Chapter 16 Study Guide-- Jyoti
Chapter 16 Study Guide-- Jyoti BIO 2600
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This 13 page Study Guide was uploaded by Markiesha Notetaker on Tuesday April 26, 2016. The Study Guide belongs to BIO 2600 at Wayne State University taught by Dr. Jyoti Nautiyal in Winter 2016. Since its upload, it has received 127 views. For similar materials see Intr To Cell Biology in Biology at Wayne State University.
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Date Created: 04/26/16
Chapter 16 Study Guide Cell Communication This chapter is about how cells send signals and interpret the signals they receive. In signal transduction, the signaling cell produces a type of extracellular signal molecule that is detected by the target cell. Target cells have receptors that respond to the signal molecule. Signal transduction begins when the receptor on a target cell receives an incoming extracellular signal and converts it to the intracellular signaling molecules. Types of Signaling/Range 1. Endocrine signaling a. Longest range of signaling b. Hormones are sent out by endocrine cells and sent to the rest of the body 2. Paracrine signaling a. Second longest range of signaling b. The signal molecules diffuse locally through the extracellular fluid remaining in the immediate area of the cell that secretes it. 3. Neuronal signaling a. Short distance; not shortest. (<100nm) b. The signal molecule diffuses through the synapse during an action potential 4. Contact dependent signaling a. Shortest range. No molecules secreted b. Cells make direct physical contact through signal molecules in both plasma membranes c. Example: Nervecell production in fruit fly Drosophila Slide 5 Mechanisms of Hormones 1. Down regulation when the hormone triggers a decrease of hormone receptors. A high amount of the hormone causes the cell to be less sensitive to it. (Less receptors=less sensitivity) 2. Up regulation absence of the hormone triggers an increase in receptors. Low amounts of the hormone makes the cell more sensitive to it. (High receptors=high sensitivity) The same signal molecule can induce different responses in different target cells. The information conveyed by the signal depends on how the target cell interprets it. Example: Different cell types respond to AcH in different ways. 1. Pacemaker cells when attached to acetylcholine, it slows down the rate of firing 2. Salivary gland secretes components of saliva 3. Skeletal muscles causes the cell to contract Slide 10 The presence of one signal will often modify the effects of another. For example, one combo may enable a cell to survive, another may cause it to divide and another may cause it to differentiate. In the absence of any signals, most cells kill themselves. Slide 11 Some cell responses can be fast or slow Example acetylcholine can stimulate a muscle cell to contract in milliseconds. When cell growth and cell division is triggered by a signal, it can take many hours to execute because this response requires changes in gene expression and proteins. Hormones crossing the plasma membrane Extracellular signal molecules 1. Too big to go through plasma membrane/hydrophilic a. Rely on receptors on the surface of the target cell to relay the message across the plasma membrane 2. Small and hydrophobic a. Able to go through plasma membrane and into the cytosol. Once inside, they activate intracellular enzymes or intracellular receptor proteins that regulate gene expression. b. Example) Steroids can pass through the membrane and binds to receptor proteins either in the cytosol or the nucleus. i. Whether they are in the cytosol or nucleus, they are called nuclear receptors because once activated, they act as transcriptional regulators in the nucleus. ii. Cortisol, estradiol, testosterone and thyroxine. These receptors are usually in an inactive form. Once activated by the hormone, the receptor undergoes a conformational change allowing it to promote or inhibit the transcription of specific target genes. Slide 14 Example) Nitric Oxide (NO) gas. Some gases can diffuse across the membrane and regulate specific intracellular proteins. NO is synthesized from AA arginineand diffuses into neighboring cells. The NO causes smooth muscle cells in vessel walls to relax, allowing the vessel to dilate so that blood can flow more freely through it. The majority of signal molecules are too large to cross the plasma membrane. These molecule bind to cellsurface receptor proteins that span the plasma membrane. These receptors recognize the extracellular signal and generate new intracellular signals in response. The message is then relayed downstream from one intracellular signaling molecule to another, each activating the next signal. This is a pathway. These intracellular signaling pathways perform the functions of: 1. Relaying a message onward 2. Amplifying the signal received,making it stronger 3. Detect signals from one intracellular pathway and integrate them before relaying the signal onward 4. Distribute the signal to more than one effector protein Positive Feedback a component that lies downstream in the pathway acts on an earlier component to enhance the response to the initial signal Negative Feedback a downstream component acts to inhibit an earlier component in the pathway to diminish the response to the initial signal Many intracellular signals act as molecular switches. Receiving the signal cause them to go from inactive to active and vice versa. Once activated, the proteins can stimulate or suppress other proteins in the signal pathway. Proteins that are activated by phosphorylation have that phosphate added by a p rotein kinase A protein phosphatase takes the phosphate off again. A protein can either be activated or inactivated by either action (having a phosphate bound versus having a phosphate removed) GTP binding proteins toggle between an active and inactive state depending on whether they have GTP or GDP bound to them. Once bound by GTP, they have GTPhydrolyzing (GTPase) activity. (They can cleave off the GTP). They shut themselves off by hydrolyzing the GTP to GDP. Two main types of GTP binding proteins are: 1. Trimeric GTPbinding proteins 2. Small Monomeric GTPases The small monomeric GTPases are aided by two set of regulatory proteins called: 1. Guanine nucleotide exchange factors (GEFs) that activate the switch proteins by promoting the exchange of GDP for GTP. 2. GTPaseactivating proteins(GAPs) that turn them off by promoting GTP hydrolysis Slide 21 Types of Cell Surface Receptor There are 3 classes of cell surface receptors that interpret extracellular signals. 1. Ion channelcoupled receptors a. They change the permeability of the plasma membrane to selected ions, altering the membrane potential and can produce an electrical current b. Turns chemical signals into electrical ones 2. Gprotein coupled receptors a. These activate the membrane bound trimeric GTPbinding proteins which then activate or inhibit an enzyme or ion channel in the plasma membrane, thus activating an intracellular signaling cascade 3. Enzymecoupled receptors a. These act as an enzyme or associate with enzymes inside the cell, thus activating a wide variety of intracellular signaling pathways. Slide 22 Enzyme coupled receptors These are transmembrane that display their ligand binding domains on the outer surface of the plasma membrane. The cytoplasmic domain either act as an enzyme itself or forms a complex with another protein that acts as an enzyme. These receptors have a role in responses to growth factors (extracellular signal) that regulate the growth, proliferation, differentiation and survival of cells in animal tissues. (SEE PREVIOUS PICTURE/ SLIDE 22 LAST RECEPTOR) The largest class of enzyme coupled receptors consists of receptors with a cytoplasmic domain that functions as a tyrosine protein kinase (meaning it phosphorylates particular tyrosines on specific intracellular signaling proteins.) These receptors are eceptor tyrosine kinase (RTK) a. Inside has EGF, insulin, NGF, PDGF, and FGF receptors b. Outside has a cysteine rich domain, immunoglobulin domain, and fibronectin like domain. These receptors usually only have a single transmembrane segment that spans the lipid bilayer as a alpha helix. In many cases, the binding of an extracellular signal molecule causes two receptor molecules to come together in the plasma membrane. The pairing brings the two intracellular tails of the receptor together, activating their kinase domains so that the tails phosphorylate each other. This is callutophosphorylation. The phosphorylation occurs on specific tyrosines in RTKs. This tyrosine phosphorylation then triggers an elaborate intracellular signaling complex on the cytosolic tails of the receptor. The phosphorylated tyrosines serve as docking sites for a whole zoo of intracellular signaling proteins. . Notes from lecture Growth factor receptor tyrosine kinases have a ligand binding ectodomain (on the outside of plasma membrane) and a dimerization ectodomain. On the inside of the plasma membrane, RTKs have ErbB/HER domains In tumor cells, the EGFRs signal transduction in many different pathways. This is especially important for cancer cells because if a drug is used to limit one pathway, it may work for a while, but the cell just uses another pathway to make the affected pathway work again, thus combating the efforts of the drug. This cross talking between pathways is what makes cancer so hard to treat. Some of the pathways used in cancer cells are shown below. Slide 38 RTKs activate the monomeric GTPase Ras RTKs recruit and activate many kinds of intracellular signaling proteins leading to large complexes on the cytosolic tail of the RTK. One of the major members of the complexes Ras, a small GTPbinding protein that is bound by a lipid tail to the cytoplasmic face of the plasma membrane. All RTKs activate Ras including: 1. PDGF receptors mediate cell proliferation in wound healing 2. NGF receptors play a part in development of vertebrate neurons Ras is a monomeric GTPase. (mentioned earlier). It is recruited to the RTK by an adaptor protein already bound to the RTK. It cycles between two distinct conformational states, active when GTP is bound to it and inactive when GDP is bound to it. In its active state, Ras initiates a phosphorylation cascade in whc series of serine, threonine protein kinases phosphorylate and activate each other in sequence. his system, which carries a signal from the plasma membrane to the nucleus is called the MAPkinase module, named after the last kinase in the chain, MAPkinase. 1. The Ras protein activates MAPkinase kinase kinase 2. MAPkinase kinase kinase phosphorylates and activates MAPkinase kinase 3. MAPkinase kinase phosphorylates and activates MAPkinase (last kinase) 4. MAPkinase activates various effector proteins including certain transcriptional regulators altering their ability to control gene transcription. This change may stimulate cell proliferation, promote cell survival, or induce cell differentiation . Akt Pathway One of the pathways that RTKs activate to promote cell growth and survival relies on the enzyme phosphoinositide 3kinase (PI 3kinase). 1. This enzyme phosphorylates inositol phospholipids in the plasma membrane. 2. These lipids then serve as docking sites for specific intracellular signaling proteins which relocate from the cytosol to the plasma membrane where they can activate one another 3. One of the most important of these relocated proteins is the serine/threonine protein kinase Akt. Akt promotes the growth and survival of many cell types, often by inactivating the signaling proteins it phosphorylates. Example) Akt phosphorylates and inactivates a cytosolic protein called Bad. In its active state(when Bcl2 is bound to it), Bad encourages the cell to kill itself by activating apoptosis. When Akt phosphorylates Bad, it releases the Bcl2 and goes to an inactive state. Thus Akt promotes cell survival by inactivating a protein that promotes cell death. Akt alsopromotes cells to grow in sizet does so by activating the Tor pathway. Tor stimulates cells to grow by: 1. Enhancing protein synthesis 2. Inhibiting protein degradation Not all receptors trigger cascades to carry a message to the nucleus. Some use a more direct route to control gene expression. Example is the protein Notch. ● Controls development of neural cells in Drosophila ● The receptor acts as a transcriptional regulator ● When binded to Delta ( a signal protein on the surface of a neighboring cell) the Notch receptor is cleaved ● The cleavage releases the cytosolic tail of the receptor which is then free to move to the nucleus
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