Section principles of cell signaling feedback if the

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Section: Principles of Cell SignalingFeedback: If the signaling pathways for the signals converge (or overlap), adaptation toone signal can also desensitize the cell to the others by affecting the common signalingmolecules. In this example, convergence occurs downstream of the receptors; therefore,the first three mechanisms do not apply. Note that the first three adaptation mechanismsact on the receptor for the original signal, not on other receptors.18. Answer: ADifficulty: 2Section: Signaling through G-Protein-Coupled ReceptorsFeedback: The GPCR and the RGS act in opposite directions with regards to theactivation of trimeric G proteins, by acting as a GEF and a GAP, respectively.19. Answer: TFFFTDifficulty: 1Section: Signaling through G-Protein-Coupled Receptors
Feedback: All GPCRs (found in multicellular as well as unicellular organisms) belong to alarge family of proteins that share a similar structure with seven transmembrane helices.Despite this similarity, they recognize a wide variety of ligands. The GPCR activates the αsubunit of a trimeric G protein, which is then released; this is usually followed by thebinding of another α subunit to repeat the cycle for as long as the receptor is activated.Insulin is recognized by a receptor tyrosine kinase.20. Answer: CDifficulty: 2Section: Signaling through G-Protein-Coupled ReceptorsFeedback: While cholera toxin causes constitutive Gsactivation, pertussis toxin preventsGiactivation. Both of these result in a net activation of adenylyl cyclase, leading to highercAMP levels. Note that adenylyl cyclase is not the only target of Gi.21. Answer: DBCADifficulty: 1Section: Signaling through G-Protein-Coupled ReceptorsFeedback: Upon cAMP production by activated adenylyl cyclase, cAMP-dependentprotein kinase (PKA) is activated through the release of its catalytic subunits as a result ofcAMP binding to the regulatory subunits. The catalytic subunits then enter the nucleuswhere they phosphorylate CREB (cAMP response element-binding protein), whichrecruits other proteins to activate transcription of target genes.22. Answer: CDifficulty: 1Section: Signaling through G-Protein-Coupled ReceptorsFeedback: Stimulation of protein kinase C (PKC) activity near the plasma membraneresults from elevated cytosolic Ca2+[induced by inositol 1,4,5-trisphosphate (IP3)production] and from binding to diacylglycerol and negatively charged phospholipids. IP3and diacylglycerol are second messengers in the cytosol and plasma membrane,respectively, and are produced from phosphatidylinositol 4,5-bisphosphate by the action ofPLCβ.23. Answer: IDifficulty: 3Section: Signaling through G-Protein-Coupled ReceptorsFeedback: The effect of channel sensitization to Ca2+is comparable to that of increasedCa2+concentration, i.e. higher-frequency oscillations. The frequency as well as theamplitude of the Ca2+spikes can also be modulated by other mechanisms.24. Answer: calmodulin
Difficulty: 1Section: Signaling through G-Protein-Coupled ReceptorsFeedback: Calmodulin is an important Ca2+-binding protein that helps mediate manycellular responses to Ca2+signals.25. Answer: VOBVDifficulty: 2

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