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: Reception: target cell detection Transduction: single-step or series of changes 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 G-protein-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 G-protein binds to the receptor protein, the G-protein changes shape Step 4: The new conformation of the G-protein causes GTP to displace GDP (G-protein is now active)

G-Protein-Linked Receptors Step 5: Activate G-protein 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”