Protein Trafficking Flashcards
All proteins begin their synthesis on ribosomes in the Blank (the liquid component of the cytoplasm), except for those few that are synthesized
on the ribosomes of mitochondria and plastids.
cytosol
There are three ways by which proteins are transported from one location or compartment of the cell to another.
- gated transport
- transmembrane transport, and
- vesicular transport
- Proteins and RNA molecules move between the
cytosol and the nucleus through nuclear pore
complexes in the nuclear envelope.
Gated Transport
- Transmembrane protein translocators directly
transport-specific proteins across a membrane
from the cytosol into a topologically distinct
space. - The transported protein molecule usually must
unfold to snake through the translocator. - The initial transport of selected proteins from the
cytosol into the ER lumen or mitochondria, for
example, occurs in this way. - Integral membrane proteins often use the same
translocators but translocate only partially across
the membrane, so that the protein becomes
embedded in the lipid bilayer.
Transmembrane Transport
- Membrane-enclosed transport intermediates – which may be small, spherical transport vesicles or larger, irregularly shaped organelle fragments – ferry proteins from one topologically equivalent compartment to another.
- The transport vesicles and fragments become loaded with a cargo of molecules derived from the lumen of one compartment as they bud and pinch off from its membrane; they discharge their cargo into a second compartment by fusing with the membrane enclosing the compartment.
Vesicular Transport
Each mode of protein transfer is usually guided by Blank in the transported protein, which are recognized by complementary Blank.
sorting signals, sorting receptors
True or False
Signal sequences can also be external stretches of amino acids, which remain part of the protein.
○ Such signals are used in gated transport into the nucleus.
False: internal stretches
True or False
Sorting signals can also be composed of two internal amino acid sequences that form a specific three-dimensional arrangement of atoms on the protein’s surface.
False: multiple internal amino acid sequences
Such signal patches are always used for nuclear
import and in vesicular transport.
sometimes
True or False
Proteins destined for initial transfer to the ER usually have a signal sequence at their C-terminus that characteristically includes a sequence composed of about 5-10 hydrophobic amino acids.
False: N-terminus
True or False
Proteins destined for mitochondria have signal sequences of yet another type, in which positively charged amino acids alternate with hydrophilic ones
False: hydrophobic
True or False
Finally, many proteins destined for peroxisomes have a signal sequence of three characteristic amino acids at their N-terminus.
False: C terminus
True or False
The nuclear envelope consists of an inner and outer nuclear membrane that is continuous with each other and with the ER membrane, and the space between the inner and outer nuclear membrane is discontinuous with the ER lumen.
False: continuous
True or False
Not all proteins that function in the nucleus are
synthesized in the cytosol and are then imported.
False: All the proteins
Each Blank is composed of a set of proteins called
nucleoporins.
NPC
True or False
Large molecules diffuse passively through the NPCs, but small molecules have to be actively transported.
False: Small, large
Proteins containing sorting signals called nuclear
localization signals are recognized by Blank and are actively transported into the nucleus through NPCs, while proteins containing nuclear export signals are recognized by Blank and are transported out of the nucleus to the cytosol.
nuclear import receptors (sometimes called importins), nuclear export receptors or exportins
Chaperone proteins of the cytosolic Blank maintain the precursor proteins in an unfolded state, and the second
set of hsp70 proteins in the matrix space or stroma pulls
the polypeptide chain into the organelle.
hsp70 family
True or False
In chlorophyll, import from the stroma into the thylakoid
can occur by several routes, distinguished by the
chaperones and energy source used.
False: chloroplast
complex transfers proteins across the outer membrane
TOM
transfer proteins across the inner
membrane
TIM complexes (TIM23 and TIM22)
The Blank complex transports some soluble proteins into the matrix space ad
helps to insert transmembrane proteins into the inner membrane.
TIM23
The Blank complex mediates the insertion of a subclass of inner membrane
proteins, including the transporter that moves ADP, ATP, and phosphate in and
out of mitochondria.
TIM22
the Blank
complex, mediates the insertion of those inner membrane proteins synthesized
within mitochondria.
○ It also helps to insert some imported inner membrane proteins that are
initially transported into the matrix space by the other complexes.
OXA
chloroplasts have an extra membrane-enclosed
compartment called the Blank.
thylakoid
Many chloroplast proteins, including the protein subunits of the photosynthetic system of the ATP synthase, are located in the Blank.
thylakoid membrane
uses components that are homologs of Sec proteins, which mediate protein translocation across the bacterial plasma membrane.
Sec pathway
uses a chloroplast homolog signal
recognition particle (SRP).
SRP-like pathway
TAT pathway, TAT stands for
“twin arginine translocation”
The two arginines are critical in the signal sequences
that direct proteins into this pathway, which depends
on the H+ gradient across the thylakoid membrane.
TAT pathway
seems not to require any protein translocator.
Spontaneous insertion pathway
are specialized for carrying out oxidation
reactions using molecular oxygen.
Peroxisomes
True or False
Not just proteins that carry a special ER signal sequence are imported into
the ER.
False: Only proteins
The signal sequence is recognized by a Blank, which binds both the growing polypeptide chain and the ribosome and directs them to a receptor protein on the cytosolic surface of the rough
ER membrane.
signal-recognition particle (SRP)
