Essential Cell Biology 5th edition

Answers A:39
D. Prokaryotic cells do not perform endocytosis. A
prokaryotic cell therefore does not contain any receptors
with appropriate cytosolic tails that could mediate
adaptin binding. Therefore, no clathrin can bind and no
clathrin coats can assemble.
ANSWER 15–6 The preassembled sugar chain allows
better quality control. The assembled oligosaccharide
chains can be checked for accuracy before they are
added to the protein; if a mistake were made in adding
sugars individually to the protein, the whole protein
would have to be discarded. Because far more energy
is used in building a protein than in building a short
oligosaccharide chain, this is a much more economical
strategy. The difficulty of modifying oligosaccharides
precisely becomes apparent as the protein moves to the cell
surface: although sugar chains are continually modified by
enzymes in various compartments of the secretory pathway,
these modifications are often incomplete and result in
considerable heterogeneity of the glycoproteins that leave
the cell. This heterogeneity is largely due to the restricted
access that the enzymes have to the sugar trees attached
to the surface of proteins. The heterogeneity also explains
why glycoproteins are more difficult to study and purify than
nonglycosylated proteins.
ANSWER 15–7 Aggregates of the secretory proteins would
form in the ER, just as they do in the trans Golgi network.
As the aggregation is specific for secretory proteins, ER
proteins would be excluded from the aggregates. The
aggregates would eventually be degraded.
ANSWER 15–8 Transferrin without Fe bound does not
interact with its receptor and circulates in the bloodstream
until it catches an Fe ion. Once iron is bound, the iron–
transferrin complex can bind to the transferrin receptor on
the surface of a cell and be endocytosed. Under the acidic
conditions of the endosome, the transferrin releases its
iron, but the transferrin remains bound to the transferrin
receptor, which is recycled back to the cell surface, where
it encounters the neutral pH environment of the blood. The
neutral pH causes the receptor to release the transferrin
into the circulation, where it can pick up another Fe ion to
repeat the cycle. The iron released in the endosome, like the
LDL in Figure 15−33, moves on to lysosomes, from where it
is transported into the cytosol.
The system allows cells to take up iron efficiently even
though the concentration of iron in the blood is extremely
low. The iron bound to transferrin is concentrated at the
cell surface by binding to transferrin receptors; it becomes
further concentrated in clathrin-coated pits, which collect
the transferrin receptors. In this way, transferrin cycles
between the blood and endosomes, delivering the iron that
cells need to grow.
ANSWER 15–9
A. True.
B. False. The signal sequences that direct proteins to
the ER contain a core of eight or more hydrophobic
amino acids. The sequence shown here contains many
hydrophilic amino acid side chains, including the charged
amino acids His, Arg, Asp, and Lys, and the uncharged
hydrophilic amino acids Gln and Ser.
C. True. Otherwise they could not dock at the correct
target membrane or recruit a fusion complex to a
docking site.
D. True.
E. True. Lysosomal proteins are selected in the trans Golgi
network and packaged into transport vesicles that
deliver them to the late endosome. If not selected, they
would enter by default into transport vesicles that move
constitutively to the cell surface.
F. False. Lysosomes also digest internal organelles by
autophagy.
G. False. Mitochondria do not participate in vesicular
transport, and therefore N-linked glycoproteins,
which are exclusively assembled in the ER, cannot be
transported to mitochondria.
H. False. The outer nuclear membrane is continuous with
the ER and all proteins made by ribosomes bound there
end up in the ER lumen.
ANSWER 15–10 They must contain a nuclear localization
signal as well. Proteins with nuclear export signals shuttle
between the nucleus and the cytosol. An example is the A1
protein, which binds to mRNAs in the nucleus and guides
them through the nuclear pores. Once in the cytosol, a
nuclear localization signal ensures that the A1 protein is reimported
so that it can participate in the export of further
mRNAs.
ANSWER 15–11 Influenza virus enters cells by endocytosis
and is delivered to endosomes, where it encounters
an acidic pH that activates its fusion protein. The viral
membrane then fuses with the membrane of the endosome,
releasing the viral genome into the cytosol (Figure A15–11).
NH 3 is a small molecule that readily penetrates membranes.
Thus, it can enter all intracellular compartments, including
endosomes, by diffusion. Once in a compartment that
has an acidic pH, NH 3 binds H + to form NH + 4 , which is a
charged ion and therefore cannot cross the membrane
by diffusion. NH
+ 4 ions therefore accumulate in acidic
compartments, raising their pH. When the pH of the
endosome is raised, viruses are still endocytosed, but
because the viral fusion protein cannot be activated, the
virus cannot enter the cytosol. Remember this the next time
you have the flu and have access to a stable.
endocytosis
Figure A15–11
EXTRACELLULAR
SPACE
plasma membrane
CYTOSOL
endosomal membrane
H + H +
activation of
viral fusion
protein
fusion of viral
and endosomal
membranes
release of viral
genome into cell
ANSWER 15–12
A. The problem is that vesicles having two different kinds
of v-SNAREs in their membrane could dock on either of
two different membranes.
B. The answer to this puzzle is currently not known, but
we can predict that cells must have ways of turning
the docking ability ECB5 of SNAREs eA15.11-A15.11
on and off. This may be
achieved through other proteins that are, for example,