Essential Cell Biology 5th edition

A:40 Answers
co-packaged in the ER with SNAREs into transport
vesicles and facilitate the interactions of the correct
v-SNARE with the t-SNARE in the cis Golgi network.
ANSWER 15–13 Synaptic transmission involves the release
of neurotransmitters by exocytosis. During this event, the
membrane of the synaptic vesicle fuses with the plasma
membrane of the nerve terminals. To make new synaptic
vesicles, membrane must be retrieved from the plasma
membrane by endocytosis. This endocytosis step is blocked
if dynamin is defective, as the protein is required to pinch
off the clathrin-coated endocytic vesicles.
ANSWER 15–14 The first two sentences are correct. The
third is not. It should read: “Because the contents of the
lumen of the ER, or any other compartment in the secretory
or endocytic pathways, never mix with the cytosol, proteins
that enter these pathways will never need to be imported
again.”
ANSWER 15–15 The protein is translocated into the ER.
Its ER signal sequence is recognized as soon as it emerges
from the ribosome. The ribosome then becomes bound
to the ER membrane, and the growing polypeptide chain
is transferred through the ER translocation channel. The
nuclear localization sequence is therefore never exposed to
the cytosol. It will never encounter nuclear import receptors,
and the protein will not enter the nucleus.
ANSWER 15–16 (1) Proteins are imported into the
nucleus after they have been synthesized, folded, and,
if appropriate, assembled into complexes. In contrast,
unfolded polypeptide chains are translocated into the ER
as they are being made by the ribosomes. Ribosomes are
assembled in the nucleus yet function in the cytosol, and
the enzyme complexes that catalyze RNA transcription
and splicing are assembled in the cytosol yet function
in the nucleus. Thus, both ribosomes and these enzyme
complexes need to be transported through the nuclear
pores intact. (2) Nuclear pores are gates, which are always
open to small molecules; in contrast, translocation channels
in the ER membrane are normally closed, and open only
after the ribosome has attached to the membrane and the
translocating polypeptide chain has sealed the channel from
the cytosol. It is important that the ER membrane remain
impermeable to small molecules during the translocation
process, as the ER is a major store for Ca 2+ in the cell, and
Ca 2+ release into the cytosol must be tightly controlled
(discussed in Chapter 16). (3) Nuclear localization signals
are not cleaved off after protein import into the nucleus; in
contrast, ER signal peptides are usually cleaved off. Nuclear
localization signals are needed to repeatedly re-import
nuclear proteins after they have been released into the
cytosol during mitosis, when the nuclear envelope breaks
down.
ANSWER 15–17 The transient intermixing of nuclear
and cytosolic contents during mitosis supports the idea
that the nuclear interior and the cytosol are indeed
evolutionarily related. In fact, one can consider the nucleus
as a subcompartment of the cytosol that has become
surrounded by the nuclear envelope, with access only
through the nuclear pores.
ANSWER 15–18 The actual explanation is that the single
amino acid change causes the protein to misfold slightly
so that, although it is still active as a protease inhibitor, it
is prevented by chaperone proteins in the ER from exiting
this organelle. It therefore accumulates in the ER lumen
and is eventually degraded. Alternative interpretations
might have been that (1) the mutation affects the stability
of the protein in the bloodstream so that it is degraded
much faster in the blood than the normal protein, or (2) the
mutation inactivates the ER signal sequence and prevents
the protein from entering the ER. (3) Another explanation
could have been that the mutation altered the sequence to
create an ER retention signal, which would have retained the
mutant protein in the ER. One could distinguish between
these possibilities by using fluorescently tagged antibodies
against the protein or by expressing the protein as a fusion
with GFP to follow its transport in the cells (see How We
Know, pp. 520–521).
ANSWER 15–19 Critique: “Dr. Outonalimb proposes to
study the biosynthesis of forgettin, a protein of significant
interest. The main hypothesis on which this proposal is
based, however, requires further support. In particular, it is
questionable whether forgettin is indeed a secreted protein,
as proposed. ER signal sequences are normally found at
the N-terminus. C-terminal hydrophobic sequences will be
exposed outside the ribosome only after protein synthesis
has already terminated and can therefore not be recognized
by an SRP during translation. It is therefore unlikely that
forgettin will be translocated by an SRP-dependent
mechanism; it is more likely that it will remain in the cytosol.
Dr. Outonalimb should take these considerations into
account when submitting a revised application.”
ANSWER 15–20 The Golgi apparatus may have evolved
from specialized patches of ER membrane. These regions of
the ER might have pinched off, forming a new compartment
(Figure A15–20), which still communicates with the
ER by vesicular transport. For the newly evolved Golgi
compartment to be useful, transport vesicles would also
have to have evolved.
Figure A15–20
ANSWER 15–21 This is a chicken-and-egg question. In fact,
the situation never arises in present-day cells, although
it must have posed a considerable problem for the first
ECB5 eA15.20-A15.20