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Cell Communication Signal-transduction pathway Cell signaling is the way that cells have to respond to external stimuli • Increase in temperature • Fight of flight response • Increased aerobic activity In each case, signal transduction pathways are a key component in cell communication Signal-transduction pathway Signal on a cell’s surface is converted into a specific cellular response (binds to a receptor) Transduced = changed (protein changes shape) Cell junctions connect cell to cell • Animal – gap junctions • Plant – plasmodesmata Direct Contact communication • Cell –cell recognition, important in development and immune system Local signaling (short distance): Paracrine (Local regulators - growth factors) Synaptic (neurotransmitters – electrical to chemical signal) Long distance: hormones Hormones Endocrine System Travel through the circulatory system Plant hormones = growth regulators Signal transduction Stages of cell signaling 3 steps: 1. Reception: target cell detection 2. Transduction: single-step or series of changes 3. Response: triggering of a specific cellular response Cell signaling Step 1: Reception Only Target Cells have the receptor to bind the signal molecule Signal molecule is a ligand – only binds to specific molecules Binding Usually changes shape of receptor • Activates receptor • Causes aggregation (clumping) Step 2: Transduction Usually multistep Amplifies response Mostly involves proteins Relay molecules are activated or deactivated by phosphorylation Protein phosphorylation Protein activity regulation (ON/OFF switch) Adding phosphate from ATP to a protein activates proteins Enzyme: protein kinases (1% of all our genes) Reversal enzyme: protein phosphatases • Removes the phosphate group • Phosphorylation (activation) is only temporary • Do not want the protein to be continually “ON” Second messengers Non-protein signaling pathway Ex: cyclic AMP (cAMP) Ex: Glycogen breakdown with epinephrine • Epinephrine activates receptor in membrane Enzyme: adenylyl cyclase G-protein-linked receptor in membrane (guanosine di- or tri- phosphate) Cellular responses to signals Cytoplasmic activity regulation Cell metabolism regulation Nuclear transcription regulation Plant and Animal Hormones G-Protein-Linked Receptors One example of secondary messenger is the Gprotein-linked receptors We will discuss the steps as an example • Step 1: There is a “loosely” bound protein bound to the cytosol side of the plasma membrane (G-protein) – Inactive if bound with GDP – Active if bound with GTP (like ATP, but with guanine) G-Protein-Linked Receptors Step 2: Another protein acts as the receptor protein • When a chemical signal binds to the receptor protein, changes shape and allows the G-protein to bind G-Protein-Linked Receptors Step 3: When the Gprotein binds to the receptor protein, the G-protein changes shape Step 4: The new conformation of the Gprotein causes GTP to displace GDP (Gprotein is now active) G-Protein-Linked Receptors Step 5: Activate Gprotein now moves free of the receptor protein Step 6: The G-protein moves along the plasma membrane until it binds to another membrane protein (enzyme) G-Protein-Linked Receptors Step 7: The G-protein binds to the membrane-bound enzyme and changes its shape (activates the enzyme) Step 8: Allows the enzyme to perform its function (ie. Convert ATP to cAMP) G-Protein-Linked Receptors Note: The G-protein is also an enzyme • It acts as a GTPase enzyme (catalyzes the breakdown of GTP to GDP) • This takes a little bit of time so the G-protein can be active for a limited time • This allows the G-protein to “regenerate”