termination of a signaling pathway.
Important Topics to Remember about Signal Transduction
All cells respond to extracellular signals/stimuli that activate plasma membrane or cytosolic receptors.
Activated receptors function as transcription factors or activate G protein switches that regulate a variety of downstream pathways or induce generation of intracellular second messengers that do so.
Protein phosphorylation by kinases and dephosphorylation by phosphatases regulate protein activity in cellular pathways and can amplify intracellular signaling.
Overview of Cell Signaling
Extracellular Signaling Molecules
Also known as ligands or first messengers.
Synthesized, packaged into secretory vesicles, and secreted by specialized signaling cells within multicellular organisms.
Exogenously derived molecules serving as ligands for cellular receptors (e.g., bacterial or fungal proteins activating the immune system).
The signal produces a specific response only in target cells expressing receptor proteins that bind the signal.
Roles of Molecules in Signal Transduction
The conversion of an impulse or stimulus from one physical or chemical form to another, through which cells respond to extracellular signals.
The activated receptor relays this information to cytoplasmic proteins and other molecules that:
Relay and amplify the signal.
Integrate it with other signals.
Perform a cell response.
Overview of Hydrophobic Signaling Molecules
Examples include steroids and related molecules.
Step-by-step mechanism:
Diffuse through the plasma membrane.
Bind to cytosolic receptors.
Receptor-signal complex moves into the nucleus, binding transcription-control regions in DNA to activate or repress gene expression.
Hydrophilic Signaling Molecules
Includes small molecules (e.g., adrenaline, acetylcholine), peptides (e.g., yeast mating factors, glucagon), and proteins (e.g., insulin, growth hormone).
Require transmembrane receptors to transduce signals due to their size and hydrophilicity.
Step-by-step mechanism:
Bind to specific cell-surface receptor proteins, triggering a conformational change.
Activated receptor activates downstream signal transduction proteins or small-molecule second messengers.
Signal transduction proteins or second messengers activate one or more effector proteins.
Effector proteins stimulate modifications of cytosolic proteins (short-term changes) or move into the nucleus to trigger long-term changes in gene expression.
Termination of Cellular Response
Step 8: Negative feedback/feedback repression from intracellular signaling molecules.
Step 9: Destruction of the extracellular signal.
Feedback regulation can positively or negatively regulate proteins in a signal transduction cascade.
Other mechanisms include:
GTP hydrolysis inactivating G proteins.
Secondary messenger degradation or sequestration (e.g., cAMP by cAMP phosphodiesterase).
Dephosphorylation of proteins by phosphatases.
Receptor endocytosis, which can involve degradation or recycling.
Desensitization through receptor phosphorylation, with arrestin bound to inactivate the receptor.
Types of Extracellular Signaling
Endocrine Signaling
Involves hormones (e.g., epinephrine, insulin).
Signaling molecules synthesized by signaling cells (e.g., endocrine glands) and transported through the circulatory system to distant target cells.
Paracrine Signaling
Involves signaling molecules (e.g., neurotransmitters, growth factors) that affect only nearby target cells.
Neuronal Signaling
A special case where signals are delivered to individual cells over long distances via electrical impulses along axons to synapses, releasing neurotransmitters to diffuse across to target cells.
Autocrine Signaling
Cells respond to signals it secretes, often seen in tumor cells overproducing growth factors.
Juxtacrine Signaling
Involves signals transmitted through direct contact with surface receptors or through small conduits (gap junctions) into adjacent cells.
Basic Elements of Cell Signaling Systems
Receptors on target cells receive messages, generating intracellular second messengers through associated effector enzyme functions.
Second messengers: Small soluble molecules regulating specific target proteins through cascade signaling pathways.
Molecular switches: Two main types controlled by phosphorylation state and GTP-binding proteins (G proteins).
Each signaling pathway involves a series of protein interactions causing cellular responses by altering protein conformations and functions.
Regulation of Protein Activity
Kinase/Phosphatase Switch
Involves kinase phosphorylation and phosphatase dephosphorylation to regulate target protein activity.
Protein Kinase: Transfers terminal phosphate from ATP to specific hydroxyl groups on serine or threonine residues.
Protein Phosphatase: Hydrolyzes phosphate off proteins, restoring the original state.
Example:
Unphosphorylated: inactive.
Phosphorylated: active.
GTPase Switch Proteins
Cycling between Active and Inactive States
GTP-binding proteins act as on-off switches in signal transduction pathways, regulated by GEFs and GAPs.
GTPase superfamily includes:
Heterotrimeric G proteins: Activated by direct interaction with surface receptors.
Monomeric G proteins: Activated by GEFs that are stimulated by surface receptors or other proteins.
Mechanisms and Switch Domains
Monomeric G proteins, like Ras, utilize switch I and switch II regions to promote binding to downstream signaling proteins. Variability in GTP hydrolysis rates affects conformation and interaction with effectors.
Intracellular Second Messengers
Types
cAMP: Generated from ATP by adenylyl cyclase, activates PKA.
cGMP: Generated by guanylyl cyclase, activates PKG and specific cation channels.
IP3 and DAG: Produced from PIP2 by phospholipase C.
IP3 opens channels to release Ca2+ from the ER.
DAG activates PKC alongside Ca2+.
Calcium Ions (Ca2+): Released from stores or transported into cells, activating calmodulin and other regulatory proteins.
Nitric Oxide (NO): Catalyzed by nitric oxide synthase, easily diffuses across membranes and acts in autocrine or paracrine signaling.
Characteristics
Second messengers are typically small and diffuse quickly, allowing high local concentration for effective signaling relay and amplification.
Studying Cell-Surface Receptors and Signal Transduction Proteins
Near-maximal cellular responses to a ligand can occur at concentrations where less than 100% of the receptors are bound.
Signal receptors are often targets for drugs.
Various experimental approaches are implemented for studying receptors, including affinity chromatography, Western blotting, immunoprecipitation, and pull-down assays.
Binding Assays
Determine dissociation constant (Kd) and receptor numbers per cell.
Experiment methodology:
Use labeled ligand at varying concentrations to experimental and control cells.
Measure the amount of bound ligand and plot against concentration.
Results demonstrate that physiological responses can occur even when only a fraction of receptors are bound.
G Protein-Coupled Receptors (GPCRs)
Structure and Mechanism
GPCRs are part of a large family of proteins responding to many extracellular signals, sustaining trimeric G protein activation.
Composed of seven a-helical transmembrane domains, with four extracellular and four cytosolic segments.
Ligands include hormones, neurotransmitters, and chemoattractants.
Activation Mechanism
Upon activation by ligands, G proteins composed of three subunits (α, β, γ) exchange GDP for GTP, releasing subunits that interact with effector proteins.
Signal Cascade Example
Involves steps from hormone binding through G protein activation, second messenger generation, to downstream effects altering cellular activity and gene expression.
Termination Mechanisms
Utilizes GAPs for rapid GTP hydrolysis, altering secondary messenger concentrations and deactivating GPCRs through phosphorylation and arrestin binding, blocking further signaling.
Specificity and Diversity of GPCR Responses
Diverse ligands and receptors yield a wide array of activation signals.
G proteins subunits contribute to signal specificity, allowing different cellular responses based on receptor types and signaling context.
Key GPCR Functions
Regulation through adenylyl cyclase can either stimulate or inhibit cAMP production, impacting downstream effector processes significantly.
Hormonal effects (e.g., epinephrine) on glycogen metabolism highlight how GPCR signaling intricately connects with metabolic pathways.
Next Lecture: Signal Transduction and Cell Signaling, Part 2
Reading Assignments
Chapter 15: pages 690-704
Chapter 16: pages 705-726
Reminder of the next adaptive quiz availability until Tuesday (Oct 28th) 8:00 am.