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The outer membrane of the nucleus is continuous with the membrane of which other organelle?
endoplasmic reticulum
Which organelle is the major site of new membrane synthesis in a cell?
endoplasmic reticulum
Which organelle receives proteins and lipids from the endoplasmic reticulum, modifies them, and then dispatches them to other destinations in the cell?
golgi apparatus
Soluble proteins and pieces of membrane enter the Golgi network via
transport vesicles derived from the ER
Proteins exit from the Golgi network in transport vesicles destined for either
the cell surface or another organelle of the endomembrane system
How do the interiors of the ER, Golgi apparatus, endoscopes, and lysosomes communicate with each other?
by small vesicles that bud off of one organelle and fuse with another
Which membrane-enclosed organelles most likely evolved in a similar manner?
the nucleus and the ER
The nuclear membranes and the membranes of the ER most likely originated by
invagination of the plasma membrane
The outer nuclear membranes is continuous with the
rough ER
In muscle cells, which organelle sequesters Ca2+ from the cytosol?
smooth ER
In muscle cells, the specialized smooth ER is called the
sarcoplasmic reticulum
In a typical human secretory cell, what membrane has the largest surface area?
rough ER
The rough ER is folded up to form
an extensive maze of interconnected spaces
mostly composes about ½ of the total membrane present in the cell
IkB blocks the nuclear localization signal of
NFkB until it is ubiquitinated and degraded after stimulation
Once IkB is degraded, the nuclear localization signal on NFkB can be
recognized by the nuclear import receptor
Blocking IkB degradation would inhibit
NFkB transport
Adding excess nuclear location signal peptide would also block import of NFkB because
the set amount of nuclear import receptor would be binding the large excess of peptides instead of the actual proteins to be imported
Once the nuclear import receptor transports charge into the nucleus,
ran-GTP helps export the nuclear receptor out of the cytosol
Ran is converted to Ran-GDP by the
Ran-GAP protein in the cytosol and then moves back to the nucleus
Proteins that lack a sorting signal remain as permanent residents of which part of a eukaryotic cell?
cytosol
What would happen to a protein that is engineered to contain both a nuclear localization signal and a nuclear export signal?
it would shuttle in and out of the nucleus
Proteins that escort mature mRNAs from the nucleus to the cytosol need
an export signal to leave the nucleus and a localization signal to re-enter
Which proteins bind to nuclear localization signals on newly synthesized proteins?
nuclear import receptors
The nuclear localization signal on proteins destined for the nucleus is recognized by cytosol in proteins called
nuclear import receptors
Nuclear import receptors help direct newly synthesized proteins to a nuclear pore by
interacting with the tentacle-like fibrils that extend from the rim of the pore into the cytosol
What actions occur commonly at a nuclear pore complex?
nuclear import receptors are exported to the cytosol
mRNA molecules are exported to the cytosol
nuclear import receptors enter from the cytosol
proteins with a nuclear localization signal enter at the same time that proteins with a nuclear export signal exit
Proteins destined for the Golgi apparatus, endosomes, lysosomes, and even the cell surface must pass through which organelle?
endoplasmic reticulum
Which is true of ribosomes?
a common pool of ribosomes is used to synthesize both cytosol if proteins and proteins destined for the ER
When a ribosome is making a protein that has an ER signal sequence,
that signal sequence directs the ribosome to the ER membrane
As a polypeptide is being translocated across the membrane of the ER, a stop-transfer sequence can halt the process. What eventually becomes of this stop-transfer sequence?
it forms an alpha-helical membrane spanning segment of the protein
The ER signal sequence on a growing polypeptide chain is recognized by a signal-recognition particle (SRP) in the cytosol. What does this interaction accomplish?
it guided the ribosome and its polypeptide to the ER
The SRP binds to both the
exposed ER signal sequence and the ribosome
the SRP-ribosome complex then binds to an SRP receptor in the ER membrane
Free ribosomes are structurally identical to those that
are bound to the ER
When a ribosome happens to be making a protein with an ER signal sequence,
the signal sequence directs the ribosome to the ER membrane
Membrane-bound ribosomes and free ribosomes are structurally and functionally
identical; they differ only in the proteins they are making at a particular time
Which organelle cannot receive proteins directly from the cytosol?
Golgi apparatus
Proteins are delivered to the Golgi apparatus from the
ER or from other components of the endomembrane system
What is the source of energy for protein translocation into the ER?
synthesis of the protein
Proteins with an ER signal sequence are translocated as they are being made;
the elongation of the polypeptide provides the thrust needed to push the growing chain through the ER membrane
Nuclear import is driven by the hydrolysis of GTP, which is triggered by an accessory protein called Ran-GAP (GTPase-activating protein). What is true of this process?
nuclear receptors carry Ran-GTP from the nucleus to the cytosol
What enables proteins destined for nuclear import to pass through the nuclear pore?
they are recognized by receptors that interact with repeated amino acid sequences in proteins lining the nuclear pore
Since glycosylation is on the extracellular side of the plasma membrane, the C term is
in the extracellular space and the N terminus is in the cytosol
Glycosylation occurs inside the
lumen of the ER and Golgi apparatus
Where in the cell are some proteins initially decorated with an oligosaccharide tree on asparagine residues?
ER
Glycosylation begins with the addition of a
preformed oligosaccharide in the ER
If proteins cannot leave the ER, they will never make it to
the Golgi apparatus
What best describes a pathway that a protein might follow from synthesis to secretion?
Cytosol → ER → transport vesicle → Golgi apparatus → transport vesicle → plasma membrane
How are newly made lipids supplied to the plasma membrane?
via the constitutive pathway of exocytosis
How are proteins destined to function in the ER retained there?
they contain a C-terminal ER retention signal
Increasing Ca2+ concentration directly or through opening pores will trigger
vesicle fusion with the plasma membrane increasing muscle contraction
Cleaving SNAREs and blocking GTP binding inactivate
SNARE and Rab respectively, leading to decreased muscle contraction
What accurately describes a step in GTP-driven nuclear transport?
binding of Ran-GTP to the receptor releases the cargo protein