Cell Bio Ch 15

15-25 Which of the following statements about vesicle budding from the Golgi is false? (a) Clathrin molecules are important for binding to and selecting cargos 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 cargos for transport. Cargo binds to cargo receptors. Adaptin molecules capture cargo receptors, which bind to the appropriate cargo molecules for incorporation into the vesicle.

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.

15-15 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

15-38 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 from 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. Vesicles for regulated exocytosis bud from the trans Golgi network and accumulate at the plasma membrane until the appropriate signal has been received (choice (b)). There are no signal sequences for proteins destined for exocytosis (choice (c)). Those proteins that are to be secreted by regulated exocytosis aggregate in the trans Golgi as a result of the acidic pH and high Ca2+ concentrations (choice (d)); those proteins that do not aggregate are packed into transport vesicles for constitutive exocytosis.

15-41 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. Normally, cholesterol dissociates from the LDL receptor in the acidic environment of the endosomes and is released into the cytosol. If the drug prevents cholesterol from dissociating from the LDL receptor in acidic conditions, cholesterol may not become released into the cytosol, and thus cytosolic cholesterol levels are likely to decrease relative to those in normal cells. There is no reason to believe that the LDL receptor will remain on the plasma membrane (choice (c)), because the cytosolic region of the receptor is not directly altered by the drug. Vesicle uncoating is also unlikely to be altered (choice (d)), because this occurs after vesicles are pinched off from the membrane.

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.

15-35 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. The receptors for the unfolded proteins are on the ER membrane, and they sense the misfolded proteins using their luminal domains.

10 Which of the following statements is true? (a) The signal sequences on mitochondrial proteins are usually C-terminal. (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.

15-17 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.

15-11 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.

15-13 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

5. Which of the following organelles are not part of the endomembrane system? (a) Golgi apparatus (b) the nucleus (c) mitochondria (d) lysosomes

(c) mitochondria

15-18 You are interested in Fuzzy, a soluble protein that functions within the ER lumen. Given that information, which of the following statement 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.

15-12 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

8. 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

9 What would happen in each of the following cases? Assume in each case that the protein involved is a soluble protein, not a membrane protein. A. You add a signal sequence (for the ER) to the N-terminal end of a normally cytosolic protein. B. You change the hydrophobic amino acids in an ER signal sequence into charged amino acids. C. You change the hydrophobic amino acids in an ER signal sequence into other, hydrophobic, amino acids. D. You move the N-terminal ER signal sequence to the C-terminal end of the protein.

. B. The altered signal sequence will not be recognized and the protein will The protein will now be transported into the ER lumen. remain in the cytosol. C. The protein will still be delivered into the ER. It is the distribution of hydrophobic amino acids that is important, not the actual sequence. D. The protein will not enter the ER. Because the C-terminus of the protein is the last part to be made, the ribosomes synthesizing this protein will not be recognized by the SRP and carried to the ER.

Which of the following statements about vesicle budding from the Golgi is false? A. Clathrin molecules are important for binding to and selecting cargos 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 cargos for transport.

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.

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.

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 from 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.

15-21 Using genetic engineering techniques, you have created a set of proteins that contain two (and only two) conflicting signal sequences that specify different compartments. Predict which signal would win out for the following combinations. Explain your answers. A. Signals for import into the nucleus and import into the ER. B. Signals for export from the nucleus and import into the mitochondria. C. Signals for import into mitochondria and retention in the ER.

A. The protein would enter the ER. The signal for a protein to enter the ER is recognized as the protein is being synthesized and will end up either in the ER or on the ER membrane. Cytosolic nuclear transport proteins recognize proteins destined for the nucleus once those proteins are fully synthesized and fully folded. B. The protein would enter the mitochondria. For a nuclear export signal to work, the protein would have to end up in the nucleus first and thus would need a nuclear import signal for the nuclear export signal to be used. C. The protein would enter the mitochondria. To be retained in the ER, the protein needs to enter the ER. Because there is no signal for ER import, the ER retention signal would not function.

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. Uncoating of vesicles will not occur.

B. Cytosolic cholesterol levels will decrease relative to normal cells.

Which of the following statements about the endoplasmic reticulum 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 other nuclear membrane

B. Proteins to be delivered to the ER lumen are synthesized on smooth ER

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.

which of the following statements is true? A. The signal sequences on mitochondrial proteins are usually C-terminal. 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.

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

Which of the following organelles are not part of the endomembrane system? A. Golgi apparatus B. The nucleus C. mitochondria D. lysosomes

C. Mitochondria

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.

Most proteins destined to enter the ER 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 ER

C. begin to cross the membrane while still being synthesized.

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

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

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.

You are interested in Fuzzy, a soluble protein that functions within the ER lumen. Given that information, which of the following statement 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.

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

Where are proteins in the chloroplast synthesized? A. in the cytosol B. in the chloroplast C. on the ER D. in both the cytosol and the chloroplast

D. in both the cytosol and the chloroplast

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.

new membrane synthesis occurs in

ER

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

ER → Golgi → plasma membrane

photosynthesis occurs in

chloroplast

modification of secreted proteins occurs in

golgi apparatus and rough ER

degradation of worn-out organelles occurs in

lysosome

oxidative phosphorylation occurs in

mitochondrion

transcription occurs in

nucleus

breakdown of lipids and toxic molecules occurs in

peroxisome

steroid hormone synthesis occurs in

smooth ER

An individual transport vesicle

will fuse with only one type of membrane.