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Cell Biology > Cell Biology Chapter 9: The Endomembrane System and Membrane Trafficking > Flashcards

Cell Biology Chapter 9: The Endomembrane System and Membrane Trafficking Flashcards

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1
Q

The Endomembrane System Controls Molecular Transport In/Out Of Cell

A

True

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2
Q

vesicle-mediated transport

A

Existing membrane pinches off to form a vesicle

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3
Q

Vesicle-mediated Transport - 9 Steps

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

Exocytosis

A

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

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5
Q

Cargo Selection

A

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
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6
Q

Coat proteins

A

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

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7
Q

The 3 common Coat proteins are

A

Clathrin, COPI and COPII

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8
Q

Budding

A

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

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9
Q

Scission

A

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

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10
Q

Uncoating

A 
Coat proteins (like COPI and COPII) are no longer needed once the vesicle is free.
They are recycled
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11
Q

Transport

A

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

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12
Q

Tethering

A

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?

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13
Q

Docking

A

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

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14
Q

Fusion

A

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

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15
Q

Disassembly

A

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

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16
Q

Exocytosis Begins In The ER

A

True

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17
Q

Exocytosis

A

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.

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18
Q

Peptidase

A

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

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19
Q

N-oligosaccharide transferase

A

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

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20
Q

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?

A

) 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.

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21
Q

What would happen if a cell had mutated COPI proteins

A

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

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22
Q

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

A

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

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23
Q

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

A

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

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24
Q

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

A

rab

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25
Q

COPII-Coated Vesicles

A

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

26
Q

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
A

Ligand entry into the cell via a membrane pore

27
Q

Uncoating

A

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

28
Q

Transport

A

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

29
Q

Tethering

A

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

30
Q

For docking

A

The Bet1p v-SNARE

31
Q

Disassembly

A

mediated by NSF and SNAP

32
Q

Resident ER Proteins

A

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

33
Q

THE Golgi apparatus is divided into

A

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

34
Q

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
A

E) all but choice c

35
Q

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

A

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

36
Q

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
A

E) answers A, B and C

37
Q

ER retention signals are _______

A
  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)
38
Q

Resident ER Proteins in yeast

A

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

39
Q

The Golgi Apparatus Is Subdivided Into _______

A

Cis, Medial, And Trans Cisternae.

Each cisternae has a unique set of enzymes
Site for post-translational modification (PTM) like glycosylation

40
Q

What serves in Resident Golgi Proteins Serve as a Retention Signal

A

The Transmembrane Region.

41
Q

Golgi retention sequences

A

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

42
Q

Golgi retention sequences models

A

Kin recognition model

Bilayer thickness model

43
Q

Kin recognition model

A

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

44
Q

Trans Golgi Network (TGN)

A

Sorts Proteins Exiting The Golgi Apparatus

45
Q

Bilayer thickness model

A

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

46
Q

Sorting Mechanisms at the TGN

A

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

47
Q

Budding Of Vesicles At The TGN

A

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

48
Q

Scission of Vesicles Budding from the TGN

A

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
49
Q

2 Mechanisms For Controlling The Final Steps Of Exocytosis

A

Constitutive secretion
- constant

Regulated secretion

  • Controlled by signaling proteins
  • Secretory vesicles (zymogen granules)
  • Condensing vacuole
50
Q

Endocytosis Begins _______

A

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

51
Q

Clathrin

A

Stabilizes The Formation Of Endocytic Vesicles

52
Q

Endocytic sorting

A

requires multiple compartments unlike exocytosis

Separation and sorting occurs in early and late endosomes

53
Q

The Endosome Is Subdivided

A

Into Early And Late Compartments

54
Q

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

A

EEA-1,Syntaxin,Rab 5, PIP3

55
Q

pH during endocytosis

A

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

56
Q

Proton Pumps

A

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

57
Q

Endocytosis Ends At _______

A

The Lysosome

58
Q

Mamo-6-phosphate

A

lysosome retrieval sequence

59
Q

TGN

A

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)

60
Q

pH Is Used In 3 Ways To Control Endocytosis

A

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

61
Q

Digestion In The Lysosome

A

Proteases
Glycosidases
DNaseII
Phosphatases

62
Q

Ub-mediated Degradation Occurs In MVBs

A

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

Cell Biology (7 decks)

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