Lodish 8e Ch15 Test Bank PDF

15 - 1 15 Signal Transduction and G-Protein Coupled Receptors Sections 15.1 1. In paracrine signaling, the signali...

15 - 1 15 Signal Transduction and G-Protein Coupled Receptors Sections 15.1 1. In paracrine signaling, the signaling molecule: a. acts on cells in close proximity to the secreting cell. b. acts on target cells far away from the secreting cell. c. acts on the same cells that secreted the signaling molecule. d. is carried to the target cells via the circulation. Ans: a Question Type: Multiple choice Section 15.1 Blooms: Understanding Difficulty: Easy 2. GTPases serve in many signal transduction pathways and the presence of GTP or GDP dictates whether the pathway is on or off, respectively. Which of the following statements is TRUE regarding guanine nucleotide exchange factors (GEF) and the role in these signaling pathways? a. They hydrolyze GTP into GDP and Pi. b. They decrease the GTPase activity of the G-protein. c. They catalyze the dissociation of GDP on the G-protein and promote the replacement of GTP. d. none of the above Ans: c Question Type: Multiple choice Section 15.1 Blooms: Understanding Difficulty: Moderate 3. Explain the differences between endocrine, paracrine, and autocrine signaling. Ans: In endocrine signaling, signaling molecules are synthesized by one organ and act on distant target cells. In animals, the signaling molecule is carried to target cells via the circulation or other extracellular fluids. In paracrine signaling, the signaling molecules are released and affect only target cells in close proximity. In autocrine signaling, the cell that releases the signaling molecule is also affected by the released signaling molecule. Question Type: Essay Section 15.1 Blooms: Understanding Difficulty: Easy Sections 15.2 4. Cell sensitivity to an external signal is determined by: a. kon. b. koff. c. Kd.  15 - 2 d. the number of surface receptors. Ans: d Question Type: Multiple choice Section 15.2 Blooms: Remembering Difficulty: Moderate 5. If [R] = the free receptor concentration and [L] = the free ligand concentration, Kd is: a. [R]/[L]. b. [L]/[R]. c. [R][L]/[RL]. d. [RL]/[R] [L]. Ans: c Question Type: Multiple choice Section 15.2 Blooms: Analyzing Difficulty: Moderate 6. Describe the differences between an agonist and an antagonist. Ans: Agonists are molecules that mimic the function of a natural ligand by binding to the receptor and inducing the normal response. In contrast, an antagonist binds to the receptor but does not induce a response. An antagonist can block the binding of the natural ligand, thus reducing the normal physiological response to the ligand. Question Type: Essay Section 15.2 Blooms: Applying Difficulty: Moderate 7. A particular antagonist for an epinephrine-receptor protein is under consideration as a new drug. What values would you use to measure how tightly the drug binds to the target protein compared with epinephrine binding? What technique would you use to measure drug binding? Ans: The dissociation constant, Kd, is a measure of how tightly a ligand binds to its receptor. If the Kd for the drug is less than that for epinephrine, that would mean the antagonist binds more tightly to the receptor protein than does epinephrine. You could use a competitive binding assay to measure drug binding. Question Type: Essay Section 15.2 Blooms: Analyzing Difficulty: Difficult 8. What experimental approach was used to identify functional domains of G protein–coupled receptors? Ans: The functional domains of G protein–coupled receptors were determined by experiments using recombinant chimeric receptor proteins containing parts of the b2 and a2 adrenergic receptors. These chimeric receptors were tested for their ligand-binding specificity and their ability to activate or inhibit adenylyl cyclase. The results of these studies demonstrated that the cytosolic loop (C3 loop) between a helices 5 and 6 interacts with G proteins. Question Type: Essay Section 15.2 Blooms: Applying Difficulty: Difficult 9. Pull-down assays can be used to detect when small G proteins like Rac1 have been activated by various growth factors. If you were given two lysates, one having been treated with platelet-derived growth factor (PDGF) and the  15 - 3 other treated with PDGF that had been heat-inactivated, briefly describe the pull-down assay and what you would expect to see following the assay. Ans: Active Rac1 (i.e., in the GTP-bound form) binds specifically to the binding domain (PBD) of p21-activated protein kinase (PAK1). This PAK1-PBD bound to agarose beads can therefore be used as a tool to pull down active Rac GTP from protein lysates by centrifugation. To assay for levels of Rac GTP, PAK1-PDB-agarose is added to each of the lysates described above. The tubes are mixed and centrifuged, the proteins re-suspended and analyzed by SDS-polyacrylamide gel electrophoresis, and then transferred to a membrane and blotted against anti-Rac and anti-actin antibodies. The actin antibody is used to ensure equal amounts of protein were loaded in each lane. The blot probed with the anti-Rac antibody should only show a signal from the lysate of cells that were treated with PDGF, because PAK1-PDD agarose beads will recognize and pull down the GTP-bound, active form of Rac during centrifugation. Rac is not activated by the heat-inactivated PDGF and therefore little to no signal should be seen in these lysates. Question Type: Essay Section 15.2 Blooms: Analyzing Difficulty: Difficult 10. Pull-down assays using proteins that ONLY bind to the GTP-bound version of the small G-protein can be used to detect when small G proteins like Rac1 have been activated by various growth factors. You were given two lysates, one having been treated with platelet-derived growth factor (PDGF) and the other not treated with PDGF. This method is similar to performing co-IPs, so based on that knowledge, which of the following steps is erroneous? a. To assay for levels of active Rac1, PAK1-beads (the protein that only binds GTP-bound Rac1-conjugated to beads similar to Nickel beads) are added to each of the lysates described above and incubated on a rocking platform (on ice). b. The tubes are vortexed well, loading buffer is added, and the samples are heated to aid in protein denaturation. c. The samples are loaded and analyzed by SDS-polyacrylamide gel electrophoresis, then the proteins are transferred to a membrane. d. The membrane (a.k.a. blot) will be probed with the anti-Rac antibody. Ans: b Question Type: Multiple choice Section 15.2 Blooms: Analyzing Difficulty: Moderate Sections 15.3 11. Of the components of a heterotrimeric G protein, which subunit(s) is(are) able to activate downstream responses? a. only the alpha subunit b. only the beta/gamma subunits c. only the delta subunit d. both the alpha subunit and the beta/gamma subunits Ans: d Question Type: Multiple choice Section 15.3 Blooms: Remembering Difficulty: Easy 12. In trimeric G proteins, GTP binds to: a. the a subunit. b. the b subunit. c. the g subunit. d. the activated trimer.  15 - 4 Ans: a Question Type: Multiple choice Section 15.3 Blooms: Remembering Difficulty: Easy 13. Which of the following is NOT a common intracellular second messenger? a. inositol 1,4,5-trisphosphate (IP3) b. 1,2 diacylglycerol (DAG) c. adenosine triphosphate (ATP) d. 3´–5´ cyclic guanine monophosphate (cGMP) Ans: c Question Type: Multiple choice Section 15.3 Blooms: Understanding Difficulty: Moderate 14. Phospholipase C is activated by: a. Gas. b. Gai. c. Gaq. d. none of the above Ans: c Question Type: Multiple choice Section 15.3 Blooms: Remembering Difficulty: Easy 15. Which of the following general statement(s) about a G protein–coupled receptor is (are) TRUE? a. It contains 12 transmembrane domains. b. It is positioned with the N-terminus on the cytoplasmic face of the membrane. c. It is positioned with the C-terminus on the cytoplasmic face of the membrane. d. all of the above Ans: c Question Type: Multiple choice Section 15.3 Blooms: Applying Difficulty: Easy 16. All the following statement(s) about cholera toxin are TRUE, except: a. it chemically modifies the Gas protein. b. it is a G protein-coupled receptor. c. it prevents hydrolysis of bound GTP to GDP. d. it leads to continuous activation of adenylyl cyclase. Ans: b Question Type: Multiple choice Section 15.3 Blooms: Remembering Difficulty: Moderate 17. Phosphatidylinositol 4,5-bisphosphate (PIP2) is cleaved by phospholipase C into:  15 - 5 a. 1,2-diacylglycerol (DAG). b. phosphatidylinositol (PI). c. inositol 1,4,5-trisphosphate (IP3). d. 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3). Ans: d Question Type: Multiple choice Section 15.3 Blooms: Remembering Difficulty: Easy 18. 1,2-diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) are cleaved from phosphatidylinositol 4,5- bisphosphate (PIP2) by the enzyme: a. adenylyl cyclase. b. phosphodiesterase. c. phospholipase C. d. protein kinase C. Ans: c Question Type: Multiple choice Section 15.3 Blooms: Remembering Difficulty: Easy Sections 15.4 19. Summarize the steps in the cycling of GTPase switch proteins from active to inactive states. Ans: GTPase switch proteins cycle from inactive to active states depending upon whether GTP (active) or GDP (inactive) is bound. Signal-induced receptor activation leads to the conversion of the GTPase switch protein from an inactive to an active state mediated by a guanine nucleotide exchange factor (GEF). GDP is released and replaced by GTP, resulting in a conformational change that allows the protein to activate downstream effector proteins. The intrinsic GTPase activity causes hydrolysis of GTP to GDP and the conversion back to the inactive state. Question Type: Essay Section 15.3 Blooms: Applying Difficulty: Moderate 20. cGMP phosphodiesterase catalyzes the conversion of: a. cAMP to cGMP. b. cGMP to 5´-GMP. c. GTP to cGMP. d. cGMP to GDP. Ans: b Question Type: Multiple choice Section 15.4 Blooms: Applying Difficulty: Easy 21. Which enzyme plays a role in regulating rhodopsin-induced closing of cation channels? a. guanylyl cyclase b. adenylyl cyclase c. phosphodiesterase d guanylyl cyclase and phosphodiesterase  15 - 6 Ans: d Question Type: Multiple choice Section 15.4 Blooms: Remembering Difficulty: Easy 22. Which of the following is a common step in the opening/closing of ion channels by acetylcholine and rhodopsin binding to their receptors? a. The Ga·GTP subunit dissociates from the Gbg complex. b. The released Ga·GTP subunit interacts with the ion channel. c. The released Gbg complex interacts with the ion channel. d. The released Ga·GTP subunit activates cGMP phosphodiesterase. Ans: a Question Type: Multiple choice Section 15.4 Blooms: Applying Difficulty: Moderate 23. Rhodopsin, a light-sensitive GPCR whose role in vision is dependent on its phosphorylation status, is influenced in part by the protein arrestin. Which of the following is TRUE with regard to rhodopsin and vision? a. Rhodopsin phosphatase increases the degree of rhodopsin phosphorylation. b. Arrestin binds to the completely phosphorylated opsin to inhibit signaling. c. Rhodopsin activation promotes the opening of a cGMP-gated ion channel. d. none of the above Ans: b Question Type: Multiple choice Section 15.4 Blooms: Applying Difficulty: Moderate 24. Describe the proposed mechanism for the opening of K+ channels by cardiac muscarinic acetylcholine receptors. Ans: Cardiac muscarinic acetylcholine receptors are linked to K+ channels by a trimeric G protein. Binding of acetylcholine to its receptor triggers the activation and release of the Gia-GTP subunit from the Gbg subunit. The released Gbg subunit, rather than the Gia subunit, then binds to and opens the K+ channel. Activation is terminated by the hydrolysis of GTP to GDP and reformation of the ternary complex. Question Type: Essay Section 15.4 Blooms: Applying Difficulty: Moderate 25. The G protein stimulated by light via the Rhodopsin receptor is ____, while the effector is ____. a. Gαi; sodium channels b. Gαo; adenylyl cyclase c. Gαt; PDE d. Gαq; guanylate cyclase Ans: c Question Type: Multiple choice Section 15.4 Blooms: Remembering Difficulty: Easy  15 - 7 Sections 15.5 26. Which of the following statements about adenylyl cyclase stimulation/inhibition in adipose cells is TRUE? a. Prostaglandin E1 stimulates adenylyl cyclase. b. Glucagon inhibits adenylyl cyclase. c. Epinephrine stimulates adenylyl cyclase. d. Glucagon inhibits adenylyl cyclase and epinephrine stimulates adenylyl cyclase. Ans: c Question Type: Multiple choice Section 15.5 Blooms: Understanding Difficulty: Moderate 27. Which of the following events occur during the epinephrine-stimulated conversion of glycogen to glucose-1- phosphate? a. activation of PKA by cAMP b. inhibition of glycogen synthase c. activation of glycogen phosphorylase d. all of the above Ans: d Question Type: Multiple choice Section 15.5 Blooms: Applying Difficulty: Moderate 28. The activity of b-adrenergic receptors is regulated by: a. b-adrenergic receptor kinase (BARK). b. calmodulin. c. b-tubulin. d. all of the above Ans: a Question Type: Multiple choice Section 15.5 Blooms: Understanding Difficulty: Easy 29. Which of the following mechanisms can terminate an intracellular signaling pathway once the concentration of an external signal decreases? a. degradation of the second messenger b. desensitization of receptors c. deactivation of a signal transduction protein d. all of the above Ans: d Question Type: Multiple choice Section 15.5 Blooms: Understanding Difficulty: Easy 30. Describe experimental evidence supporting that intrinsic GTPase activity of the Ga subunit is important for terminating effector activation.  15 - 8 Ans: The role of the intrinsic GTPase activity of the Ga subunit has been investigated using GTP analogs that can bind to the Ga subunit but cannot be hydrolyzed by its intrinsic GTPase activity. In these nonhydrolyzable compounds, the terminal phosphodiester (P–O–P) bond in GTP is replaced with P–CH2–P or P–NH–P bonds. In the presence of these nonhydrolyzable GTPs, the response of the effector protein is prolonged upon ligand-induced activation of the receptor. This is because the displacement of GDP with the modified GTP results in continuous activation of the effector protein. Because the bound GTP analog cannot be hydrolyzed to GDP, the effector remains in an active state permanently. Question Type: Essay Section 15.5 Blooms: Applying Difficulty: Moderate 31. How does the body respond to decreases in blood glucose levels below about 5 mM? Ans: Reduced blood glucose levels induce a cells in the pancreas to release glucagon into the blood. Glucagon binds to the glucagon receptor in liver cell membranes. The glucagon receptor is coupled to a Gas protein. Stimulation of the glucagon signal pathway in liver cells activates PKA. The overall effect is to inhibit glycogen synthesis and increase glycogenolysis. Glycogen breakdown yields glucose 1-phosphate, which liver cells convert to glucose. Glucose is released into the blood, bringing the blood glucose level back up to normal. Question Type: Essay Section 15.5 Blooms: Applying Difficulty: Moderate 32. When the regulatory domain of PKA is bound to cAMP: a. the released catalytic subunit travels to the nucleus where it can phosphorylate CREB. b. the catalytic subunit acts on CRE to stimulate gene transcription. c. the AKAP adaptor protein sequesters the catalytic subunit. d. PDE is inhibited to cause cAMP levels to rise. Ans: a Question Type: Multiple choice Section 15.5 Blooms: Understanding Difficulty: Moderate 33. Muscarinic acetylcholine receptors are GPCRs that slow the rate of heart muscle contraction upon ligand binding/activation. Activation of this receptor leads to opening of potassium channels triggered by decreases in cAMP levels. The muscarinic acetylcholine receptor likely couples to: a. Gαq. b. Gαs. c. Gαi. d. Gαo. Ans: c Question Type: Multiple choice Section 15.5 Blooms: Applying Difficulty: Easy 34. Cholera toxin can make you sick through overactivation of a Gas pathway even if your body stops making the ligand associated with activation of the pathway. Knowing this, which of the following CANNOT be true about cholera toxin? a. It chemically modifies the Gas protein. b. Cholera toxin activates a ligand-activated G protein-coupled receptor.  15 - 9 c. It prevents hydrolysis of bound GTP to GDP. d. It leads to continuous activation of adenylyl cyclase. Ans: b Question Type: Multiple choice Section 15.5 Blooms: Analyzing Difficulty: Moderate 35. Signal amplification is an important part of GPCR-mediated signaling. Which of the following steps do NOT directly amplify the signal? a. effector activation b. binding of second messenger to target protein/ion channel c. kinase acting on substrates d. second messenger generation Ans: b Question Type: Multiple choice Section 15.5 Blooms: Understanding Difficulty: Moderate 36. One form of receptor desensitization can be mediated by negative feedback involving phosphorylation of the receptor itself by a kinase activated downstream of a second messenger. The phosphorylated receptor likely: a. would be resensitized by a kinase. b. would activate heterotrimeric G proteins at a faster rate. c. would be unable to bind ligand. d. could be resistant to desensitization if Ser/Thr phosphatases were acting on the same receptor, Ans: d Question Type: Multiple choice Section 15.5 Blooms: Applying Difficulty: Moderate 37. Which type of experimental evidence shows that the intrinsic GTPase activity of the Ga subunit is important for terminating effector activation? a. A nonhydrolyzable GTP analog that can bind to the Ga subunit but cannot be hydrolyzed by the intrinsic GTPase, thereby activating the effector protein longer upon ligand-induced activation of the receptor. b. A nonhydrolyzable GTP analog causes displacement of GDP with the modified GTP resulting in continuous activation of the receptor because the bound GTP analog cannot be hydrolyzed to GDP. c. A dominant active (constant activity) GEF causes stimulation of the effector protein for longer upon ligand- induced activation of the receptor. d. A dominant negative (no activity) GEF causes stimulation of the effector protein for longer upon ligand-induced activation of the receptor. Ans: a Question Type: Multiple choice Section 15.5 Blooms: Analyzing Difficulty: Difficult