Cell Communication – Signal Transduction External signal is received and converted to another form to elicit a response
Signal Transduction & G Protein-coupled Receptors Topics Signal Trans.: From Extracellular Signal to Cellular Response Cell-Surface Receptors & Signal Transduction Proteins G Protein-coupled Receptors (GPCRs): Structure and Mechanism GPCRs That Regulate Ion Channels GPCRs That Regulate Adenylyl Cyclase GPCRs That Regulate Cytosolic Calcium
Learn the G protein cycle of reactions involved in GPCR signaling. Learning Objectives Learn the general properties of signaling molecules (ligands), cell-surface receptors, & intracellular signal transduction components. Learn the G protein cycle of reactions involved in GPCR signaling. Learn the epinephrine receptor signal trans pathway used for control of glycogen degradation. Learn about the GPCR-stimulated IP3/DAG signaling pathway.
General Principles of Signal Transduction refers to the overall process of converting extracellular signals into intracellular responses . Key players in signal transduction are : signaling molecules, receptors, signal transduction proteins second messengers, and effector proteins.
Cells respond to signals by changing the activity of existing enzymes (fast) and/or the levels of expression of enzymes and cell components (slower) by gene regulation (Steps 7a & 7b). Receptors and signal transduction systems have evolved to detect and respond to hormones, growth factors, drugs & neurotransmitters.
Intercellular Communication
Intercellular Communication Communication between cells requires: ligand: the signaling molecule receptor protein: the molecule to which the receptor binds -may be on the plasma membrane or within the cell
Structure and function of receptors Globular proteins acting as a cell’s ‘letter boxes’ Located mostly in the cell membrane Receive messages from chemical messengers coming from other cells Transmit a message into the cell leading to a cellular effect Different receptors specific for different chemical messengers Each cell has a range of receptors in the cell membrane making it responsive to different chemical messengers
Mechanism Receptors contain a binding site (hollow or cleft in the receptor surface) that is recognised by the chemical messenger Binding of the messenger involves intermolecular bonds Binding results in an induced fit of the receptor protein Change in receptor shape results in a ‘domino’ effect Domino effect is known as Signal Transduction, leading to a chemical signal being received inside the cell Chemical messenger does not enter the cell. It departs the receptor unchanged and is not permanently bound
Overall process of receptor/messenger interaction Signal transduction Binding interactions must be: - strong enough to hold the messenger sufficiently long for signal transduction to take place - weak enough to allow the messenger to depart Implies a fine balance Drug design - designing molecules with stronger binding interactions results in drugs that block the binding site - antagonists
Messenger binding Bonding forces Ionic H-bonding van der Waals Example: vdw interaction Phe H-bond Binding site Ser O H ionic bond Asp CO2 Receptor
How does the Binding Site Change Shape? Substrate binding Bonding forces Induced fit - Binding site alters shape to maximise intermolecular bonding Phe Ser O H Asp CO2 Phe Ser O H Asp CO2 Induced Fit Intermolecular bonds not optimum length for maximum binding strength Intermolecular bond lengths optimised
External signals are converted to Internal Responses Cells sense and respond to the environment Prokaryotes: chemicals Humans: light - rods & cones of the eye sound – hair cells of inner ear chemicals in food – nose & tongue Cells communicate with each other Direct contact Chemical signals
General principles 1. Signals act over different ranges. 2. Signals have different chemical natures. 3. The same signal can induce a different response in different cells. 4. Cells respond to sets of signals. 5. Receptors relay signals via intracellular signaling cascades.
Cells detect signal & respond 1º messenger Cells detect signal & respond Effector Enzymes Target Enzymes 2º messengers Signal transduction: ability of cell to translate receptor-ligand interaction into a change in behavior or gene expression
Relay molecules in a signal transduction pathway Primary Messenger Secondary Messengers Target Enzymes EXTRACELLULAR FLUID Receptor Signal molecule Relay molecules in a signal transduction pathway Plasma membrane CYTOPLASM Activation of cellular response Reception 1 Transduction 2 Response 3 Cascade Effect
activates 100 control factors Each protein in a signaling pathway Amplifies the signal by activating multiple copies of the next component in the pathway - activates an enzyme activity, processes 100 substrates /second primary signal Primary enzyme activates 100 target enzymes Each of the 100 enzymes activates an additional 100 downstream target enzymes Each of the 10,000 downstream targets activates 100 control factors so rapidly have 1,000,000 active control factors
1 102 104 105 106 108 A Signal Cascade amplification Glucose-1-phosphate (108 molecules) Glycogen Active glycogen phosphorylase (106) Inactive glycogen phosphorylase Active phosphorylase kinase (105) Inactive phosphorylase kinase Inactive protein kinase A Active protein kinase A (104) ATP Cyclic AMP (104) Active adenylyl cyclase (102) Inactive adenylyl cyclase Inactive G protein Active G protein (102 molecules) Binding of epinephrine to G-protein-linked receptor (1 molecule) Transduction Response Reception A Signal Cascade amplification 1 102 104 105 106 108
Receptors relay signals via intracellular SIGNALING CASCADES amplification
Main Types of Receptors ION CHANNEL RECEPTORS G-PROTEIN-COUPLED RECEPTORS KINASE-LINKED RECEPTORS INTRACELLULAR RECEPTORS
ion-channel-linked Trimeric G-protein-linked enzyme-linked Cell-surface receptors -large &/or hydrophilic ligands ion-channel-linked Trimeric G-protein-linked enzyme-linked (tyrosine kinase)
Receptor protein is part of an ion channel protein complex Signal transduction Control of ion channels Receptor protein is part of an ion channel protein complex Receptor binds a messenger leading to an induced fit Ion channel is opened or closed Ion channels are specific for specific ions (Na+, Ca2+, Cl- , K+) Ions flow across cell membrane down concentration gradient Polarises or depolarises nerve membranes Activates or deactivates enzyme catalysed reactions within a cell
Nerve Cell communication Ion channel receptors Cellular response Gate open Gate close Ligand-gated ion channel receptor Plasma Membrane Signal molecule (ligand) Gate closed Ions Examples: Muscle Contraction Nerve Cell communication
Review: Na+ Cl- - + Remember the Na+/K+ ATPase (Na+/K+ pump)? [Na+] inside ~10mM; outside ~150mM [K+] inside ~150mM; outside ~5mM cell has membrane potential ~ -60mV -60mV K+ A- Na+ Cl- - +
Intercellular Communication How ?
Intercellular Communication There are four basic mechanisms for cellular communication: 1. direct contact 2. paracrine signaling 3. endocrine signaling 4. synaptic signaling
Intercellular Communication Direct contact – molecules on the surface of one cell are recognized by receptors on the adjacent cell
Intercellular Communication Paracrine signaling – signal released from a cell has an effect on neighboring cells local ex. nitric oxide, histamines, prostaglandins
Intercellular Communication Endocrine signaling – hormones released from a cell affect other cells throughout the body long distance ex. Estrogen, Thyroxine, GH Epinephrine ….
Intercellular Communication Synaptic signaling – nerve cells release the signal (neurotransmitter) which binds to receptors on nearby cells
Intercellular Communication When a ligand binds to a receptor protein, the cell has a response. signal transduction: the events within the cell that occur in response to a signal Different cell types can respond differently to the same signal.
Receptor Types There are 3 subclasses of membrane receptors: 1. channel linked receptors – ion channel that opens in response to a ligand 2. enzymatic receptors – receptor is an enzyme that is activated by the ligand 3. G protein-coupled receptor – a G-protein (bound to GTP) assists in transmitting the signal
Intercellular Communication A cell’s response to a signal often involves activating or inactivating proteins. Phosphorylation is a common way to change the activity of a protein. protein kinase – an enzyme that adds a phosphate to a protein phosphatase – an enzyme that removes a phosphate from a protein
Signal Transduction Components: Kinases/Phosphatases Proteins that participate in intracellular signal transduction fall into two main classes--protein kinases/phosphatases and GTPase switch proteins. Kinases use ATP to phosphorylate amino acid side-chains in target proteins. Kinases typically are specific for tyrosine or serine/threonine sites. Phosphatases hydrolyze phosphates off of these residues. Kinases and phosphatases act together to switch the function of a target protein on or off .
Kinases - Phosphorylation Phosphatase - Dephosphorylation Tyrosine-OH Tyr-Kinases Serine-OH Ser/Thr-Kinases Threonine-OH „dual specificity“ Kinases
There are about 600 kinases and 100 phosphatases encoded in the human genome. Activation of many cell-surface receptors leads directly or indirectly to changes in kinase or phosphatase activity. Note that some receptors are themselves kinases (e.g., the insulin receptor).
Growth hormone receptor Tetrameric complex constructed in presence of growth hormone GH GH binding & dimerisation OH Binding of kinases HO OP PO ATP ADP Activation and phosphorylation GH receptors (no kinase activity) OH HO kinases Kinase active site opened by induced fit Growth hormone binding site Kinase active site
Intracellular receptors Chemical messengers must cross cell membrane Chemical messengers must be hydrophobic Example-steroids and steroid receptors CO2H H2N Steroid binding region Zinc DNA binding region (‘zinc fingers’) Zinc fingers contain Cys residues (SH) Allow S-Zn interactions
Intracellular Receptors steroid hormones -have a nonpolar, lipid-soluble structure -can cross the plasma membrane to a steroid receptor -usually affect regulation of gene expression An inhibitor blocks the receptor from binding to DNA until the hormone is present.
Intracellular receptor Mechanism Co-activator protein Receptor Cell membrane DNA Receptor-ligand complex Dimerisation Messenger 1. Messenger crosses membrane 2. Binds to receptor 3. Receptor dimerisation 5. Complex binds to DNA 6. Transcription switched on or off 7. Protein synthesis activated or inhibited 4. Binds co-activator protein
Intracellular Receptors A steroid receptor has 3 functional domains: 1. hormone-binding domain 2. DNA binding domain 3. domain that interacts with coactivators to affect gene expression
END PART I