Cell Communication Chapter 11

What you need to know! The 3 stages of cell communication. How G-protein-coupled receptors receive cell signals and start transduction How receptor tyrosine kinase receive cell signals and start transduction. How a phosphorylation cascade amplifies a cell signal during transduction. How a cell response in the nucleus turns on genes while 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: Reception: target cell’s detection of signal molecule Transduction: converting the signal into a cellular response Response: The specific cellular response to the signal

Signaling Overview Media\11_06SignalingOverview_A.swf

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: Ligand binds to a protein receptor causing a conformational change which binds the receptor protein to an inactive G protein A GTP molecule now replaces a GDP molecule in the G protein which activates the G protein 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) 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

Receptor Tyrosine Kinases Two receptor tyrosine kinases work in tandem to create multiple responses from a single ligand: Ligands bind to both tyrosine kinases The two tyrosine kinases combine to form a dimer this activates the tyrosine region of the protein The tyrosine regions of the dimer are each phosphoralized by ATP molecules Each tyrosine substructure activates a specific relay protein, each relay protein undergoes a conformational change and in-turn activates a transduction pathway

Receptor Tyrosine Kinases

Ligand Gated Ion Channels Ion channels are simple gates that open when ligands bind to them The ions trigger the cellular response upon entry This 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 Ca2+ can also act as a second messenger which, when released from the ER, activate various proteins

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 Media\11_13SignalTransduction_A.swf

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