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Cell Communication Chapter 5.6+ Cellular Biology What you need to know! • The 3 stages of cell communication. • How G-protein-coupled receptors receive cell signals and start transduction • How a cell signal is amplified by a phosphorylation cascade. • An example of a second messenger and its role in a signal transduction pathway. • How a cell response in the nucleus turns on genes, whereas in the cytoplasm it activates enzymes. • What apoptosis means and why it is important to normal functioning of multicellular organisms. Local Signals • Adjacent cells have a variety of junctions: tight, desmosomes, gap • These adjacent cells communicate via gap junctions (plasmodesmata in plants), or cell-cell recognition (antigen and receptor protein) • Local regulators work to influence several localized cells • Neurotransmitters are released into the synapse and stimulate the target cell Long-distance Signaling • Hormones released by the endocrine system travel through the blood and activate cells elsewhere in the body • Neurons can send electrical impulses from the brain to any part of the body External Signals Responses • Animal cells communicate by contact, secreting regulators (hormones), or neurotransmitters There are three stages of cell signaling: 1. Reception: target cell’s detection of signal molecule 2. Transduction: converting the signal into a cellular response 3. Response: The specific cellular response to the signal Signaling Overview Reception • The ligand or signaling molecule can be sent throughout the entire body • Only cells with the appropriate receptor protein “hear” the signal • Most receptors are membrane proteins – Water soluble ligands that cannot cross the plasma membrane • Others receptors are intracellular – Lipid soluble ligands can cross membranes Examples: • G Protein-Coupled Receptors • Receptor Tyrosine Kinases • Ligand-gated ion channels G Protein-Coupled Receptors • Membrane imbedded protein receptor works with a G protein: 1. Ligand binds to a protein receptor causing a conformational change which binds the receptor protein to an inactive G protein 2. A GTP molecule now replaces a GDP molecule in the G protein which activates the G protein 3. The G protein leaves the receptor protein and binds to an enzyme at the allosteric site, this activates the enzyme (which in turn activates transduction) 4. The G protein hydrolyzes the GTP to GDP, this inactivates the G protein, which in turn detaches it from and deactivates the enzyme G Protein-Coupled Receptors Ligand Gated Ion Channels • Ion channels are simple gates that open when ligands bind to them • The ions trigger the cellular response upon entry • These are most common in the nervous system where ligands are neurotransmitters and the ions change the polarity of the cell Intracellular Receptors • Intracellular receptors require ligands that can cross the plasma membrane – Steroids and small gas molecules qualify • The hormone-receptor complex move into the nuclease and act as transcrption factors • These transcription factors turn on certain genes by stimulating RNA polymerase to transcribe the gene(s) • The gene activation triggers the response • The receptor carries out the entire transduction Transduction: Phosphorylation Cascade • The enzyme activated at the end of reception is a relay protein that phosphorylates (activates) protein kinases • The activated protein kinases in-turn activate other protein kinases (potentially amplifying the signal) • The cascade ends with the phosphorylation of a protein that triggers the cell’s response • Protein phosphatases (PP) remove the phosphate from the kinases and the final protein so they can all be reused Transduction Transduction: Second Messengers • Not all transduction molecules are proteins • cAMP is a converted ATP (NA) that cause a phosphorylation cascade Response • Change in cellular function based on a signal transduction induced by a ligand • The final product of many transduction are transcription factors that activate genes • Other pathways simply activate proteins (i.e. enzymes) already in the cytoplasm • Signal transductions can lead to multiple responses from a single ligand Signal Transduction Review Apoptosis • Advanced cell signaling which ends in scheduled cell suicide • Systematic dismantling and digestion of the cell • This prevents the digestive and metabolic enzymes from spilling out and damaging adjacent cells • Crucial in: brain development, the immune system, and morphogenesis of the fingers and toes