FinalExam2015_CellBio Key

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MCB104 Final Exam Student name: 1 PART 1: Cell Biology Total points: 134 1 (12 points). For each of the following sentences, select the best word or phrase from the list below to fill in the blanks. Not all words or phrases will be used; each word or phrase should be used only once. G-protein-coupled receptors (GPCRs) all have a similar structure with __________________ transmembrane domains. When a GPCR binds an extracellular signal, an intracellular G protein, composed of __________________ subunits, becomes activated. __________________ of the G-protein subunits are tethered to the plasma membrane by short lipid tails. When unstimulated, the _ subunit is bound to __________________, which is exchanged for __________________ on stimulation. The intrinsic __________________ activity of this subunit is important for inactivating the G protein. Active G protein then stimulates adenylyl cyclase, causing production of the second messenger ______ adenylyl cyclase alpha GTPase AMP diacylglycerol beta ATP five seven ATPase four three Ca2+ GDP twelve cAMP GTP two 2 points per correctly filled blank: G-protein-coupled receptors (GPCRs) all have a similar structure with seven transmembrane domains. When a GPCR binds an extracellular signal, an intracellular G protein, composed of three subunits, becomes activated. Two of the G-protein subunits are tethered to the plasma membrane by short lipid tails. When unstimulated, the alpha subunit is bound to GDP, which is exchanged for GTP on stimulation. The intrinsic GTPase activity of the alpha subunit is important for inactivating the G protein. Active G protein stimulates adenylyl cyclase, causing production of the second messenger cAMP. 2 (9 points) . Binding of a G-protein-coupled receptor to its ligand can lead to activation of protein kinase C (PKC) as depicted in the diagram below. You have discovered a novel protein, Tumor Inducing Kinase 1 (TIK1, in red), which is phosphorylated and activated by PKC. When TIK1 becomes phosphorylated, it promotes oncogenic transformation of cells. This is a potentially important finding because it implies that you could use one of the many existing GPCR inhibitors as a novel anti-cancer drug. However, you first want to learn more about how TIK1 becomes activated. In particular, which of the following conditions would lead to signal-independent activation of TIK1? Which one(s) would activate TIK1 in a signal-dependent way? Explain your reasoning. (a) the expression of a constitutively active phospholipase C

MCB104 Final Exam Student name: 2 (b) a mutation in the GPCR that binds the signal more tightly (c) a Ca2+ channel in the endoplasmic reticulum with an increased affinity for IP3 (d) a mutation in the gene that encodes TIK1 such that the enzyme can no longer be phosphorylated by PKC (a) (3 points) A constitutively active phospholipase C will lead to the constitutive production of IP3 and diacylglycerol, leading to activation of PKC in a signal- independent manner; thus, TIK1 activation and the lipid modification will be signal-independent. (b) and (c) (2 points each) will increase activity of the signal transduction pathway in a signal-dependent manner. (d) (2 points) will prevent PKC from activating TIK1 and will thus prevent the oncogenic transformation. 3 (10 points). Would each of the following events increase, decrease or not affect Receptor Tyrosine Kinase (RTK) signaling in a cancer cell? Briefly explain your answer. (a) (2 points) Chromosomal amplification of a ligand-dependent receptor (b) (2 points) Chromosomal amplification of a ligand-independent receptor (c) (2 points) Mutation of a Tyrosine in the intracellular domain to Alanine (d) (2 points) Mutation of a Serine in the intracellular domain to Alanine (e) (2 points) Mutation of an amino acid in the extracellular domain required for receptor dimerization

MCB104 Final Exam Student name: 3 2 points for each correct answer (a) Increase: amplification of a ligand-dependent RTK would increase the signaling as long as some ligand is present. (b) Increase: amplification of a ligand-independent RTK would boost signaling under all circumstances (c) Decrease: cross-phosphorylation of the intracellular tyrosines of an RTK is required for signal propagation (d) No effect: Tyrosine, not serine phosphorylation propagates the signal (e) Decrease: Receptor dimerization is required for cross-phosphorylation of the intracellular domains and activation 4 (4 points). What would be the most obvious outcome of repeated cell cycles consisting of S phase and M phase only? (a) Cells would not be able to replicate their DNA. (b) The mitotic spindle could not assemble. (c) Cells would get larger and larger. (d) The cells produced would get smaller and smaller. (d) (4 points) The cells produced would get smaller and smaller, as they would not have sufficient time to double their mass before dividing 4. (5 points) A mutant yeast strain stops proliferating when shifted from 25°C to 37°C. When these cells are analyzed at the two different temperatures, using a machine that sorts cells according to the amount of DNA they contain, the graphs in Figure Q18-3 are obtained.

MCB104 Final Exam Student name: 4 Figure 18-3 Which of the following would not explain the results with the mutant? (a) inability to initiate DNA replication (b) inability to begin M phase (c) inability to activate proteins needed to enter S phase (d) inappropriate production of a signal that causes the cells to remain in G1 (b) (4 points) At 37°C, the cells all have one genome-worth of DNA, meaning that they have not replicated their DNA and therefore have not entered S phase. Cells that are unable to begin M phase should have two genomes-worth of DNA, as they would have completed DNA replication and arrested in G2. 6. (10 points) The yeast S. pombe has been used to dissect the mechanism of cell cycle regulation. It has been observed that cells expressing higher than normal levels of active cdc25 protein exhibit a small cell size or wee phenotype, as depicted in the picture below. normal elevated cdc25 levels

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