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Section 6.docx

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BIOL 2020
Patrice Cote

Section 6 Cell Signaling and Signal Transduction: Communication Between Cells p. 605-630, 642-646, 653-657 cells communicate through extracellular messenger molecules autocrine signaling: cell that is producing messenger has receptors on its surface that responds to and inhibits the synthesis of the messenger paracrine signaling: messenger molecules travel a short distance to so that they are in close proximity to the cell that is generating the message; they’re travel is limited because they are usually unstable or are degraded by enzymes endocrine signaling: messenger molecules reach their target cells through the bloodstream. These messengers are also called hormones. Target cell is usually distant. messenger molecule outside of cell: ligand First messenger is also called a ligand; sticks to receptor outside of the cell when a ligand binds to a receptor (they are specific), the interaction causes a signal across the cytoplasmic domain that can cause 2 different pathways: Two types of cell signaling: 1. Signal transmitted by second messenger. Receptor has an effector (an enzyme), which activates a second messenger. Second messengers are small molecules that typically activate (or inactivate) specific proteins. Signal transmitted by protein recruitment. Based on phosphorylation of the receptor, and then because its phosphorylated, proteins get recruited to the cell surface. Then there is a cascade which activates target protein. phosphorylation can: activate or inactivate an enzyme increase or decrease protein-protein interactions induce a protein to move from one subcellular compartment to another act as a signal that initiates protein degradation What cell signaling can activate: transcription survival protein synthesis movement cell death metabolic change signal transduction: process of information being carried by extracellular messenger molecules that translate a change inside a cell Signal transduction is terminated by removing the ligand: by enzymes or activated receptor is internalized; then gets degraded inside cell receptor and ligand can both be degraded or ligand is separated in an endosome, and receptor is returned to cell surface CELL SURFACE RECEPTORS NO kinase activity G Protein-Coupled Receptors (GPCRs) big family of receptors that contain 7 transmembrane α-helices translate the binding of extracellular signaling molecules into the activation of GTP-binding proteins 4% of genes encode these receptors 700 different types of these receptors receptors attached to heterotrimeric G proteins amino-terminus present on the outside of the cell carboxyl-terminus on the inside three loops on the outside that make the ligand binding site three loops on the cytoplasmic side that make the binding sites for intracellular signaling proteins G proteins binds to the third intracellular loop Arrestins also binds to the third loop; competition Arrestin inactivates the receptor, and causes endocytosis to occur g proteins: 3 subunits in G proteins: alpha, beta, gamma alpha subunit, what does the action, GTP-ase, gets activated by GTP, inactive when GDP beta, gamma: inhibits when activated, G-alpha has a low affinity for the other subunits, causing its dissociation G-alpha activates an effector protein (adenyl cyclase) which creates second messengers (cAMP); second messengers activate one or more signaling proteins (PKA) 1. ligand binds to g-receptor conformation change in the extracellular ligand-binding site conformational exchange is transferred across the membrane (in the 7 loops) change in cytoplasmic loops of receptor increase in the affinity of the receptor for g proteins binding activate the alpha subunit in G protein, causing a release of GDP, which is then followed by binding of GTP ligand -> GPCR -> G Protein -> adenyl cyclase -> cAMP -> PKA - GRK GRK - G protein coupled receptor kinase other effectors: phospholipase C-beta and cyclic GMP phosphodiesterase Adenylyl Cyclase (effector) PKA activation phosphorylates cAMP binds to nucleotide binding site on inactive pKA... which ACTIVATES PKA when the sites are activated the subunits is removed from the catalytic binding site allosteric regulation Receptor Protein-Tyrosine Kinase (RTKs) second class of receptors that translate the presence of extracellular messenger molecules into into changes inside the cell. receptor dimerization; followed by activation of the receptor’s protein-kinase domain phosphorylate specific tyrosine residues of cytoplasmic substrate proteins affects their activity, localization, or their ability to interact with other proteins in a cell Ligand-gated channels conduct a flow of ions across the plasma membrane results in temporary change of membrane potential this affects the activity of other membrane proteins the influx of certain ions (Ca2+) can change the activity of cytoplasmic enzymes ex. neurotransmitters, nerve impulse Steroid Hormone Receptors ligand-regulated transcription factors steroids diffuse across the plasma membrane and bind to receptors in the cytoplasm results in a conformational change that causes the hormone-receptor complex to move into the nucleus and bind to elements present in the promoters or enhancers of hormone-responsive genes; this increases or decreases the rate of gene transcription B and T cell receptors foreign antigens Lipid Derived Second Messengers phospholipases lipid splitting enzyme cuts ester bonds in lipids phospholipid kinases lipid phosphorylating enzymes phospholipid phosphatases lipid dephosphorylating enzymes phospholipases phospholipase C is more important phospholipase C has a PH DOMAIN involved in phosphatidylinositol derived second messengers phosphatidylinositol - phospholipid the inositol rings can be phosphorylated at different places (3) when phosphates are added it is called a phosphoinositide Phosphatidylinositol (PI) derived second messengers PIP2 creates an anchoring site in the membrane for proteins that have a pH domain PLC Beta is the effector, recognizes PIP2 very specifically DAG - diacylglycerol from fatty acid chains - second messenger IP3 - second messenger DAG --> becomes a binding site for protein kinase C
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