ESSENTIAL CELL BIOLOGY, FOURTH EDITION
CHAPTER 15: INTRACELLULAR COMPARTMENTS AND PROTEIN
TRANSPORT
© 2014 GARLAND SCIENCE PUBLISHING
Membrane-Enclosed Organelles
15-1
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.
15-2
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.
15-3
Name the membrane-enclosed compartments in a eukaryotic cell where each of the
functions listed below takes place.
A.
photosynthesis = chloroplast
B.
transcription = nucleus
C.
oxidative phosphorylation = mitochondrion
D.
modification of secreted proteins = GA & ER
E.
steroid hormone synthesis = smooth ER
F.
degradation of worn-out organelles = lysosome
G.
new membrane synthesis = ER
H.
breakdown of lipids and toxic molecules = peroxisome
15-4
Label the structures of the cell indicated by the lines in Figure Q15-4.
Page 1 of 20
Figure Q15-4
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
15-5
nucleus
peroxisome
rough endoplasmic reticulum
Golgi apparatus
cytosol
endosome
plasma membrane
lysosome
mitochondrion
smooth endoplasmic reticulum
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.
The cytosol makes up about half of the total cell volume of a typical eukaryotic cell.
Ingested materials within the cell will pass through a series of compartments called
endosome on their way to the lysosome, which contains digestive enzymes and will
ultimately degrade the particles and macromolecules taken into the cell and will also
degrade worn-out organelles. The GA has a cis and trans face and receives proteins and
lipids from the ER, a system of interconnected sacs and tubes of membranes that
typically extends throughout the cell.
cytosol
endoplasmic reticulum
endosomes
15-6
Golgi apparatus
lysosome
mitochondria
nucleus
peroxisomes
plasma membrane
Which of the following organelles is not part of the endomembrane system?
(a)
Golgi apparatus
(b)
the nucleus
Page 2 of 20
(c)
(d)
15-8
mitochondria
lysosomes
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.
Protein Sorting
15-9
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.
Plasma membrane proteins are inserted into the membrane in the ER. The address
information for protein sorting in a eukaryotic cell is contained in the AA sequence of
the proteins. Proteins enter the nucleus in their folded form. Proteins that remain in the
cytosol do not contain a sorting signal. Proteins are transported into the Golgi apparatus
via transport vesicles. The proteins transported into the endoplasmic reticulum by
protein translocators are in their unfolded form.
amino acid sequence
endoplasmic reticulum
folded
Golgi apparatus
plasma membrane
protein translocators
sorting signal
transport vesicles
unfolded
15-10 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
15-11 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.
15-12 Proteins that are fully translated in the cytosol and lack a sorting signal will end up in
____.
(a)
the cytosol.
(b)
the mitochondria.
Page 3 of 20
(c)
(d)
the interior of the nucleus.
the nuclear membrane.
15-13 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.
15-16 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.
15-18 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.
15-21 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.
15-24 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
15-25 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
Page 4 of 20
(d)
in the lumen of the ER
15-26 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.
15-27 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.
15-28 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.
15-29 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.
15-30 Match the components involved with ER transport with the appropriate cellular location.
Locations can be used more than once, or not at all.
Components
1. signal-recognition particle _____
2. protein translocator _____
3. mRNA _____
Location
A. cytosol
B. ER lumen
C. ER membrane
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4. SRP receptor _____
5. active site of signal peptidase ____
15-31 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 membranespanning sequences. Non-membrane-spanning regions of the protein are labeled “X,” “Y,”
and “Z.”
Figure Q15-31
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
15-34 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?
Figure Q15-34
(a)
(b)
(c)
(d)
The N-terminus of this protein is cytoplasmic.
The C-terminus of this protein is cytoplasmic.
The mature version of this protein will span the membrane twice, with both the Nand C-terminus in the cytoplasm.
None of the above.
Vesicular Transport
15-37 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.
Proteins are transported out of a cell via the secretory or exocytic pathway. Fluids and
macromolecules are transported into the cell via the Endocytic pathway. All proteins
being transported out of the cell pass through the ER and the GA. Transport vesicles link
organelles of the endomembrane system. The formation of disulfide bonds in the
endoplasmic reticulum stabilizes protein structure.
Page 6 of 20
carbohydrate
disulfide bonds
endocytic
endomembrane
endoplasmic reticulum
endosome
exocytic
Golgi apparatus
hydrogen bonds
ionic bonds
lysosome
protein
secretory
15-38 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.
15-39 Molecules to be packaged into vesicles for transport are selected by ________.
(a)
clathrin.
(b)
adaptins.
(c)
dynamin.
(d)
SNAREs.
15-40 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
Secretory Pathways
15-48 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 14sugar oligosaccharide to the asparagine of the only Asn-X-Ser sequence found on GP1
(Figure Q15-48). Unfortunately, immune cells do not seem to recognize this synthesized
glycoprotein. Which of the following statements is a likely explanation for this problem?
Page 7 of 20
Figure Q15-48
(a)
(b)
(c)
(d)
The oligosaccharide should have been added to the serine instead of the
asparagine.
The oligosaccharide should have been added one sugar at a time.
The oligosaccharide needs to be further modified before it is mature.
The oligosaccharide needs a disulfide bond.
15-49 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.
15-53 Match the set of labels below with the numbered label lines on Figure Q15-53.
Page 8 of 20
Figure Q15-53
A.
B.
C.
D.
E.
cisterna
Golgi stack
secretory vesicle
trans Golgi network
cis Golgi network
15-57 Figure Q15-57 shows the orientation of the Krt1 protein on the membrane of a Golgiderived vesicle that will fuse with the plasma membrane.
Figure Q15-57
Given this diagram, which of the following statements is true?
(a)
When this vesicle fuses with the plasma membrane, the entire Krt1 protein will be
secreted into the extracellular space.
(b)
When this vesicle fuses with the plasma membrane, the C-terminus of Krt1 will
be inserted into the plasma membrane.
(c)
When this vesicle fuses with the plasma membrane, the N-terminus of Krt1 will
be in the extracellular space.
(d)
When this vesicle fuses with the plasma membrane, the N-terminus of Krt1 will
be cytoplasmic.
Page 9 of 20
Endocytic Pathways
15-60 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.
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.
chaperone
cholesterol
clathrin
endosome
Golgi apparatus
mycobacterium
phagocytosis
pinocytosis
pseudopods
rough ER
SNARE
transcytosis
15-64 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.
ANSWERS
15-1
Choice (b) is false. Proteins to be delivered to the ER lumen are synthesized on rough
ER; these areas appear “rough” because ribosomes are attached to the cytosolic surface of
these ER regions.
15-2
Choice (a) is true; the area of the endoplasmic reticulum membrane is 20–30 times that of
the plasma membrane in a typical cell. Chloroplasts and mitochondria are also
surrounded by a double membrane [choice (b)]. The cytosol is about half the volume of a
typical eukaryotic cell, with membrane-enclosed organelles making up the other half of
Page 10 of 20
the volume [choice (c)]. Chloroplasts and mitochondria also carry their own genome,
whereas the nucleus carries the genome of the organism [choice (d)].
15-3
A
B.
C.
D.
E.
F.
G.
H.
15-4
photosynthesis = chloroplast
transcription = nucleus
oxidative phosphorylation = mitochondrion
modification of secreted proteins = Golgi apparatus and rough endoplasmic
reticulum (ER)
steroid hormone synthesis = smooth ER
degradation of worn-out organelles = lysosome
new membrane synthesis = ER
breakdown of lipids and toxic molecules = peroxisome
See Figure A15-4.
Figure A15-4
15-5
The cytosol makes up about half of the total cell volume of a typical eukaryotic cell.
Ingested materials within the cell will pass through a series of compartments called
endosomes on their way to the lysosome, which contains digestive enzymes and will
ultimately degrade the particles and macromolecules taken into the cell and will also
degrade worn-out organelles. The Golgi apparatus has a cis and trans face and receives
proteins and lipids from the endoplasmic reticulum, a system of interconnected sacs and
tubes of membranes that typically extends throughout the cell.
15-6
(c) Mitochondria are not part of the endomembrane system, which is thought to have
arisen initially through invagination of the plasma membrane. Instead, mitochondria (and
chloroplasts) are thought to have evolved from a bacterium that was engulfed by a
primitive eukaryotic cell.
15-7
A genome, a double membrane, ribosomes, and proteins similar to those found in bacteria
are evidence for an organelle having evolved from an engulfed bacterium.
Page 11 of 20
15-8
Choice (b) is correct. Lysosomes are part of the endomembrane system and are not
thought to have come from the engulfment of an ancient prokaryotic cell [choice (a)].
Bacteria use their plasma membrane for ATP production [choice (c)]. Chloroplasts and
mitochondria have their own DNA and do not share [choice (d)].
15-9 Plasma membrane proteins are inserted into the membrane in the endoplasmic reticulum.
The address information for protein sorting in a eukaryotic cell is contained in the amino
acid sequence of the proteins. Proteins enter the nucleus in their folded form. Proteins
that remain in the cytosol do not contain a sorting signal. Proteins are transported into
the Golgi apparatus via transport vesicles. The proteins transported into the endoplasmic
reticulum by protein translocators are in their unfolded form.
15-10 (d) Proteins in the chloroplast are synthesized in the cytosol and in the chloroplast. The
chloroplast proteins that are encoded by the nuclear DNA are synthesized in the cytosol,
and the sorting signals on the protein direct them to the chloroplast. The chloroplast
proteins encoded by the chloroplast DNA are synthesized on ribosomes inside the
chloroplast.
15-11 (d) Proteins destined for transport vesicles will be translated on ribosomes associated
with the endoplasmic reticulum.
15-12 (a) Proteins produced in the cytosol that lack sorting signals remain in the cytosol.
Proteins produced in the cytosol and destined for the mitochondria [choice (b)] or the
interior of the nucleus [choice (c)] will have a sorting signal to direct the protein to its
proper location. Proteins destined for the nuclear membrane [choice (d)] are not
translated in the cytosol.
15-13 (c) Signal sequences are found within the amino acid sequence of proteins. They are
sometimes removed when the protein is at the correct destination [choice (d)], but not all
are removed. For example, nuclear import signals are not removed once a protein is
inside the nucleus. A protein translocator resides in the membrane and helps transport
soluble proteins across the membrane [choice (b)], but does not add signal sequences to
proteins.
15-14 Choice (a) is correct. The nuclear localization signal typically contains positively charged
amino acids, not hydrophobic ones [choice (b)]. Proteins are not unfolded as they enter
the nucleus [choice (c)]. Proteins are actively transported in and out of the nucleus and do
not diffuse through the nuclear pores [choice (d)].
15-15 Choice (c) is correct. mRNAs and proteins can move through the same nuclear pore
[choice (a)]. Nuclear import receptors bind to proteins in the cytosol and transit with
them across the nuclear pore into the nucleus [choice (b)]. Nuclear pores are made up of
many copies of multiple proteins [choice (d)].
Page 12 of 20
15-16 Choice (a) is correct. The nuclear import receptor interacts with the fibrils of the nuclear
pore [choice (c)] and Ran [choice (d)].
15-17 The data on the gel show that protein A is always found in the nucleus in the absence of
protein B. Therefore, any mechanism that is proposed must explain this result.
One possible answer is that protein B binds protein A and masks the nuclear localization
signal. In the presence of hormone, protein B interacts with the hormone, which changes
its conformation so that it can no longer bind protein A. When protein B no longer binds
to protein A, the nuclear localization signal on protein A is now exposed and protein A
can enter the nucleus. Therefore, in the absence of protein B, the nuclear localization
signal on protein A is always exposed and protein A resides in the nucleus.
Another possible answer is that protein B binds protein A and sequesters it by keeping
protein A in some subcellular compartment, away from the nucleus. In the presence of
hormone, protein B interacts with the hormone, changing its conformation so that it can
no longer bind to protein A. When protein B is not present, protein A can enter the
nucleus in the presence or absence of hormone.
15-18 (c) When Ran-GTP binds to the nuclear transport receptor, cargo is released. If Ran could
not exchange its GDP for GTP, this would not happen. Ran-GTP is not needed for cargo
binding, for nuclear entry, or for interactions with the nuclear pore fibrils during nuclear
import.
15-19 Choice (c) is correct. The signal sequences on a protein destined for the mitochondria are
on its N-terminus [choice (a)]. Although some mitochondrial proteins are synthesized
inside the mitochondria from the mitochondrial genome, most mitochondrial proteins are
encoded by genes in the nucleus and imported into the mitochondria after synthesis in the
cytosol [choice (b)]. Mitochondrial proteins are unfolded as they enter the mitochondria
through protein translocators[choice (d)].
15-20 (b) Once a protein is bound to the import receptor, the protein—in a complex that
includes the protein translocator—will diffuse along the outer membrane until it reaches a
specialized site where the inner and outer membranes contact each other, and will then be
translocated simultaneously across the inner and outer membranes.
15-21 (a) Although peroxisomes can get some membrane-embedded proteins from the ER, most
peroxisomal proteins are imported from the cytosol.
15-22 The peroxisomal targeting sequence lies between amino acids number 100 and number
125. Any fusion protein containing this sequence can be targeted for import into the
peroxisome (because the yeast cannot grow on a medium lacking histidine), whereas the
fusion proteins lacking this region do not target the fusion protein for import into the
peroxisome (because the yeast do grow on medium lacking histidine). The most
important pieces of data are from the fusion protein containing amino acids 100–200 of
the thiolase protein fused to HDH and the fusion protein containing amino acids 1–125 of
Page 13 of 20
the thiolase protein fused to HDH. Neither of these fusion proteins allow growth on
medium lacking histidine and can be used to define the minimal region necessary for
targeting thiolase for import into the peroxisome.
(Note that although these experiments show that amino acids 100–125 are necessary,
these experiments do not show that this region is sufficient for peroxisomal targeting. It is
possible that the region consisting of amino acids 100–125 is sufficient, or it could be that
this region collaborates with redundant signals between amino acids 1 and 100 or
between amino acids 125 and 200.)
15-23 (c) Proteins destined to enter the endoplasmic reticulum have an N-terminal signal
sequence that leads to the docking of the ribosome synthesizing the protein onto the ER
and the entry of the protein across the ER membrane as the polypeptide chain is being
synthesized.
15-24 (d) The rough ER consists of ER membranes and polyribosomes that are in the process of
translating and translocating proteins into the ER membrane and lumen. Thus, all proteins
that end up in the lysosome, Golgi apparatus, or plasma membrane, or are secreted, will
be encoded by the RNAs associated with the rough ER.
15-25 (c) Ribosomes are cytoplasmic proteins and thus their protein components are translated
in the cytosol.
15-26 A.
B.
C.
D.
The protein will now be transported into the ER lumen.
The altered signal sequence will not be recognized and the protein will remain in
the cytosol.
The protein will still be delivered into the ER. It is the distribution of hydrophobic
amino acids that is important, not the actual sequence.
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 signal-recognition particle (SRP) and hence not carried to the ER.
15-27 Choice (d) is correct. ER signal sequences are typically at the N-terminus [choice (a)].
More than one ribosome can bind to an mRNA molecule [choice (b)]. Hydrophobic stoptransfer sequences are found on membrane-inserted proteins and not on soluble proteins
[choice (c)].
15-28 (d) Plasma membrane proteins come from proteins in the ER membrane, not from the ER
lumen.
15-29 (c)
15-30 1—A; 2—C; 3—A; 4—C; 5—B
15-31 (a) The final topology of the protein on the ER membrane is diagrammed in Figure A1531.
Page 14 of 20
Figure A15-31
15-32 The N-terminal signal sequence initiates translocation and the protein chain starts to
thread through the translocation channel. When the stop-transfer sequence enters the
translocation channel, the channel discharges both the signal sequence and the stoptransfer sequence sideways into the lipid bilayer. The signal sequence is then cleaved, so
that the protein remains held in the membrane by the hydrophobic stop-transfer sequence.
15-33 A.
B.
C.
The protein would enter the ER. The signal for a protein to enter the ER is
recognized as the protein is being synthesized and the protein 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.
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.
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.
15-34 (b) The mature version of this protein will span the membrane once, with membranespanning segment 2 in the membrane and the C-terminus facing the cytoplasm.
15-35 A.
B.
C.
Deleting the first signal sequence completely would convert the next membranespanning domain into an internal start-transfer signal and would invert the
orientation of the protein (see Figure A15-35A).
Changing the hydrophobic amino acids to charged amino acids destroys the
ability of the sequence both to act as a signal sequence and to become a
membrane-spanning sequence. Therefore, the adjacent membrane-spanning
domain will now become an internal start-transfer sequence and the protein will
be inverted, as seen above in part A. The mutated signal sequence would not get
cleaved off, because it would remain on the cytoplasmic side of the membrane
and signal peptidase is found only inside the ER (see Figure A15-35B).
Mutating every other membrane-spanning region so that they are now charged
(and thus cannot span the membrane) would decrease the number of
transmembrane regions and increase the size of the loops between membranespanning regions (see Figure A15-35C).
Page 15 of 20
Figure A15-35
15-36 (d)
15-37 Proteins are transported out of a cell via the secretory or exocytic pathway. Fluid and
macromolecules are transported into the cell via the endocytic pathway. All proteins
being transported out of the cell pass through the endoplasmic reticulum and the Golgi
apparatus. Transport vesicles link organelles of the endomembrane system. The
formation of disulfide bonds in the endoplasmic reticulum stabilizes protein structure.
15-38 (a) Cargo binds to cargo receptors. Adaptin molecules capture cargo receptors, which
bind to the appropriate cargo molecules for incorporation into the vesicle.
15-39 (b)
15-40 (d) Adaptins are involved in vesicle budding and are removed during the uncoating
process, and thus should not be present when the vesicle reaches its target.
15-41 (c) Given that coated pits can form but no vesicle budding is seen, dynamin is the most
likely answer. Since coated pits are formed, clathrin and adaptin are unlikely to be the
answer, because they are involved in the initial shaping of the vesicle into the pit [choices
(a) and (d)]. Rab proteins are involved in the recognition of the transport vesicle with its
target membrane and not with vesicle budding [choice (b)].
15-42 Choice (b) is correct. An individual vesicle may contain more than one type of protein in
its lumen [choice (a)], all of which will contain the same sorting signal (or will lack
specific sorting signals). Endocytic vesicles [choice (c)] generally move away from the
plasma membrane. The vesicle membrane will not necessarily contain the same lipid and
protein composition as the donor organelle, because the vesicle is formed from a selected
subsection of the organelle membrane from which it budded [choice (d)].
15-43 (c) Rab proteins are important for docking, but are not involved in the catalysis of
membrane fusion.
15-44 To get maximal levels of vacuolar vesicle fusion, vesicles from each strain must carry
both v-SNAREs and t-SNAREs. Experiment 1, which represents the normal scenario, is
the only experiment in which 100% alkaline phosphatase activity is measured. However,
as long as complementary SNAREs are present on the vesicles, some vesicle fusion does
occur (see experiments 3, 4, 6, 7, 8, and 9). If both vesicles are missing v-SNAREs
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(experiment 2) or t-SNAREs (experiment 5) or both SNAREs (experiments 10 and 11),
the level of fusion is very low. It does not matter whether a t-SNARE or a v-SNARE is
on the vesicle of a particular strain, as long as the vesicle from the other strain contains a
complementary SNARE (compare experiments 3 and 4, 6 and 7, and 8 and 9).
15-45 (d)
15-46 1.
2.
Proteins in the ER can undergo disulfide bond formation. (This does not occur in
the cytosol because of its reducing environment.)
Proteins in the ER can undergo glycosylation. (Glycosylating enzymes are not
found in the cytosol.)
(Signal-sequence cleavage is also an acceptable answer, although not really what
this question is referring to.)
15-47 (d) An enzyme in the ER lumen catalyzes disulfide bond formation.
15-48 (c) Oligosaccharides are usually further modified by enzymes in the ER and the Golgi
before the glycoprotein is inserted into the plasma membrane. The other choices are
untrue, and thus are not good explanations. Oligosaccharides are added to the Asn and
not the Ser [choice (a)] and are added as a branched 14-sugar oligosaccharide [choice (b)].
Disulfide bonding occurs between cysteines of proteins and not in sugars [choice (d)].
15-49 (d)
15-50 The protein would end up in the extracellular space. Normally, the protein would go from
the ER to the Golgi apparatus, get captured because of its ER retention signal, and return
to the ER. However, without the ER retention signal, the protein would evade capture,
ultimately leave the Golgi via the default pathway, and become secreted into the
extracellular space. The protein would not be retained anywhere else along the secretory
pathway: it presumably has no signals to promote such localization because it normally
resides in the ER lumen.
15-51 (b) Proteins that are misfolded are exported from the ER into the cytosol, where they are
degraded.
15-52 (c) The receptors for the unfolded proteins are on the ER membrane, and they sense the
misfolded proteins using their luminal domains.
15-53 A—3; B—1; C—5; D—4; E—2
15-54 (c)
15-55 A—3 (oligosaccharide protein transferase = ER)
B—1 (galactose transferase = central Golgi cisternae)
C—4 (SA transferase = trans Golgi network)
D—2 (GlcNAc transferase = cis Golgi network
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Proteins are modified in a stepwise fashion in the Golgi apparatus, with early steps taking
place in the cis Golgi network, intermediate steps taking place in the central Golgi
cisternae, and late steps occurring in the trans Golgi network. If each enzyme produces
the substrate for the next step, then a mutant lacking the enzyme that catalyzes the
addition of the first sugar will be missing all of the sugars, a mutant lacking the enzyme
that catalyzes the addition of the second sugar will contain the first sugar but will lack the
other three, and so on. By this logic, mannose and GlcNAc must be the first sugars added,
additional GlcNAc is the second added, galactose the third, and SA the last. Hence, the
oligosaccharide protein transferase must be in the ER, the GlcNAc transferase in the cis
Golgi network, the galactose transferase in the central Golgi cisternae, and the SA
transferase in the trans Golgi network.
15-56 Choice (a) is correct. 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 network 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.
15-57 (c) The orientation of Krt1 as the vesicle fuses with the plasma membrane is shown in
Figure A15-57. The darker-colored lines in the membrane represent the membranes
contributed by the vesicle during fusion.
Figure A15-57
15-58 New plasma membrane reaches the plasma membrane by the constitutive exocytosis
pathway. New plasma membrane proteins reach the plasma membrane by the
constitutive exocytosis pathway. Insulin is secreted from pancreatic cells by the
regulated exocytosis pathway. The interior of the trans Golgi network is acidic. Proteins
that are constitutively secreted do not aggregate in the trans Golgi network.
15-59 The three main classes of protein that must be sorted before they leave the trans Golgi
network in a cell capable of regulated secretion are (1) those destined for lysosomes, (2)
those destined for secretory vesicles, and (3) those destined for immediate delivery to the
cell surface.
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15-60 Eukaryotic cells are continually taking up materials from the extracellular space by the
process of endocytosis. One type of endocytosis is pinocytosis, which uses clathrin
proteins to form small vesicles containing fluids and molecules. After these vesicles have
pinched off from the plasma membrane, they will fuse with the endosome, where
materials that are taken into the vesicle are sorted. A second type of endocytosis is
phagocytosis, 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
pseudopods (sheetlike projections of their plasma membrane) to surround the invading
microorganisms.
15-61 (a) Although some unicellular eukaryotes ingest food particles by phagocytosis,
phagocytosis is not involved in digestion in the animal gut.
15-62 1.
2.
3.
recycled to the original membrane
destroyed in the lysosome
transcytosed across the cell to a different membrane
15-63 The lysosomal enzymes are all acid hydrolases, which have optimal activity at the low
pH (about 5.0) found in the interior of lysosomes. If a lysosome were to break, the acid
hydrolases would find themselves at pH 7.2, the pH of the cytosol, and would therefore
do little damage to cellular constituents.
15-64 (b) 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.
15-65 W—3 (defect in mannose-6-phosphate receptor)
X—2 (defect in phosphotransferase)
Y—1; Z—1 (defect in lysosomal hydrolases); these will be defects in two different
lysosomal acid hydrolases
A cell that has no mannose-6-phosphate receptor will be able to make all the lysosomal
hydrolases properly but will not be able to send them to the lysosome and will also not be
able to scavenge hydrolases from the external media. Hence, this cell line cannot be
rescued by a culture medium that has had lysosomal hydrolases secreted into it and thus
will not be rescued by any of the media tested here. A cell line that has no
phosphotransferase will be able to scavenge hydrolases from the external medium, but
because all of the cell’s own hydrolases will lack the mannose-6-phosphate tag, it will be
rescued only by medium from a cell line that is able to make all of the hydrolases. Cell
lines lacking one hydrolase will be rescued by medium from any cell line that is able to
Page 19 of 20
secrete that hydrolase in a mannose-6-phosphate-tagged form; in addition, media from
cultures of cells lacking a hydrolase will rescue any cell line with another type of defect.
15-66 Strain A has protein accumulating in the ER, which means that this cell has a mutation
that blocks transport from the ER to the Golgi apparatus. Strain B has secreted protein,
and therefore is the wild-type control. Strain C has protein accumulating in the Golgi
apparatus, and thus has a mutation that blocks exit of proteins from the Golgi apparatus.
Strain D has protein accumulating in the cis Golgi network, and thus has a mutation that
blocks the travel of proteins through the Golgi cisternae.
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