Cell Communication Talking on cell phones Signaltransduction pathway

Cell Communication: Talking on “cell phones”

Signal-transduction pathway b Cell signaling is the way that cells have to respond to external stimuli • • • Increase in temperature Fight of flight response Increased aerobic activity b In each case, signal transduction pathways are a key component in cell communication

Signal-transduction pathway b Signal on a cell’s surface is converted into a specific cellular response (binds to a receptor) b Transduced = changed (protein changes shape) b Cell junctions connect cell to cell • Animal – gap junctions • Plant – plasmodesmata b 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 b Endocrine System b Travel through the circulatory system b Plant hormones = growth regulators

Signal transduction

Stages of cell signaling b 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 b b b 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 b Usually multistep b Amplifies response b Mostly involves proteins b Relay molecules are activated or deactivated by phosphorylation

Protein phosphorylation b Protein activity regulation (ON/OFF switch) b Adding phosphate from ATP to a protein activates proteins b Enzyme: protein kinases (1% of all our genes) b Reversal enzyme: protein phosphatases • • • Removes the phosphate group Phosphorylation (activation) is only temporary Do not want the protein to be continually “ON”

Second messengers b b b Non-protein signaling pathway Ex: cyclic AMP (c. AMP) Ex: Glycogen breakdown with epinephrine • Epinephrine activates receptor in membrane b b Enzyme: adenylyl cyclase G-protein-linked receptor in membrane (guanosine di- or tri- phosphate)

Cellular responses to signals b Cytoplasmic activity regulation b Cell metabolism regulation b Nuclear transcription regulation

Plant and Animal Hormones

G-Protein-Linked Receptors b One example of secondary messenger is the G- protein-linked receptors b 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 b 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 b Step 3: When the G- protein binds to the receptor protein, the G -protein changes shape b Step 4: The new conformation of the Gprotein causes GTP to displace GDP (Gprotein is now active)

G-Protein-Linked Receptors b Step 5: Activate Gprotein now moves free of the receptor protein b Step 6: The G-protein moves along the plasma membrane until it binds to another membrane protein (enzyme)

G-Protein-Linked Receptors b Step 7: The G-protein binds to the membrane-bound enzyme and changes its shape (activates the enzyme) b Step 8: Allows the enzyme to perform its function (ie. Convert ATP to c. AMP)

G-Protein-Linked Receptors b 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”

Step 3: Response b Usually the response is the activation of DNA in some way b We will be discussing this more in depth when we begin protein synthesis