Deck 15: Intracellular Compartments and Protein Transport

Question 1

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.

Answer 1

C

Question 2

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.

Answer 2

C

Question 3

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 the smooth ER.
C)Steroid hormones are synthesized on the smooth ER.
D)The ER membrane is contiguous with the outer nuclear membrane.

Answer 3

B

Question 4

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 contain proteins with disordered segments that fill the channel and allow small water-soluble molecules to pass through in a non-selective fashion.
D)Nuclear pores are made up of many copies of a single protein.

Question 5

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

A)Golgi apparatus
B)the endosome
C)mitochondria
D)lysosomes

Question 6

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.

Question 7

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.

Question 8

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

Question 9

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.

Question 10

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.

Question 11

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.

Question 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 these answers are correct

Question 13

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

A)cytosol.
B)mitochondria.
C)interior of the nucleus.
D)nuclear membrane.

Question 14

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.

Question 15

Most proteins destined to enter the endoplasmic reticulum

A)are transported across the membrane after their synthesis is complete.
B)are completely translated on free ribosomes in the cytosol.
C)begin to cross the membrane while still being synthesized.
D)remain within the endoplasmic reticulum.

Question 16

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 contain enzymes that help inactivate toxins.
D)Proteins do not need to unfold to enter the peroxisome.

Question 17

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.

Question 18

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.

Question 19

Where are chloroplast proteins translated?

A)in the cytosol
B)in the chloroplast
C)on the endoplasmic reticulum
D)in both the cytosol and the chloroplast

Question 20

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.

Question 21

Cells have oligosaccharides displayed on their cell surface that are important for cell-cell recognition.Your friend discovered a transmembrane glycoprotein, GP1, on a pathogenic yeast cell that is recognized by human immune cells.He decides to purify large amounts of GP1 by expressing it in bacteria.To his purified protein he then adds a branched 14-sugar oligosaccharide to the asparagine of the only Asn-X-Ser sequence found on GP1 (Figure 15-32).Unfortunately, immune cells do not seem to recognize this synthesized glycoprotein.Which of the following statements is a likely explanation for this problem? Figure 15-32

A)The oligosaccharide should have been added to the serine instead of the asparagine.
B)The oligosaccharide should have been added one sugar at a time.
C)The oligosaccharide needs to be further modified before it is mature.
D)The oligosaccharide needs a disulfide bond.

Question 22

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.

Question 23

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

Question 24

Figure 15-22 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? Figure 15-22

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 these answers are correct.

Question 25

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 was not listening carefully when he was told about the molecular defect of this cell line.He is 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

Question 26

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.

Question 27

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.

Question 28

Which of the following is NOT a process that delivers material to the lysosome?

A)pinocytosis
B)phagocytosis
C)transcytosis
D)autophagy

Question 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.

Question 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.

Question 31

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

Question 32

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.

Question 33

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.

Question 34

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.

Question 35

Vesicles from the ER enter the Golgi at the

A)medial cisternae.
B)trans Golgi network.
C)cis Golgi network.
D)trans cisternae.

Question 36

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.

Question 37

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.

Question 38

Molecules to be packaged into vesicles for transport are selected by

A)clathrin.
B)adaptins.
C)dynamin.
D)SNAREs.

Question 39

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.

Question 40

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.

Question 41

Briefly describe the mechanism by which an internal stop-transfer sequence in a protein causes the protein to become embedded in the lipid bilayer as a transmembrane protein with a single membrane-spanning region.Assume that the protein has an N-terminal signal sequence and just one internal hydrophobic stop-transfer sequence.

Question 42

If you remove the ER retention signal from a protein that normally resides in the ER lumen, where do you predict the protein will ultimately end up? Explain your reasoning.

Question 43

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; each word or phrase should be used only once.
chaperone Golgi apparatus pseudopods
cholesterol mycobacterium rough ER
clathrin phagocytosis SNARE
endosome pinocytosis transcytosis
Eukaryotic cells are continually taking up materials from the extracellular space by the process of endocytosis.One type of endocytosis is __________, which uses __________ proteins to form small vesicles containing fluids and molecules.After these vesicles have pinched off from the plasma membrane, they will fuse with the __________, where materials that are taken into the vesicle are sorted.A second type of endocytosis is __________, which is used to take up large vesicles that can contain microorganisms and cellular debris.Macrophages are especially suited for this process, as they extend __________ (sheetlike projections of their plasma membrane) to surround the invading microorganisms.

Question 44

A gene regulatory protein, A, contains a typical nuclear localization signal but surprisingly is usually found in the cytosol.When the cell is exposed to hormones, protein A moves from the cytosol into the nucleus, where it turns on genes involved in cell division.When you purify protein A from cells that have not been treated with hormones, you find that protein B is always complexed with it.To determine the function of protein B, you engineer cells lacking the gene for protein B.You compare normal and defective cells by using differential centrifugation to separate the nuclear fraction from the cytoplasmic fraction, and then separating the proteins in these fractions by gel electrophoresis.You identify the presence of protein A and protein B by looking for their characteristic bands on the gel.The gel you run is shown in Figure 15-4.
Figure 15-4
On the basis of these results, what is the function of protein B? Explain your conclusion and propose a mechanism for how protein B works.

Question 45

v-SNAREs and t-SNAREs mediate the recognition of a vesicle at its target membrane so that a vesicle displaying a particular type of v-SNARE will only fuse with a target membrane containing a complementary type of t-SNARE.In some cases, v-SNAREs and t-SNAREs may also mediate the fusion of identical membranes.In yeast cells, right before the formation of a new cell, vesicles derived from the vacuole will come together and fuse to form a new vacuole destined for the new cell.Unlike the situation we have discussed in class, the vacuolar vesicles contain both v-SNAREs and t-SNAREs.Your friend is trying to understand the role of these SNAREs in the formation of the new vacuole and consults with you regarding the interpretation of his data.
Your friend has designed an ingenious assay for the fusion of vacuolar vesicles by using alkaline phosphatase.The protein alkaline phosphatase is made in a "pro" form that must be cleaved for the protein to be active.Your friend has designed two different strains of yeast: strain A produces the "pro" form of alkaline phosphatase (pro-Pase), whereas strain B produces the protease that can cleave pro-Pase into the active form (Pase).Neither strain has the active form of the alkaline phosphatase, but when vacuolar vesicles from the strains A and B are mixed, fusion of vesicles generates active alkaline phosphatase, whose activity can be measured and quantified (Figure 15-9A).
Figure 15-9
Your friend has taken each of these yeast strains and further engineered them so that they express only the v-SNAREs, only the t-SNAREs, both SNAREs (the normal situation), or neither SNARE.He then isolates vacuolar vesicles from all strains and tests the ability of each variant form of strain A to fuse with each variant form of strain B, by using the alkaline phosphatase assay.The data are shown in the graph in Figure 15-9B.On this graph, the SNARE present on the vesicle of the particular yeast strain is indicated as "v" (for the presence of the v-SNARE) and "t" (for the presence of the t-SNARE).
What do his data say about the requirements for v-SNAREs and t-SNAREs in the vacuolar vesicles? Is it important to have a specific type of SNARE (that is, v-SNARE or t-SNARE) on each vesicle?

Question 46

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; use each word or phrase only once.
carbohydrate endosome lysosome
disulfide bonds exocytic protein
endocytic Golgi apparatus secretory
endomembrane hydrogen bonds
endoplasmic reticulum ionic bonds
Proteins are transported out of a cell via the __________ or __________ pathway.Fluids and macromolecules are transported into the cell via the __________ pathway.All proteins being transported out of the cell pass through the __________ and the __________.Transport vesicles link organelles of the __________ system.The formation of __________ in the endoplasmic reticulum stabilizes protein structure.

Question 47

The interior of the organelle contains ribosomes.
How might this organelle have arisen?

Question 48

You are trying to identify the peroxisome-targeting sequence in the thiolase enzyme in yeast.The thiolase enzyme normally resides in the peroxisome and therefore must contain amino acid sequences that are used to target the enzyme for import into the peroxisome.To identify the targeting sequences, you create a set of hybrid genes that encode fusion proteins containing part of the thiolase protein fused to another protein, histidinol dehydrogenase (HDH).HDH is a cytosolic enzyme required for the synthesis of the amino acid histidine and cannot function if it is localized in the peroxisome.You genetically engineer a series of yeast cells to express these fusion proteins instead of their own versions of these enzymes.If the fusion proteins are imported into the peroxisome, the HDH portion of the protein cannot function and the yeast cells cannot grow on a medium lacking histidine.You obtain the results shown in Figure 15-5.
Figure 15-5
What region of the thiolase protein contains the peroxisomal targeting sequence? Explain your answer.

Question 49

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.

Question 50

For each of the following sentences, fill in the blanks with the best word or phrase selected from the list below.Not all words or phrases will be used; use each word or phrase only once.
amino acid sequence Golgi apparatus sorting signal
endoplasmic reticulum plasma membrane transport vesicles
folded protein translocators unfolded
Plasma membrane proteins are inserted into the membrane in the __________.The address information for protein sorting in a eukaryotic cell is contained in the __________ of the proteins.Proteins enter the nucleus in their __________ form.Proteins that remain in the cytosol do not contain a __________.Proteins are transported into the Golgi apparatus via __________.The proteins transported into the endoplasmic reticulum by __________ are in their __________form.

Question 51

In a cell capable of regulated secretion, what are the three main classes of proteins that must be separated before they leave the trans Golgi network?

Question 52

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.

Question 53

Name a type of protein modification that can occur in the ER but not in the cytosol.

Question 54

You have created a green fluorescent protein (GFP) fusion to a protein that is normally secreted from yeast cells.Because you have learned about the use of temperature-sensitive mutations in yeast to study protein and vesicle transport, you obtain three mutant yeast strains, each defective in some aspect of the protein secretory process.Being a good scientist, you of course also obtain a wild-type control strain.You decide to examine the fate of your GFP fusion protein in these various yeast strains and engineer the mutant strains to express your GFP fusion protein.However, in your excitement to do the experiment, you realize that you did not label any of the mutant yeast strains and no longer know which strain is defective in what process.You end up numbering your strains with the numbers 1 to 4, and then you carry out the experiment anyway, obtaining the results shown in Figure 15-17 (the black dots represent your GFP fusion protein).
Figure 15-17
Name the process that is defective in each of these strains.Remember that one of these strains is your wild-type control.

Question 55

If a lysosome breaks, what protects the rest of the cell from lysosomal enzymes?