Cell Biology Chapter 9: The Endomembrane System and Membrane Trafficking

Question 1

The Endomembrane System Controls Molecular Transport In/Out Of Cell

Answer 1

True

Question 2

vesicle-mediated transport

Answer 2

Existing membrane pinches off to form a vesicle

Question 3

Vesicle-mediated Transport - 9 Steps

Answer 3

1. Cargo selection
2. Budding
3. Scission
4. Uncoating
5. Transport
6. Tethering
7. Docking
8. Fusion
9. Disassembly

Question 4

Exocytosis

Answer 4

Is a one way journey from ER —> Golgi —> to the plasma membrane

Question 5

Cargo Selection

Answer 5

Sorting mechanisms are used to ensure only the right molecules are transported

Cytosolic tails of membrane proteins
Anterograde signal can consist of two leucines (-LL-)

Coat Proteins (Clathrin, COPI and COPII are examples)
Cause receptors and other proteins with signal sequences to cluster into a patch on the surface of the donor compartment

Question 6

Coat proteins

Answer 6

cause the receptors to cluster in a patch the surface of the donor compartment.

Question 7

The 3 common Coat proteins are

Answer 7

Clathrin, COPI and COPII

Question 8

Budding

Answer 8

Clustered coat proteins interact with cytosolic adaptor proteins

Together the coat and the adaptor proteins form a mesh-like lining. Makes the formation of vesicles energetically favorable

*Membranes would never pinch off vesicles by themselves

Question 9

Scission

Answer 9

The budded vesicle must be cut from the membrane it originated from
Scission proteins surround the “neck” and tighten it until the membrane breaks

Question 10

Uncoating

Answer 10

Coat proteins (like COPI and COPII) are no longer needed once the vesicle is free.
They are recycled

Question 11

Transport

Answer 11

Motor proteins that attach to microtubules or actin filaments (see Chapter 5) transport vesicles with cargo
Not absolutely required, but increases the efficiency of delivery

Question 12

Tethering

Answer 12

Vesicles have small GTP-binding proteins called Rab proteins on them

Rabs bind tether proteins and this forms a temporary complex on the acceptor membrane bringing the vesicle closer to it
Like a search and rescue mission….Is it our vesicle?

Question 13

Docking

Answer 13

After the “Rab” protein helps the vesicle find their targets, they have to dock.

t-SNARES (on target membrane)
v-SNARES (on vesicle membrane)

Are complementary and bind one another, stabilizing the temporary connection made between Rabs and tether proteins

Question 14

Fusion

Answer 14

Several v-SNARE and t-SNARE interactions will lead to the fusion of the vesicle membrane with the acceptor compartment

Question 15

Disassembly

Answer 15

This triggers uncoupling of the SNARE pairs by NSF and SNAP proteins
(they can be used again)

V-SNARES will be recycled back to the donor compartment in vesicles moving in the opposite direction

Question 16

Exocytosis Begins In The ER

Answer 16

True

Question 17

Exocytosis

Answer 17

Newly synthesized endomembrane proteins are modified in the ER.

Signaling sequences in the newly-made proteins signal their secondary destinations. Some have retention signals that keep them in the ER.

Proteins that leave the ER enter the cis-Golgi network via COPII-coated vesicles.

Retrograde vesicle transport returns ER-resident proteins from the Golgi apparatus to the ER.

Question 18

Peptidase

Answer 18

Signal Peptidase cleaves the amino-terminal signal sequence from most proteins that have one

Question 19

N-oligosaccharide transferase

Answer 19

adds a core oligosaccharide to the side chain of Asn residues in an N-linked glycosylation signal

Question 20

Patients with I-cell disease fail to form proper lysosomes, and thus accumulate inclusion bodies in their cells. In vitro (cell culture), cells from some I-cell patients can be “rescued” (restored to normal phenotype) by adding normal lysosomal enzymes to the culture medium. However, this treatment does not eliminate inclusions in cells from other I-cell patients. Which statement best explains this discrepancy?

Answer 20

) Cells that respond to the added enzymes have functional endosome proton pumps but faulty lysosomal enzymes; cells that do not respond have functional endosome proton pumps but faulty mannose-6-phosphate receptors.

Question 21

What would happen if a cell had mutated COPI proteins

Answer 21

ER proteins that were accidentally carried to the Golgi complex would not return to the ER.

Question 22

Choose the proper temporal sequence of events taking place for formation of a lysosome (first–> last).

Answer 22

Budding of vesicles from the trans Golgi network, dissociation of mannose-6-phosphate receptors from their ligands, activation of acidic hydrolases.

Question 23

Recently, a t-SNARE was found in the outer membrane of mitochondria of a cell. What are the implications of this finding?

Answer 23

This suggests that vesicles budding from the Golgi fuse with mitochondria.

Question 24

Which of the following is NOT a type of vesicle coat protein? rab, clathrin, COPI, COPII

Answer 24

rab

Question 25

COPII-Coated Vesicles

Answer 25

Cells harbor either ER export sites or transitional ER ER export signals are recognized by coat proteins called COPII COPII proteins interact with adaptor proteins = budding Scission mechanism is not well understood

Question 26

Which is NOT a characteristic of receptor-mediated endocytosis? ``` A) Sorting B) Selectivity C) Ability to concentrate ligand D) Dynamin E) Ligand entry into the cell via a membrane pore ```

Answer 26

Ligand entry into the cell via a membrane pore

Question 27

Uncoating

Answer 27

Sec23p in the COPII complex hydrolyzes GTP causing disassembly of the COPII coat

Question 28

Transport

Answer 28

Sec23p binds dynactin, which can couple vesicles to dynein or kinesin

Question 29

Tethering

Answer 29

Occurs when Sec23p interacts with at least two tethering proteins on cis-Golgi causing them to bind a Rab

Question 30

For docking

Answer 30

The Bet1p v-SNARE

Question 31

Disassembly

Answer 31

mediated by NSF and SNAP

Question 32

Resident ER Proteins

Answer 32

Not all proteins that arrive at the CGN (cis Golgi network) are supposed to be there Golgi-to-ER retrograde transport retrieves them KDEL ER-retention signal K-K-X-X or K-X-K dilysine ER retention signaL

Question 33

THE Golgi apparatus is divided into

Answer 33

The Cis, Medial and Trans Cisternae Vesicle trafficking within the Golgi occurs in both antergrade and retrograde directions Each cisternae has a unique set of enzymes Site for post-translational modification (PTM) like glycosylation

Question 34

Which of the following are examples of posttranslational processing? ``` A) glycosylation B) protein folding C) 5' CAP D) signal sequence removed E) all but choice c ```

Answer 34

E) all but choice c

Question 35

The Transmembrane Region of Resident Golgi Proteins Serve as a Retention Signal

Answer 35

Golgi retention sequences For Transmembrane proteins this includes the signal anchor sequence that initially targets it to the ER Two models proposed to explain how these sequences work and neither require a receptor! Kin recognition model Bilayer thickness model

Question 36

Which of the following could be an ER retention signal? ``` A) arg-Arg (R-R) B) lys-asp-glu-leu (KDEL) C) lys-x-lys (KXK) D) answers A and B E) answers A, B and C ```

Answer 36

E) answers A, B and C

Question 37

ER retention signals are _______

Answer 37

1. Lys-Asp-Glu-Leu (KDEL) 2. Lys-Lys-X-X or Lys-X-Lys (KKXX/KXK) 3. Arg-Arg or Arg-X-Arg (RR/RXR)

Question 38

Resident ER Proteins in yeast

Answer 38

In Yeast, a protein that functions as a t-SNARE during ER-Golgi transport also functions as a v-SNARE for retrograde transport These receptors interact with COPI (not COPII) coat complex Vesicles are transported back by kinesin on MTs

Question 39

The Golgi Apparatus Is Subdivided Into _______

Answer 39

Cis, Medial, And Trans Cisternae. Each cisternae has a unique set of enzymes Site for post-translational modification (PTM) like glycosylation

Question 40

What serves in Resident Golgi Proteins Serve as a Retention Signal

Answer 40

The Transmembrane Region.

Question 41

Golgi retention sequences

Answer 41

For Transmembrane proteins this includes the signal anchor sequence that initially targets it to the ER

Question 42

Golgi retention sequences models

Answer 42

Kin recognition model Bilayer thickness model

Question 43

Kin recognition model

Answer 43

Protein recognize each other via their membrane spanning domains, cluster to prevent forward movement.

Question 44

Trans Golgi Network (TGN)

Answer 44

Sorts Proteins Exiting The Golgi Apparatus

Question 45

Bilayer thickness model

Answer 45

Proteins move along the pathway (cis---> medial----> trans) until membrane spanning domain matches the thickness of the bilayer.

Question 46

Sorting Mechanisms at the TGN

Answer 46

Post-translational modifications can act as sorting signals N- and O-linked glycosylation, phosphorylation and ubiquitination Protein aggregation Traditional signal-receptor method Lipid raft hypothesis TGN may assemble patches of cholesterol and lipid that some proteins have a very high affinity for

Question 47

Budding Of Vesicles At The TGN

Answer 47

3 different mechanisms have been proposed Curvature-inducing proteins Modification of membrane phospholipids by Phospholipase A2 Phospholipid asymmetry that could arise from flippases or lipid transfer proteins

Question 48

Scission of Vesicles Budding from the TGN

Answer 48

Often Requires Microtubule Motor-Based Forces that Provide Traction for Curvature * Budding is an energy dependent process. Dynamin uses GTP to regulate scission of a vesicle from a donor compartment

Question 49

2 Mechanisms For Controlling The Final Steps Of Exocytosis

Answer 49

Constitutive secretion - constant Regulated secretion - Controlled by signaling proteins - Secretory vesicles (zymogen granules) - Condensing vacuole

Question 50

Endocytosis Begins _______

Answer 50

at the Plasma Membrane. Endocytosis removes material from the extracellular space Can destroy dangerous objects (like viruses, foreign cells) A method to remodel the cell surface

Question 51

Clathrin

Answer 51

Stabilizes The Formation Of Endocytic Vesicles

Question 52

Endocytic sorting

Answer 52

requires multiple compartments unlike exocytosis Separation and sorting occurs in early and late endosomes

Question 53

The Endosome Is Subdivided

Answer 53

Into Early And Late Compartments

Question 54

during endocytosis Specific interacting molecules ensure correct vesicles fuse with vesicles from TGN or early endosomes

Answer 54

EEA-1,Syntaxin,Rab 5, PIP3

Question 55

pH during endocytosis

Answer 55

pH of 6.4-6.8 allows cargo to dissociate from receptors (maintained by proton pumps)

Question 56

Proton Pumps

Answer 56

Transmembrane proton pumps move protons to the interior of the vesicle interior Pumps accumulate in the membrane over time as vesicles continue to fuse and the vesicle becomes increasingly more acidic When pH drops below 6.0 the early endosome becomes a late endosome Fig.9.14

Question 57

Endocytosis Ends At _______

Answer 57

The Lysosome

Question 58

Mamo-6-phosphate

Answer 58

lysosome retrieval sequence

Question 59

TGN

Answer 59

At the TGN a mannose-6-phosphate “tag” is recognized as a lysosomal sorting signal At the lysosome, the mannose 6-phosphate receptors are then sorted and returned to the TGN, while the hydrolases and pumps remain there The pumps are responsible for triggering the drop in pH in the endosomes. Hydrolases are only active at these low pHs (protective)

Question 60

pH Is Used In 3 Ways To Control Endocytosis

Answer 60

1) the acidic environment in endosomes helps sort cargo from receptors 2) the relatively neutral pH of the ER and Golgi apparatus keeps the hydrolytic enzymes from digesting these organelles 3) the enzymes’ requirement for a strong acid environment protects the endomembrane system from digesting itself

Question 61

Digestion In The Lysosome

Answer 61

Proteases Glycosidases DNaseII Phosphatases

Question 62

Ub-mediated Degradation Occurs In MVBs

Answer 62

Ubiquitin tags proteins for degradation by cytosolic proteosomes Also: Ub-tags on the cytosolic portion of membrane proteins in the early endosome helps trigger formation of a multivesicular body (MVB) MVBs fuse with lysosomes for digestion