cell molec exam 4 Flashcards


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1

Which of the following statements about the endoplasmic reticulum (ER) is false?

(a) The ER is the major site for new membrane synthesis in the cell.

(b) Proteins to be delivered to the ER lumen are synthesized on smooth ER.

(c) Steroid hormones are synthesized on the smooth ER.

(d) The ER membrane is contiguous with the outer nuclear membrane.

(b) Proteins to be delivered to the ER lumen are synthesized on smooth ER.

2

Which of the following statements about membrane-enclosed organelles is true?

(a) In a typical cell, the area of the endoplasmic reticulum membrane far exceeds the area of plasma membrane.

(b) The nucleus is the only organelle that is surrounded by a double membrane.

(c) Other than the nucleus, most organelles are small and thus, in a typical cell, only about 10% of a cell’s volume is occupied by membrane-enclosed organelles; the other 90% of the cell volume is the cytosol.

(d) The nucleus is the only organelle that contains DNA.

(a) In a typical cell, the area of the endoplasmic reticulum membrane far exceeds the area of plasma membrane.

3

Which of the following organelles is not part of the endomembrane system?

(a) Golgi apparatus

(b) the nucleus

(c) mitochondria

(d) lysosomes

(c) mitochondria

4

Which of the following statements is true?

(a) Lysosomes are believed to have originated from the engulfment of bacteria specialized for digestion.

(b) The nuclear membrane is thought to have arisen from the plasma membrane invaginating around the DNA.

(c) Because bacteria do not have mitochondria, they cannot produce ATP in a membrane-dependent fashion.

(d) Chloroplasts and mitochondria share their DNA.

(b) The nuclear membrane is thought to have arisen from the plasma membrane invaginating around the DNA.

5

Where are proteins in the chloroplast synthesized?

(a) in the cytosol

(b) in the chloroplast

(c) on the endoplasmic reticulum

(d) in both the cytosol and the chloroplast

(d) in both the cytosol and the chloroplast

6

Proteins that are fully translated in the cytosol do not end up in _______.

(a) the cytosol.

(b) the mitochondria.

(c) the interior of the nucleus.

(d) transport vesicles.

(d) transport vesicles.

7

Proteins that are fully translated in the cytosol and lack a sorting signal will end up in ____.

(a) the cytosol.

(b) the mitochondria.

(c) the interior of the nucleus.

(d) the nuclear membrane.

(a) the cytosol.

8

Signal sequences that direct proteins to the correct compartment are _________.

(a) added to proteins through post-translational modification.

(b) added to a protein by a protein translocator.

(c) encoded in the amino acid sequence and sufficient for targeting a protein to its correct destination.

(d) always removed once a protein is at the correct destination.

(c) encoded in the amino acid sequence and sufficient for targeting a protein to its correct destination.

9

What is the role of the nuclear localization sequence in a nuclear protein?

(a) It is bound by cytoplasmic proteins that direct the nuclear protein to the nuclear pore.

(b) It is a hydrophobic sequence that enables the protein to enter the nuclear membranes.

(c) It aids in protein unfolding so that the protein can thread through nuclear pores.

(d) It prevents the protein from diffusing out of the nucleus through nuclear pores.

(a) It is bound by cytoplasmic proteins that direct the nuclear protein to the nuclear pore.

10

Which of the following statements about nuclear transport is true?

(a) mRNAs and proteins transit the nucleus through different types of nuclear pores.

(b) Nuclear import receptors bind to proteins in the cytosol and bring the proteins to the nuclear pores, where the proteins are released from the receptors into the pores for transit into the nucleus.

(c) Nuclear pores have water-filled passages that small, water-soluble molecules can pass through in a nonselective fashion.

(d) Nuclear pores are made up of many copies of a single protein.

(c) Nuclear pores have water-filled passages that small, water-soluble molecules can pass through in a nonselective fashion.

11

A large protein that passes through the nuclear pore must have an appropriate _________.

(a) sorting sequence, which typically contains the positively charged amino acids lysine and arginine.

(b) sorting sequence, which typically contains the hydrophobic amino acids leucine and isoleucine.

(c) sequence to interact with the nuclear fibrils.

(d) Ran-interacting protein domain.

(a) sorting sequence, which typically contains the positively charged amino acids lysine and arginine.

12

Your friend works in a biotechnology company and has discovered a drug that blocks the ability of Ran to exchange GDP for GTP. What is the most likely effect of this drug on nuclear transport?

(a) Nuclear transport receptors would be unable to bind cargo.

(b) Nuclear transport receptors would be unable to enter the nucleus.

(c) Nuclear transport receptors would be unable to release their cargo in the nucleus.

(d) Nuclear transport receptors would interact irreversibly with the nuclear pore fibrils.

(c) Nuclear transport receptors would be unable to release their cargo in the nucleus.

13

Which of the following statements is true?

(a) The signal sequences on mitochondrial proteins are usually at the C-terminus.

(b) Most mitochondrial proteins are not imported from the cytosol but are synthesized inside the mitochondria.

(c) Chaperone proteins in the mitochondria facilitate the movement of proteins across the outer and inner mitochondrial membranes.

(d) Mitochondrial proteins cross the membrane in their native, folded state.

(c) Chaperone proteins in the mitochondria facilitate the movement of proteins across the outer and inner mitochondrial membranes.

14

Which of the following statements about transport into mitochondria and chloroplasts is false?

(a) The signal sequence on proteins destined for these organelles is recognized by a receptor protein in the outer membrane of these organelles.

(b) After a protein moves through the protein translocator in the outer membrane of these organelles, the protein diffuses in the lumen until it encounters a protein translocator in the inner membrane.

(c) Proteins that are transported into these organelles are unfolded as they are being transported.

(d) Signal peptidase will remove the signal sequence once the protein has been imported into these organelles.

(b) After a protein moves through the protein translocator in the outer membrane of these organelles, the protein diffuses in the lumen until it encounters a protein translocator in the inner membrane.

15

Which of the following statements about peroxisomes is false?

(a) Most peroxisomal proteins are synthesized in the ER.

(b) Peroxisomes synthesize phospholipids for the myelin sheath.

(c) Peroxisomes produce hydrogen peroxide.

(d) Vesicles that bud from the ER can mature into peroxisomes.

(a) Most peroxisomal proteins are synthesized in the ER.

16

Most proteins destined to enter the endoplasmic reticulum _________.

(a) are transported across the membrane after their synthesis is complete.

(b) are synthesized on free ribosomes in the cytosol.

(c) begin to cross the membrane while still being synthesized.

(d) remain within the endoplasmic reticulum.

(c) begin to cross the membrane while still being synthesized.

17

After isolating the rough endoplasmic reticulum from the rest of the cytoplasm, you purify the RNAs attached to it. Which of the following proteins do you expect the RNA from the rough endoplasmic reticulum to encode?

(a) soluble secreted proteins

(b) ER membrane proteins

(c) plasma membrane proteins

(d) all of the above

(d) all of the above

18

In which cellular location would you expect to find ribosomes translating mRNAs that encode ribosomal proteins?

(a) the nucleus

(b) on the rough ER

(c) in the cytosol

(d) in the lumen of the ER

(c) in the cytosol

19

You are interested in Fuzzy, a soluble protein that functions within the ER lumen. Given that information, which of the following statements must be true?

(a) Fuzzy has a C-terminal signal sequence that binds to SRP.

(b) Only one ribosome can be bound to the mRNA encoding Fuzzy during translation.

(c) Fuzzy must contain a hydrophobic stop-transfer sequence.

(d) Once the signal sequence from Fuzzy has been cleaved, the signal peptide will be ejected into the ER membrane and degraded.

(d) Once the signal sequence from Fuzzy has been cleaved, the signal peptide will be ejected into the ER membrane and degraded.

20

Which of the following statements about a protein in the lumen of the ER is false?

(a) A protein in the lumen of the ER is synthesized by ribosomes on the ER membrane.

(b) Some of the proteins in the lumen of the ER can end up in the extracellular space.

(c) Some of the proteins in the lumen of the ER can end up in the lumen of an organelle in the endomembrane system.

(d) Some of the proteins in the lumen of the ER can end up in the plasma membrane.

(d) Some of the proteins in the lumen of the ER can end up in the plasma membrane.

21

Which of the following statements is true?

(a) Proteins destined for the ER are translated by a special pool of ribosomes whose subunits are always associated with the outer ER membrane.

(b) Proteins destined for the ER translocate their associated mRNAs into the ER lumen where they are translated.

(c) Proteins destined for the ER are translated by cytosolic ribosomes and are targeted to the ER when a signal sequence emerges during translation.

(d) Proteins destined for the ER are translated by a pool of cytosolic ribosomes that contain ER-targeting sequences that interact with ER-associated protein translocators.

(c) Proteins destined for the ER are translated by cytosolic ribosomes and are targeted to the ER when a signal sequence emerges during translation.

22
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Figure Q15-31 shows the organization of a protein that resides on the ER membrane. The N- and C-termini of the protein are labeled. Boxes 1, 2, and 3 represent membrane-spanning sequences. Non-membrane-spanning regions of the protein are labeled “X,” “Y,” and “Z.”

Once this protein is fully translocated, where will region Y be?

(a) in the cytoplasm

(b) in the ER lumen

(c) inserted into the ER membrane

(d) degraded by signal peptidase

(a) in the cytoplasm

23
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Figure Q15-34 shows the organization of a protein that normally resides in the plasma membrane. The boxes labeled 1 and 2 represent membrane-spanning sequences and the arrow represents a site of action of signal peptidase. Given this diagram, which of the following statements must be true?

(a) The N-terminus of this protein is cytoplasmic.

(b) The C-terminus of this protein is cytoplasmic.

(c) The mature version of this protein will span the membrane twice, with both the N- and C-terminus in the cytoplasm.

(d) None of the above.

(b) The C-terminus of this protein is cytoplasmic.

24

Which of the following choices reflects the appropriate order of locations through which a protein destined for the plasma membrane travels?

(a) lysosome -> endosome -> plasma membrane

(b) ER -> lysosome -> plasma membrane

(c) Golgi -> lysosome -> plasma membrane

(d) ER -> Golgi -> plasma membrane

(d) ER -> Golgi -> plasma membrane

25

Which of the following statements about vesicle budding from the Golgi is false?

(a) Clathrin molecules are important for binding to and selecting cargoes for transport.

(b) Adaptins interact with clathrin.

(c) Once vesicle budding occurs, clathrin molecules are released from the vesicle.

(d) Clathrin molecules act at the cytosolic surface of the Golgi membrane.

(a) Clathrin molecules are important for binding to and selecting cargoes for transport.

26

Molecules to be packaged into vesicles for transport are selected by ­­­­­­________.

(a) clathrin.

(b) adaptins.

(c) dynamin.

(d) SNAREs

(b) adaptins.

27

Which of the following protein families are not involved in directing transport vesicles to the target membrane?

(a) SNAREs

(b) Rabs

(c) tethering proteins

(d) adaptins

(d) adaptins

28

Your friend has just joined a lab that studies vesicle budding from the Golgi and has been given a cell line that does not form mature vesicles. He wants to start designing some experiments but wasn’t listening carefully when he was told about the molecular defect of this cell line. He’s too embarrassed to ask and comes to you for help. He does recall that this cell line forms coated pits but vesicle budding and the removal of coat proteins don’t happen. Which of the following proteins might be lacking in this cell line?

(a) clathrin

(b) Rab

(c) dynamin

(d) adaptin

(c) dynamin

29

An individual transport vesicle ­­­­­­________.

(a) contains only one type of protein in its lumen.

(b) will fuse with only one type of membrane.

(c) is endocytic if it is traveling toward the plasma membrane.

(d) is enclosed by a membrane with the same lipid and protein composition as the membrane of the donor organelle.

(b) will fuse with only one type of membrane.

30

Which of the following statements about vesicular membrane fusion is false?

(a) Membrane fusion does not always immediately follow vesicle docking.

(b) The hydrophilic surfaces of membranes have water molecules associated with them that must be displaced before vesicle fusion can occur.

(c) The GTP hydrolysis of the Rab proteins provides the energy for membrane fusion.

(d) The interactions of the v-SNAREs and the t-SNAREs pull the vesicle membrane and the target organelle membrane together so that their lipids can intermix.

(c) The GTP hydrolysis of the Rab proteins provides the energy for membrane fusion.

31

N-linked oligosaccharides on secreted glycoproteins are attached to ­­­­­­________.

(a) nitrogen atoms in the polypeptide backbone.

(b) the serine or threonine in the sequence Asn-X-Ser/Thr.

(c) the N-terminus of the protein.

(d) the asparagine in the sequence Asn-X-Ser/Thr.

(d) the asparagine in the sequence Asn-X-Ser/Thr.

32

Which of the following statements about disulfide bond formation is false?

(a) Disulfide bonds do not form under reducing environments.

(b) Disulfide bonding occurs by the oxidation of pairs of cysteine side chains on the protein.

(c) Disulfide bonding stabilizes the structure of proteins.

(d) Disulfide bonds form spontaneously within the ER because the lumen of the ER is oxidizing.

(d) Disulfide bonds form spontaneously within the ER because the lumen of the ER is oxidizing.

33

Different glycoproteins can have a diverse array of oligosaccharides. Which of the statements below about this diversity is true?

(a) Extensive modification of oligosaccharides occurs in the extracellular space.

(b) Different oligosaccharides are covalently linked to proteins in the ER and the Golgi.

(c) A diversity of oligosaccharyl transferases recognizes specific protein sequences, resulting in the linkage of a variety of oligosaccharides to proteins.

(d) Oligosaccharide diversity comes from modifications that occur in the ER and the Golgi of the 14-sugar oligosaccharide added to the protein in the ER.

(d) Oligosaccharide diversity comes from modifications that occur in the ER and the Golgi of the 14-sugar oligosaccharide added to the protein in the ER.

34

Which of the following statements about the protein quality control system in the ER is false?

(a) Chaperone proteins help misfolded proteins fold properly.

(b) Proteins that are misfolded are degraded in the ER lumen.

(c) Protein complexes are checked for proper assembly before they can exit the ER.

(d) A chaperone protein will bind to a misfolded protein to retain it in the ER.

(b) Proteins that are misfolded are degraded in the ER lumen.

35

Which of the following statements about the unfolded protein response (UPR) is false?

(a) Activation of the UPR results in the production of more ER membrane.

(b) Activation of the UPR results in the production of more chaperone proteins.

(c) Activation of the UPR occurs when receptors in the cytoplasm sense misfolded proteins.

(d) Activation of the UPR results in the cytoplasmic activation of gene regulatory proteins.

(c) Activation of the UPR occurs when receptors in the cytoplasm sense misfolded proteins.

36

Vesicles from the ER enter the Golgi at the ______.

(a) medial cisternae.

(b) trans Golgi network.

(c) cis Golgi network.

(d) trans cisternae.

(c) cis Golgi network.

37

Which of the following statements about secretion is true?

(a) The membrane of a secretory vesicle will fuse with the plasma membrane when it discharges its contents to the cell’s exterior.

(b) Vesicles for regulated exocytosis will not bud off the trans Golgi network until the appropriate signal has been received by the cell.

(c) The signal sequences of proteins destined for constitutive exocytosis ensure their packaging into the correct vesicles.

(d) Proteins destined for constitutive exocytosis aggregate as a result of the acidic pH of the trans Golgi network.

(a) The membrane of a secretory vesicle will fuse with the plasma membrane when it discharges its contents to the cell’s exterior.

38

Which of the following statements about phagocytic cells in animals is false?

(a) Phagocytic cells are important in the gut to take up large particles of food.

(b) Phagocytic cells scavenge dead and damaged cells and cell debris.

(c) Phagocytic cells can engulf invading microorganisms and deliver them to their lysosomes for destruction.

(d) Phagocytic cells extend pseudopods that surround the material to be ingested.

(a) Phagocytic cells are important in the gut to take up large particles of food.

39

You are working in a biotech company that has discovered a small-molecule drug called H5434. H5434 binds to LDL receptors when they are bound to cholesterol. H5434 binding does not alter the conformation of the LDL receptor’s intracellular domain. Interestingly, in vitro experiments demonstrate that addition of H5434 increases the affinity of LDL for cholesterol and prevents cholesterol from dissociating from the LDL receptor even in acidic conditions. Which of the following is a reasonable prediction of what may happen when you add H5434 to cells?

(a) Cytosolic cholesterol levels will remain unchanged relative to normal cells.

(b) Cytosolic cholesterol levels will decrease relative to normal cells.

(c) The LDL receptor will remain on the plasma membrane.

(d) The uncoating of vesicles will not occur.

(b) Cytosolic cholesterol levels will decrease relative to normal cells.

40
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Cell lines A and B both survive in tissue culture containing serum but do not proliferate. Factor F is known to stimulate proliferation in cell line A. Cell line A produces a receptor protein (R) that cell line B does not produce. To test the role of receptor R, you introduce this receptor protein into cell line B, using recombinant DNA techniques. You then test all of your various cell lines in the presence of serum for their response to factor F, with the results summarized in Table Q16-1.

Which of the following cannot be concluded from your results above?

(a) Binding of factor F to its receptor is required for proliferation of cell line A.

(b) Receptor R binds to factor F to induce cell proliferation in cell line A.

(c) Cell line A expresses a receptor for factor F.

(d) Factor F is not required for proliferation in cell line B.

(a) Binding of factor F to its receptor is required for proliferation of cell line A.

41

When a signal needs to be sent to most cells throughout a multicellular organism, the signal most suited for this is a ___________.

(a) neurotransmitter.

(b) hormone.

(c) dissolved gas.

(d) scaffold.

(b) hormone.

42

During nervous-system development in Drosophila, the membrane-bound protein Delta acts as an inhibitory signal to prevent neighboring cells from developing into neuronal cells. Delta is involved in ______________ signaling.

(a) endocrine

(b) paracrine

(c) neuronal

(d) contact-dependent

(d) contact-dependent

43

Which of the following statements is true?

(a) Because endocrine signals are broadcast throughout the body, all cells will respond to the hormonal signal.

(b) The regulation of inflammatory responses at the site of an infection is an example of paracrine signaling.

(c) Paracrine signaling involves the secretion of signals into the bloodstream for distribution throughout the organism.

(d) The axons of neurons typically signal target cells using membrane-bound signaling molecules that act on receptors in the target cells.

(b) The regulation of inflammatory responses at the site of an infection is an example of paracrine signaling.

44

Which of the following statements is false?

(a) Nucleotides and amino acids can act as extracellular signal molecules.

(b) Some signal molecules can bind directly to intracellular proteins that bind DNA and regulate gene transcription.

(c) Some signal molecules are transmembrane proteins.

(d) Dissolved gases such as nitric oxide (NO) can act as signal molecules, but because they cannot interact with proteins they must act by affecting membrane lipids.

(d) Dissolved gases such as nitric oxide (NO) can act as signal molecules, but because they cannot interact with proteins they must act by affecting membrane lipids.

45

All members of the steroid hormone receptor family __________________.

(a) are cell-surface receptors.

(b) do not undergo conformational changes.

(c) are found only in the cytoplasm.

(d) interact with signal molecules that diffuse through the plasma membrane.

(d) interact with signal molecules that diffuse through the plasma membrane.

46

The lab you work in has discovered a previously unidentified extracellular signal molecule called QGF, a 75,000-dalton protein. You add purified QGF to different types of cells to determine its effect on these cells. When you add QGF to heart muscle cells, you observe an increase in cell contraction. When you add it to fibroblasts, they undergo cell division. When you add it to nerve cells, they die. When you add it to glial cells, you do not see any effect on cell division or survival. Given these observations, which of the following statements is most likely to be true?

(a) Because it acts on so many diverse cell types, QGF probably diffuses across the plasma membrane into the cytoplasm of these cells.

(b) Glial cells do not have a receptor for QGF.

(c) QGF activates different intracellular signaling pathways in heart muscle cells, fibroblasts, and nerve cells to produce the different responses observed.

(d) Heart muscle cells, fibroblasts, and nerve cells must all have the same receptor for QGF.

(c) QGF activates different intracellular signaling pathways in heart muscle cells, fibroblasts, and nerve cells to produce the different responses observed.

47

Acetylcholine is a signaling molecule that elicits responses from heart muscle cells, salivary gland cells, and skeletal muscle cells. Which of the following statements is false?

(a) Heart muscle cells decrease their rate and force of contraction when they receive acetylcholine, whereas skeletal muscle cells contract.

(b) Heart muscle cells, salivary gland cells, and skeletal muscle cells all express an acetylcholine receptor that belongs to the transmitter-gated ion channel family.

(c) Active acetylcholine receptors on salivary gland cells and heart muscle cells activate different intracellular signaling pathways.

(d) Heart muscle cells, salivary gland cells, and skeletal muscle cells all respond to acetylcholine within minutes of receiving the signal.

(b) Heart muscle cells, salivary gland cells, and skeletal muscle cells all express an acetylcholine receptor that belongs to the transmitter-gated ion channel family.

48

The local mediator nitric oxide stimulates the intracellular enzyme guanylyl cyclase by ________________.

(a) activating a G protein.

(b) activating a receptor tyrosine kinase.

(c) diffusing into cells and stimulating the cyclase directly.

(d) activating an intracellular protein kinase.

(c) diffusing into cells and stimulating the cyclase directly.

49
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Figure Q16-18 shows the pathway through which nitric oxide (NO) triggers smooth muscle relaxation in a blood-vessel wall. Which of the following situations would lead to relaxation of the smooth muscle cells in the absence of acetylcholine?

(a) a smooth muscle cell that has a defect in guanylyl cyclase such that it cannot bind NO

(b) a muscle cell that has a defect in guanylyl cyclase such that it constitutively converts GTP to cyclic GMP

(c) a muscle cell that has cyclic GMP phosphodiesterase constitutively active

(d) a drug that blocks an enzyme involved in the metabolic pathway from arginine to NO

(b) a muscle cell that has a defect in guanylyl cyclase such that it constitutively converts GTP to cyclic GMP

50

Which of the following statements is true?

(a) Extracellular signal molecules that are hydrophilic must bind to a cell-surface receptor so as to signal a target cell to change its behavior.

(b) To function, all extracellular signal molecules must be transported by their receptor across the plasma membrane into the cytosol.

(c) A cell-surface receptor capable of binding only one type of signal molecule can mediate only one kind of cell response.

(d) Any foreign substance that binds to a receptor for a normal signal molecule will always induce the same response that is produced by that signal molecule on the same cell type.

(a) Extracellular signal molecules that are hydrophilic must bind to a cell-surface receptor so as to signal a target cell to change its behavior.

51

Which of the following statements about molecular switches is false?

(a) Phosphatases remove the phosphate from GTP on GTP-binding proteins, turning them off.

(b) Protein kinases transfer the terminal phosphate from ATP onto a protein.

(c) Serine/threonine kinases are the most common types of protein kinase.

(d) A GTP-binding protein exchanges its bound GDP for GTP to become activated.

(a) Phosphatases remove the phosphate from GTP on GTP-binding proteins, turning them off.

52

Foreign substances like nicotine, morphine, and menthol exert their initial effects by _____.

(a) killing cells immediately, exerting their physiological effects by causing cell death.

(b) diffusing through cell plasma membranes and binding to transcription factors to change gene expression.

(c) interacting with cell-surface receptors, causing the receptors to transduce signal inappropriately in the absence of the normal stimulus.

(d) removing cell-surface receptors from the plasma membrane.

(c) interacting with cell-surface receptors, causing the receptors to transduce signal inappropriately in the absence of the normal stimulus.

53

The following happens when a G-protein-coupled receptor activates a G protein.

(a) The β subunit exchanges its bound GDP for GTP.

(b) The GDP bound to the α subunit is phosphorylated to form bound GTP.

(c) The α subunit exchanges its bound GDP for GTP.

(d) It activates the α subunit and inactivates the βγ complex.

(c) The α subunit exchanges its bound GDP for GTP.

54

Which of the following statements about G-protein-coupled receptors (GPCRs) is false?

(a) GPCRs are the largest family of cell-surface receptors in humans.

(b) GPCRs are used in endocrine, paracrine, and neuronal signaling.

(c) GPCRs are found in yeast, mice, and humans.

(d) The different classes of GPCR ligands (proteins, amino acid derivatives, or fatty acids) bind to receptors with different numbers of transmembrane domains.

(d) The different classes of GPCR ligands (proteins, amino acid derivatives, or fatty acids) bind to receptors with different numbers of transmembrane domains.

55

The length of time a G protein will signal is determined by _______.

(a) the activity of phosphatases that turn off G proteins by dephosphorylating Gα.

(b) the activity of phosphatases that turn GTP into GDP.

(c) the degradation of the G protein after Gα separates from Gβγ.

(d) the GTPase activity of Gα.

(d) the GTPase activity of Gα.

56
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During the mating process, yeast cells respond to pheromones secreted by other yeast cells. These pheromones bind GPCRs on the surface of the responding cell and lead to the activation of G proteins inside the cell. When a wild-type yeast cell senses the pheromone, its physiology changes in preparation for mating: the cell stops growing until it finds a mating partner. If yeast cells do not undergo the appropriate response after sensing a pheromone, they are considered sterile. Yeast cells that are defective in one or more components of the G protein have characteristic phenotypes in the absence and presence of the pheromone, which are listed in Table 16-34.

Which of the following models is consistent with the data from the analysis of these mutants? Explain your answer.

(a) α activates the mating response but is inhibited when bound to βγ

(b) βγ activates the mating response but is inhibited when bound to α

(c) the G protein is inactive; either free α or free βγ complex is capable of activating the mating response

(d) the G protein is active; both free α and free βγ complex are required to inhibit the mating response

(b) βγ activates the mating response but is inhibited when bound to α

57

You are interested in cell-size regulation and discover that signaling through a GPCR called ERC1 is important in controlling cell size in embryonic rat cells. The G protein downstream of ERC1 activates adenylyl cyclase, which ultimately leads to the activation of PKA. You discover that cells that lack ERC1 are 15% smaller than normal cells, while cells that express a mutant, constitutively activated version of PKA are 15% larger than normal cells. Given these results, which of the following treatments to embryonic rat cells should lead to smaller cells?

(a) addition of a drug that causes cyclic AMP phosphodiesterase to be hyperactive

(b) addition of a drug that prevents GTP hydrolysis by Gα

(c) addition of a drug that activates adenylyl cyclase

(d) addition of a drug that mimics the ligand of ERC1

(a) addition of a drug that causes cyclic AMP phosphodiesterase to be hyperactive

58

The growth factor Superchick stimulates the proliferation of cultured chicken cells. The receptor that binds Superchick is a receptor tyrosine kinase (RTK), and many chicken tumor cell lines have mutations in the gene that encodes this receptor. Which of the following types of mutation would be expected to promote uncontrolled cell proliferation?

(a) a mutation that prevents dimerization of the receptor

(b) a mutation that destroys the kinase activity of the receptor

(c) a mutation that inactivates the protein tyrosine phosphatase that normally removes the phosphates from tyrosines on the activated receptor

(d) a mutation that prevents the binding of the normal extracellular signal to the receptor

(c) a mutation that inactivates the protein tyrosine phosphatase that normally removes the phosphates from tyrosines on the activated receptor

59

The growth factor RGF stimulates proliferation of cultured rat cells. The receptor that binds RGF is a receptor tyrosine kinase called RGFR. Which of the following types of alteration would be most likely to prevent receptor dimerization?

(a) a mutation that increases the affinity of RGFR for RGF

(b) a mutation that prevents RGFR from binding to RGF

(c) changing the tyrosines that are normally phosphorylated on RGFR dimerization to alanines

(d) changing the tyrosines that are normally phosphorylated on RGFR dimerization to glutamic acid

(b) a mutation that prevents RGFR from binding to RGF

60

A protein kinase can act as an integrating device in signaling if it ___________________.

(a) phosphorylates more than one substrate.

(b) catalyzes its own phosphorylation.

(c) is activated by two or more proteins in different signaling pathways.

(d) initiates a phosphorylation cascade involving two or more protein kinases.

(c) is activated by two or more proteins in different signaling pathways.

61

Which of the following mechanisms is not directly involved in inactivating an activated RTK?

(a) dephosphorylation by serine/threonine phosphatases

(b) dephosphorylation by protein tyrosine phosphatases

(c) removal of the RTK from the plasma membrane by endocytosis

(d) digestion of the RTK in lysosomes

(a) dephosphorylation by serine/threonine phosphatases

62

Which of the following statements is true?

(a) MAP kinase is important for phosphorylating MAP kinase kinase.

(b) PI 3-kinase phosphorylates a lipid in the plasma membrane.

(c) Ras becomes activated when an RTK phosphorylates its bound GDP to create GTP.

(d) STAT proteins phosphorylate JAK proteins, which then enter the nucleus and activate gene transcription.

(b) PI 3-kinase phosphorylates a lipid in the plasma membrane.

63

The activation of the serine/threonine protein kinase Akt requires phosphoinositide 3-kinase (PI 3-kinase) to _________.

(a) activate the RTK.

(b) create phosphorylated lipids that serve as docking sites that localize Akt to the plasma membrane.

(c) directly phosphorylate Akt.

(d) to create DAG.

(b) create phosphorylated lipids that serve as docking sites that localize Akt to the plasma membrane.

64

When Ras is activated, cells will divide. A dominant-negative form of Ras clings too tightly to GDP. You introduce a dominant-negative form of Ras into cells that also have a normal version of Ras. Which of the following statements is true?

(a) The cells you create will divide less frequently than normal cells in response to the extracellular signals that typically activate Ras.

(b) The cells you create will run out of the GTP necessary to activate Ras.

(c) The cells you create will divide more frequently compared to normal cells in response to the extracellular signals that typically activate Ras.

(d) The normal Ras in the cells you create will not be able to bind GDP because the dominant-negative Ras binds to GDP too tightly.

(a) The cells you create will divide less frequently than normal cells in response to the extracellular signals that typically activate Ras.