University of Illinois, Urbana Champaign MCB 252: MCB252 EXAM 1 FORM B KEY, Questions and Answers revised and updated 2026.

University of Illinois, Urbana Champaign MCB 252: MCB252 EXAM 1 FORM B KEY, Questions and Answers revised and updated 2026. 1 NAME:_____________________________ NETWORK ID:__________________ SECTION:__________________________ TA:_____________________________ MCB 252 Exam 1 February 14, 2023 FORM B KEY This exam consists of 50 questions for a total of 175 points. Choose the BEST answer from the available choices. Questions 1-6 are 5 points each.2 1. Your research laboratory was interested in the mobility of the integral membrane receptor kinase, pTOL3, before and after it is bound to its ligand. TOL3 was fused to GFP at the extracellular end of the protein and was transiently expressed in cultures grown with or without the ligand. After a period, you exposed the cells containing the expressed tagged proteins with a laser, and you measure the recovered intensity of the photobleached region over various time points. Your data for the two experiments was charted below: What can you conclude from your data? A. TOL3 + ligand is less mobile than TOL3 w/o ligand in the membrane. B. TOL3 + ligand is more mobile than TOL3 w/o ligand in the membrane. C. TOL3 w/o ligand is degraded more than TOL3 + ligand after photobleaching. D. TOL3 + ligand recovers more quickly than TOL3 w/o ligand. E. You cannot determine anything from this data. 0 500 1000 1500 2000 2500 3000 200 Time (s) Relative Intensity (units) TOL3 w/o ligand TOL3 +ligand3 Questions 2, 3, and 4 are linked as they relate to research into signaling pathways that lead to cell differentiation. Researchers have used the developing eyes of fruit flies (D. melanogaster) to investigate signaling pathways that lead to cellular differentiation. The compound eyes of flies are composed of ~800 repeating groups of cells. In the wt fly, each of these 800 cells contains 8 photoreceptive neurons arrayed in a very repeated pattern. One of the 8 photoreceptive precursor neuronal cells, R7, is induced to differentiate into its neuronal cell-type by a neighboring cell, R8. The signaling pathway within R7 is very reminiscent of the EGF-RTK pathway that leads to epithelial cell proliferation. 2. In flies that do not express the RTK (sevenless) in R7 precursor cells, R7 cells are absent in all the 800-repeating group of cells - the precursor R7 cells never differentiate into R7 neurons. Suppose you re-supply a mutant form of RTK that cannot be phosphorylated to these cells. What effect would you expect this to have on R7 neuronal cell development? A. Re-supply of this RTK would have no affect on eye development – R7 precursor would not differentiate into R7 neuron. B. Re-supply of this RTK would "rescue" R7 differentiation because its cytoplasmic domain only acts as an adaptor for Sos, which activates Ras. C. Re-supply of this RTK would be impossible because the flies would die before reaching this stage of development. D. Re-supply of this RTK would result in the production of many R7 cells because over activating this pathway would lead to inappropriate cell proliferation. E. None of the above answers are correct. 3. Next you supply the sevenless flies with rasD, a gene that codes for a form of Ras that is not able to hydrolyze GTP to GDP. To your delight, you see that R7 precursors are differentiated into R7 neurons. Which of the following can you conclude about the role of Ras activation from this result? A. Ras phosphorylates transcription factors that drive the expression of genes necessary for R7 differentiation. B. GTP bound Ras inhibits the sevenless pathway. C. Ras acts upstream of Sevenless in this pathway. D. Ras acts downstream of Sevenless in this pathway. E. Both B and D are valid conclusions.4 4. From what you know of the EGF-RTK pathway, what would you expect to be the kinase that phosphorylates Sevenless in wt flies? A. GRB2 B. Sos C. MEK D. Sevenless E. Ras Questions 5 and 6 are linked. 5. You prepare an in vitro system to study ubiquitin-mediated degradation of proteins. You start with a physiological buffer (buffer contains all the required salts but does not contain any nucleotides) and add large poly-ubiquitinated proteins and 26S proteasomes. However, upon examination of the reactions over an hour, you still see no evidence of protein degradation. Which factor is most likely to be needed by the system? A. E3 ligases B. GTP C. ubiquitin monomers D. ATP E. 19S Regulatory Caps 6. Having identified and added the missing ingredient, you are thrilled to see strong evidence of degraded proteins after only a few minutes. However, you continuously see the accumulation of small globular proteins in your solution. Which of the following are likely candidates for the source of these proteins? A. E3 ligases B. degraded fragments of targeted proteins C. ubiquitin monomers D. 19S caps E. nondegraded targets5 Questions 7-19 are 4 points each. 7. Which type of experimental evidence shows that the intrinsic GTPase activity of the Gα subunit is important for terminating effector activation? A. A nonhydrolyzable GTP analog that can bind to the Gα subunit but cannot be hydrolyzed by the intrinsic GTPase, thereby activating the effector protein for a longer period 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 a longer period upon ligand-induced activation of the receptor. D. A dominant negative (no activity) GEF causes stimulation of the effector protein for a longer period upon ligand-induced activation of the receptor. E. All the above statements are correct. Questions 8 -10 are linked, and they relate to mechanisms involved in the regulation of controlled protein degradation. 8. In mammals, myoblast protein factor MPF1 is poly-ubiquitinated rapidly degraded in muscle cells in response to a specific extra-cellular signal. Two different proteins, X and Y, were recently shown to be involved in this process. Based on what we learnt in the class, which of the following roles would you initially rule out for these two proteins? A. E2 Ubiquitin-conjugating enzyme B. E3 Ubiquitin ligase C. Protein phosphatase D. Protein kinase E. Nucleoporin 9. While both proteins X and Y are required to control MPF1 degradation, only protein X was shown to directly interact with MPF1. Which of the following roles would you attribute to protein X? A. E1 Ubiquitin activating enzyme B. E2 Ubiquitin conjugating enzyme C. E3 Ubiquitin ligase D. All the above E. None of the above6 10. Protein X was subsequently shown to directly interact with protein Y. Interestingly, the interaction between protein X and MPF1 is prevented in myoblast cells that are lacking protein Y. As a result, the controlled degradation of MPF1 cannot occur. Which of the following roles would you attribute to protein Y? A. Protein kinase B. E2 Ubiquitin conjugating enzyme C. E3 Ubiquitin ligase D. A or B E. A or C 11. The Ras protein is a GTPase that functions in many growth-factor signaling pathways including the Receptor Tyrosine Kinase pathway. Mutant Ras protein with low binding affinity to Ras-GAP, promotes cell proliferation. Of the choices below, which other alteration of gene or protein activity can result in increased cell proliferation? A. A change that prevents Ras from being made B. A change that increases the intrinsic GTPAse activity of Ras C. Phosphorylation in the activation loop of MAP kinase by MEK D. A change that increases the affinity of 14-3-3 to Raf E. All the above-mentioned changes will result in increased cell proliferation. 12. A mutation in Raf causes the protein to remain active with or without EGF, the hormone that usually stimulates the EGFR pathway. The mutated cell constantly grows and divides, resulting in tumors. What secondary mutation would reverse this phenotype (i.e., would decrease the level of signaling produced by this pathway)? A. A mutation in the prenyltransferase, preventing attachment of prenyl groups to the CaaX motif of Ras. B. A mutation in Ras that keeps Ras locked in its GDP- bound state. C. A mutation in the EGF receptor, preventing the receptor from dimerizing. D. A mutation in Ras that keeps Ras locked in its GTP- bound state. E. A mutation in MAP kinase that severely lowers its kinase activity. 13. An SH2 domain containing adaptor protein contains a mutation that changes its binding pocket such that it binds to tyrosine and phosphotyrosine within an RTK receptor with equal affinity. As a result, MEK activity A. decreases due to changes in Raf activation. B. decreases due to allosteric inhibition of SH2-domain binding. C. increases with ligand binding-induced dimerization. D. decreases with ligand binding to the receptor. E. does not change with RTK dimerization and trans autophosphorylation.7 14. The order of events in a signaling pathway can be determined by the analysis of mutants. Cells that express a mutant defective Raf protein cannot be stimulated to proliferate uncontrollably by constitutively active RasD (dominant active). This indicates: A. quiescent cells can be induced to proliferate in the absence of growth factors if they contain a constitutively inactive mutant Raf protein. B. Raf is upstream of Ras in the signaling pathway. C. Ras is upstream of Raf in the signaling pathway. D. Ras mutants are recessive to Raf mutants. E. All the above statements are correct. 15. You have characterized a cell line that can synthesize a normal NFkB, but that is unable to enter the nucleus in response to signals activating the NFkB pathway. Which of the following mutations would you suspect this particular cell line to carry? A. A mutation that inactivates IKK. B. A mutation that promotes rapid polyubiquitination of IkB. C. A mutation that prevents the interaction of NFkB with IkB. D. A mutation that activates the IkB E3 ligase. E. None of the above are correct. 16. The micelles are rarely formed from natural phospholipids because the A. fatty acyl chains of phospholipids are generally too bulky to fit into the interior of the micelle. B. fatty acyl chains of phospholipids contain many carbon-carbon cis-double bonds. C. polar head groups of phospholipids strongly interact with cellular proteins. D. polar head groups of phospholipids are generally hydrophilic in nature. E. All the above are correct. 17. All the following statements about cholera toxin are true except that A. it chemically modifies the Gαs protein. B. it is a G protein-coupled receptor. C. it prevents hydrolysis of Gαs bound GTP to GDP. D. it leads to continuous activation of adenylyl cyclase. E. it maintains the Gαs in an active state.8 18. 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. ACTH inhibits adenylyl cyclase. E. Adenosine stimulates adenylyl cyclase. 19. The fluidity of the lipid bilayer plasma membrane is determined by A. the lipid composition of the plasma membrane. B. the length and presence/absence of C=C in the phospholipid hydrophobic tails. C. the temperature. D. All the above are correct. E. None of the above are correct. Questions 20-50 are 3 points each. 20. Which of the following features is characteristic of a Receptor tyrosine kinase (RTK)? A. It is a single transmembrane α-helical domain containing receptor. B. It directly binds to Ras when activated. C. It acts as a ligand for SOS. D. It is present on the nuclear membrane. E. It is a phosphatase. 21. Which of the following protein is generally attached to the plasma membrane using covalently attached lipid anchors? A. Ras B. Receptor tyrosine kinase (RTK) C. G Protein-coupled Receptors (GPCR) D. G-protein β subunit (Gβ) E. Adenylyl cyclase9 22. Decide whether the following statement is True or False: The phosphatidylserine is generally enriched in the exoplasmic leaflet of the plasma membrane in human erythrocytes. A. True B. False 23. Name the intracellular signaling pathway in which the signaling molecule (ligand) secreted by the signaling cells preferably interacts with the receptors present on those target cells in proximity. A. Endocrine signaling B. Paracrine signaling C. Enzyme-coupled receptor-mediated signaling D. G protein-coupled receptor mediated signaling E. Hormone signaling 24. Decide whether the following statement is True or False: The N-terminus of the G protein-coupled receptor is invariably located in the exoplasmic face and the C-terminus on the cytosolic face of the plasma membrane. A. True B. False 25. Which component in the GPCR-mediated signal transduction pathway acts as a GAP that facilitates the GTPase (hydrolysis of GTP to GDP) activity of Ga? A. Active G protein-coupled receptor (GPCR) B. Gg C. Active SOS D. Active effector molecule (example includes adenylyl cyclase) E. Active ligand 26. Name the protein/enzyme in the GPCR pathway that is involved in the conversion of cAMP to 5’-AMP. A. Protein Kinase A B. Protein Kinase C C. Receptor tyrosine kinase D. cAMP Phosphodiesterase E. Adenylyl cyclase10 27. As part of the GPCR desensitization process, b-arrestin binds to phosphorylated _____ A. ligand. B. receptor. C. Ga-protein. D. Gg-protein. E. effector. 28. During the process of nuclear import, a Ran-GEF works in the: A. cytoplasm to exchange GTP for GDP bound to Ran. B. cytoplasm to use GTP to release Ran from importin. C. nucleus to facilitate the release of GDP bound to Ran. D. nucleus to activate the intrinsic GTPase activity of Ran. E. None of the above answers are correct. 29. Among the different types of ubiquitinylation mentioned below, which type is generally linked to the degradation of the target protein? A. Monoubiquitinylation B. Multiubiquitinylation C. Polyubiquitinylation using lysine 11 inter-ubiquitin isopeptide bond D. Polyubiquitinylation using lysine 48 inter-ubiquitin isopeptide bond E. Polyubiquitinylation using lysine 63 inter-ubiquitin isopeptide bond 30. The cap particle of the 26S proteasome is called __________. A. 26S proteasome B. 20S proteasome C. 19S proteasome D. Both A and B are correct. E. None of the above are correct. 31. Both Ras and Raf proteins, involved in RTK pathways, are well-studied A. transcription factors. B. tumor suppressors. C. oncogenes. D. kinases. E. Both C and D are correct11 32. Decide whether the following statement is True or False: In signaling events, the association of Ras with a GTP always results in the activation of Ras. A. true B. false 33. Which is a protein in the GPCR pathway that acts as a GEF (Guanine nucleotide exchange factor) and activates Gα protein? A. GPCR receptor B. SOS C. Adenylyl cyclase D. BOSS E. Both A and B are correct. 34. Which of the following mechanism/(s) could amplify the GPCR-mediated intracellular signaling pathway? A. Synthesis of the second messenger within the cell. B. Removal of ligand from the receptor. C. Desensitization of receptors. D. All of the above are correct. E. Only A and B are correct. 35. Decide whether the following statement is True or False: Localization of Ran-GEF and Ran-GAP to the nucleoplasm and cytoplasm, respectively, is the basis for the unidirectional export and import of large cargo proteins across nuclear pore complex (NPC). A. True B. False 36. What is the role of the nuclear localization sequence (NLS) in a nuclear protein? A. It is bound by importins and directs the transport of nuclear proteins through the nuclear pore complex. B. It is a hydrophobic sequence that enables the protein to enter the nuclear membranes. C. It aids protein unfolding for the protein to thread through nuclear pores. D. It prevents the protein from diffusing out of the nucleus via nuclear pores. E. None of the above are correct.12 37. Decide whether the following statement is True or False: Unlike phospholipids, cholesterol is relatively evenly distributed in both leaflets (exoplasmic and cytosolic) of animal cellular membranes. A. True B. False 38. The lateral movements of specific plasma membrane proteins and lipids can be quantified by____________ analysis. A. Western blot B. Fluorescence recovery after photobleaching (FRAP) C. Immunoprecipitation D. Northern blot E. None of the above are correct. 39. Which component in the G-Protein coupled receptor (GPCR) signal transduction pathway generally interacts with adenylyl cyclase enzyme? A. Gα protein B. GPCR receptor C. Protein kinase A D. Ras E. None of the above statements are correct. 40. Decide whether the following statement is True or False: In signaling events, the phosphorylation of a protein always results in its activation. A. True B. False 41. Decide whether the following statement is True or False: Several of the subunits within the 19S cap proteasomes are proteases that are responsible for digesting the proteins into small peptides. A. True B. False13 42. During the import of proteins into the nucleus, the importin α subunit binds directly to: A. FG nucleoporins. B. Ran·GDP. C. nuclear localization signals in cargo proteins. D. Ran-GAP. E. Ran-GEF. 43. The TATA box DNA sequence sequence, recognized by the TATA binding protein (TBP) (part of the TFIID complex), is located in the _________________ of the gene. A. coding region B. core promoter C. regulatory promoter D. enhancer E. 3’UTR 44. Which of the following cellular protein is involved in the nuclear export of HIV Rev protein? A. NTF2 B. RCC1 C. Transportin 1 D. Importin 5 E. Crm1 45. Which is the amino acid in the CTD (carboxy-terminal domain) domain (YSPTSPS) that is preferentially phosphorylated during transcription initiation of RNA pol II-transcribed genes? A. serine-2 B. tyrosine-4 C. serine-5 D. serine-7 E. None of the above-mentioned residues are phosphorylated during transcription initiation. 46. Ubiquitin (Ub) is conjugated to a protein via a covalent (isopeptide) bond between the ___________ at the C-terminal end of Ub and a lysine residue on the target protein. A. glycine B. lysine C. alanine D. tyrosine E. serine14 47. Binding of hormone to an EGFR (receptor tyrosine kinase) causes all the following except: A. dimerization of the receptor. B. Tyrosine phosphorylation of the receptor. C. activation of Ras through an interaction with GRB2 and Sos. D. binding of 14-3-3 to Raf. E. binding of GRB2 to the receptor. 48. Which of the following is NOT a part of the lipid bilayer plasma membrane? A. Hydrophobic core of the membrane B. Hydrophilic face of the membrane C. Cytosolic leaflet of the membrane D. Exoplasmic leaflet of the membrane E. Hydrophilic core of the membrane 49. Which of the following molecules is NOT a “second messenger” in signaling? A. cyclic AMP (cAMP) B. cyclic GMP (cGMP) C. intracellular Ca2+ D. diacylglycerol (DAG) E. protein kinase A 50. Decide whether the following statement is True or False: The lysine-63 (K-63) linkage containing polyubiquitinylated protein will always be recognized by 26S proteasome for degradation. A. True B. False End of Exam