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1、B. Nitric oxide couples G protein-linked receptor stimulation in endothelial cells to relaxation of smooth muscle cells in blood vessels,It has been known for many years that acetylcholine dilate blood vessels by causing their smooth muscles to relax. In 1980, Furchgott concluded that blood vessels
2、are dilated because the endothelial cells produce a signal molecule that makes smooth muscle cells relax. In 1986 work by Furchgott and parallel work by Louis Ignarro identified NO as the signal that cause relaxation of the vascular smooth muscle. 1998, Received Nobel Prize,The action of Nitric oxid
3、e on blood vessels,The mechanism by which acetylcholine stimulation of the endothelial cells leads to smooth muscle relaxation also explains the mechanism of action of the chemical nitroglycerin. The drug sildenafil, sold under the trade name Viagra, is an inhibitor of a cyclic GMP-specific phosphod
4、iesterase that normally catalyzes the breakdown of cyclic GMP.,The carbon monoxide(CO) acts as a cellular messenger to stimulate the production of cGMP by stimulating G-cyclase.,3. Signal transduction mediated by the receptors on the cell surface,A. Mediated by the Ion-Linked Receptors which convert
5、 chemical signals into electrical ones,4 or 6-helix transmembrane receptor,B. Signal transduction mediated by G protein-linked receptors,The structure of G protein-linked receptors: Seven-helix transmembrane; C-terminal: Ser- and Thr-rich -the sites of phosphorylation make for the desensitization of
6、 GPLR.,Figure 15-18 Two major pathways by which G-protein-linked cell-surface receptors generate small intracellular mediators. In both cases the binding of an extracellular ligand alters the conformation of the cytoplasmic domain of the receptor, causing it to bind to a G protein that activates (or
7、 inactivates) a plasma membrane enzyme. In the cyclic AMP (cAMP) pathway the enzyme directly produces cyclic AMP. In the Ca2+ pathway the enzyme produces a soluble mediator (inositol trisphosphate) that releases Ca2+ from the endoplasmic reticulum.,The structure and activation of G proteins,Figure 1
8、5-23 A current model of how Gs couples receptor activation to adenylyl cyclase activation. As long as the extracellular signaling ligand remains bound, the receptor protein can continue to activate molecules of Gs protein, thereby amplifying the response. More important, an alphas can remain active
9、and continue to stimulate a cyclase molecule for many seconds after the signaling ligand dissociates from the receptor, providing even greater amplification.,C. Cyclic AMP signaling pathway,G-protein activation and inactivation cycle,The activation of protein kinase A by cyclic AMPs,Second messenger
10、s (cAMP), an effector, amplify the response to a single extracellular ligand by cAMP to trigger a reaction cascade. The cascade starts with the binding of cAMP to cAMP-dependent protein kinase A. PKA inhibits glycogen synthase and activates phosphorylase kinase.,Double Messenger system,Figure 15-32
11、Two intracellular pathways by which activated C-kinase can activate the transcription of specific genes. In one (red arrows) C-kinase activates a phosphorylation cascade that leads to the phosphorylation of a pivotal protein kinase called MAP-kinase, which in turn phosphorylates and activates the ge
12、ne regulatory protein Elk-1. Elk-1 is bound to a short DNA sequence in association with another DNA-binding protein. In the other pathway (green arrows) C-kinase activation leads to the phosphorylation of Ik-B, which releases the gene regulatory protein NF-kB so that it can migrate into the nucleus
13、and activate the transcription of specific genes.,The mechanisms that shut off a signal are as important as the mechanisms that turn it on.,Cholera is caused by a bacterium that multiplies in the intestine, where it produces a protein called cholera toxin. This enters the cells lining the intestine
14、and modifies the subunit of G protein so that it can no longer hydrolyze its bound GTP. The altered subunit thus remains in the active state indefinitely, continuing to transmit a signal to its target proteins: Outflow of Na+ and water into the gut.,D. The pathway through phospholipase C results in
15、a rise in intracellular Ca+,Cytolasmic calcium levels are determined by events within a membrane.,IP3-Ca2+ pathway and DG-PKC pathway,Elevation of cytosolic Ca2+ via the IP signaling pathway,SignalsGPLR GP PLC IP3 and DAG (twin signals). IP3 IP3 receptor(Ca2+ channel, located at the surface of sER)
16、Elevation of cytosolic Ca2+; DAG activates PKC to phosphoralate Ser and Thr on target proteins. Calcium binds to calcium-binding proteins(CaM) which affects other proteins.,Structure of calmodulin, a cytosolic protein of 148 amino acids that bind Ca2+ ions,Regulating calcium concentrations in plant
17、cells,Cytosolic calcium changes in response to several stimuli, including light, pressure, gravity, and hormones. Calcium signaling aids in decreasing turgor pressure in guard cells.,Ca2+/CaM dep. protein kinase (CaM-kinase) mediate many of the actions of Ca2+ in animal cells.,The functions of incre
18、ase the levels of cytosolic calcium-CaM : start-up embryo development after the fecundation. excitating contract of muscle cells; excitating secretion of endocrine and nerve cells.,G-protein-linked receptor desensitization depends on receptor phosphorylation by PKA, PKC, CaMK2 or G-protein-linked re
19、ceptor kinases(GRKs),The target cells can become desensitized to a signal molecule by five ways.,Three general ways of the desensitization: 1. Receptor inactivation by alteration; 2. Receptor sequestration by internalization; 3. Receptor down-regulation by destroying in Ls.,Sequestration; down-regul
20、ation; inactivation; inactivation; inhibitory protein,E. Receptor tyrosine kinase (RTK) and RTK-Ras signaling pathway,RTK are the second major type of cell-surface receptors,Signaling ligands of RTKs: 1.Nerve growth factor (NGF) 2.Platelet-derived growth factor (PDGF) 3.Fibroblast growth factor (FGF
21、) 4.Epidermal growth factor (EGF) 5.insulin and insulin-like GF(IGF-1) 6.ephrins(Eph) 7.vascular endothelial factor(VEGF),Six classes of enzyme-linked receptors have thus far been identified: Receptor tyrosine kinase(RTK); Tyrosine-kinase-associated receptor; Receptorlike tyrosine phosphatases; Rece
22、ptor serine/threonine kinase; Receptor guanylyl cyclases; Histidine-kinase-associated receptor,Figure 15-47 Six subfamilies of receptor tyrosine kinases. Only one or two members of each subfamily are indicated. Note that the tyrosine kinase domain is interrupted by a “kinase insert region“ in some o
23、f the subfamilies. The functional significance of the cysteine-rich and immunoglobulinlike domains is unknown.,Ligand binding leads to autophosphorylation of RTK,RTK activity stimulated by cross-phosphorylation.,Phsphorylated Tyrosine Serve as docking sites for protein with SH2 domains (Src homology
24、 region). Other protein modules such as SH3 binds to proline-rich motifs in intracellular proteins.,dimerization,Steps in activation of Ras by RTKs,Phosphotyrosines of RTK act as binding sites for a specific SH2 protein called GRB2 (Growth factor receptor binding protein in mammalian). GRB2 is not a
25、 protein with catalytic activity, but one that functions solely as an adapter molecule that links other proteins into a complex. Sos(son of sevenless) is a guanine nucleotide exchange factor for Ras (Ras-GEF) When a ligand binds to the RTK and recruits the Grb2-Sos to the inner surface of the membra
26、ne, the Sos protein binds to Ras causing GDP/GTP exchange, thus activating Ras.,In the cell, Ras activity is regulated by GAPs( GTPase Activating proteins)100 000-fold,MAP-kinase serine/threonine phosphorylation Pathway activated by Ras,Ras-activated phosphorylation cascade,MAP kinase=mitogen-activa
27、ted protein kinase; MAP-KKK=Raf (Ser/Thr-PK),RTK-Ras signaling pathway,F. Jak-STAT signaling pathway,Janus kinases (Jaks) and STATs : Jaks: A class of cytoplasmic tyrosine kinases, including Jak1, Jak2, Jak3, and Tyk2, and each is associated with particular cytokine receptors; STATs: Jaks then phosp
28、horylate and activate a set of latent gene regulatory proteins called STATs(signal transducers and activators of transcription,which have an SH2 domain),which move into the nucleus and stimulate the transcription of specific genes.,Signaling ligands and Cytokine receptors: Ligands: more than 30 cyto
29、kines and hormones activate the Jak-STAT pathway by binding to cytokine receptors. Cytokine receptors: they are composed of two or more poly peptide chains. All receptor are associated with one or more Jaks.,The Jak-STAT signaling pathway avtivated by -interferon. Providing a fast track to the nucle
30、us.,MBC 885: 15-63,4. Signals that originate from contacts between the cell surface and the substratum,A. Integrins are receptors at sites of cell-substrate and cell-cell contact.,Interaction between the extracellular domain of integrin and an extracellular ligand generate a variety of signals. The
31、interaction leads to clustering of integrins and the rapid tyrosine phosphorylation of proteins at the cytoplasmic face of focal adhesions by the tyrosine kinase,Src. Focal adhesion kinase (FAK) is an effector in integrin-mediated responses. Another signal pathway activated by integrin engagement le
32、ads to the protein-synthesizing machinery of the cytoplasm.,1.The mechanical-structural function of focal adhesion, which is carried out by actin filaments and associated proteins. 2. Signaling function from extracellular surfacenucleus (where they stimulate the transcription of genes involved in ce
33、ll growth and proliferation), which is carried out by tyrosine kinase (Src and FAK),Controlling the assembly of focal adhesions,Signal pathway that lead to the assembly of the fibers of a focal adhesion. Rho is involved in regulating the organization of the cells actin cytoskeleton.,5. Convergence,
34、divergence, and crosstalk among different signaling pathway,A. Convergence: Signals from a variety of unrelated receptors can converge to activate a common effector.,B. Divergence: Signals from the same ligand can diverge to activate a variety of different effectors.,C. Crosstalk: Signals can be passed back and forth between different pathways,6. In actual fact, signaling pathways in the cell are much more complex.,THANKS!,