Protein Trafficking

Question 1

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.

Answer 1

cytosol

Question 2

There are three ways by which proteins are transported from one location or compartment of the cell to another.

Answer 2

• gated transport
• transmembrane transport, and
• vesicular transport

Question 3

• Proteins and RNA molecules move between the
  cytosol and the nucleus through nuclear pore
  complexes in the nuclear envelope.

Answer 3

Gated Transport

Question 4

• 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.

Answer 4

Transmembrane Transport

Question 5

• 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.

Answer 5

Vesicular Transport

Question 6

Each mode of protein transfer is usually guided by Blank in the transported protein, which are recognized by complementary Blank.

Answer 6

sorting signals, sorting receptors

Question 7

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.

Answer 7

False: internal stretches

Question 8

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.

Answer 8

False: multiple internal amino acid sequences

Question 9

Such signal patches are always used for nuclear
import and in vesicular transport.

Answer 9

sometimes

Question 10

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.

Answer 10

False: N-terminus

Question 11

True or False
Proteins destined for mitochondria have signal sequences of yet another type, in which positively charged amino acids alternate with hydrophilic ones

Answer 11

False: hydrophobic

Question 12

True or False
Finally, many proteins destined for peroxisomes have a signal sequence of three characteristic amino acids at their N-terminus.

Answer 12

False: C terminus

Question 13

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.

Answer 13

False: continuous

Question 14

True or False
Not all proteins that function in the nucleus are
synthesized in the cytosol and are then imported.

Answer 14

False: All the proteins

Question 15

Each Blank is composed of a set of proteins called
nucleoporins.

Answer 15

NPC

Question 16

True or False
Large molecules diffuse passively through the NPCs, but small molecules have to be actively transported.

Answer 16

False: Small, large

Question 17

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.

Answer 17

nuclear import receptors (sometimes called importins), nuclear export receptors or exportins

Question 18

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.

Answer 18

hsp70 family

Question 19

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.

Answer 19

False: chloroplast

Question 20

complex transfers proteins across the outer membrane

Answer 20

TOM

Question 21

transfer proteins across the inner
membrane

Answer 21

TIM complexes (TIM23 and TIM22)

Question 22

The Blank complex transports some soluble proteins into the matrix space ad
helps to insert transmembrane proteins into the inner membrane.

Answer 22

TIM23

Question 23

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.

Answer 23

TIM22

Question 24

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.

Answer 24

OXA

Question 25

chloroplasts have an extra membrane-enclosed
compartment called the Blank.

Answer 25

thylakoid

Question 26

Many chloroplast proteins, including the protein subunits of the photosynthetic system of the ATP synthase, are located in the Blank.

Answer 26

thylakoid membrane

Question 27

uses components that are homologs of Sec proteins, which mediate protein translocation across the bacterial plasma membrane.

Answer 27

Sec pathway

Question 28

uses a chloroplast homolog signal
recognition particle (SRP).

Answer 28

SRP-like pathway

Question 29

TAT pathway, TAT stands for

Answer 29

“twin arginine translocation”

Question 30

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.

Answer 30

TAT pathway

Question 31

seems not to require any protein translocator.

Answer 31

Spontaneous insertion pathway

Question 32

are specialized for carrying out oxidation
reactions using molecular oxygen.

Answer 32

Peroxisomes

Question 33

True or False
Not just proteins that carry a special ER signal sequence are imported into
the ER.

Answer 33

False: Only proteins

Question 33

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.

Answer 33

signal-recognition particle (SRP)