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Cell and Developmental Biology > Membrane Trafficking > Flashcards

Membrane Trafficking Flashcards

1
Q

How are proteins trafficked from a donor compartment to a target compartment?

A
  1. Vesicle will form on the membrane
  2. Will detach - budding off from the donor organelle
  3. Transported through the cytoplasm
  4. Reaches target compartment
  5. Vesicle then fuses with target compartment and is incorporated
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2
Q

What is the difference between constitutive and regulated exocytosis?

A

Constitutive exocytosis: Occurring all the time

Regulated exocytosis: Only happens when there is an external signal

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3
Q

What are protein coats needed for?

A
  • essential for formation of vesicles

- subtle localisation where the coats operate

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4
Q

What are the 3 main types of protein coats?

A

COPI, COPII and Clathrin

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5
Q

What is COPI protein coat used for?

A

COPI coated vesicles are found in the retrograde pathway

- Going from the Golgi back to the ER

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6
Q

What is COPII protein coat used for?

A

COPII coated vesicles are the forward pathway (anterior grade trafficking)
- From ER to Golgi

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7
Q

What is Clathrin protein coat used for?

A

Clathrin - originate in the Golgi but not trafficked to the ER but elsewhere
Found in the trans golgi not the cis golgi

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8
Q

How are vesicles formed?

A
  1. Specific recognition event where soluble proteins will bind to membrane proteins
  2. Triggers recruitment of adaptor proteins
    - AP1 (More than one adaptor protein involved)
  3. Membrane curvature occurs
  4. Recruitment of protein coats
    - Formation of the vesicle
  5. Vesicle detaches from the donor membrane
  6. Protein coat detaches
  7. Left with a naked transport vehicle
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9
Q

What does AP1 do?

A
  • an adaptor protein

- AP1 is responsible for the transport of lysosomal hydrolases between the TGN and endosomes.

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10
Q

What do adaptor proteins do?

A

AP complexes connect cargo proteins and lipids to clathrin at vesicle budding sites, as well as binding accessory proteins that regulate coat assembly and disassembly

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11
Q

How are vesicles shaped in the way they are?

A

Membrane bending proteins form BAR domain dimers

  • crescent shaped
  • They interact with the head groups of the phospholipids that are in the bilayer
  • Interactions between them force the membrane into a curved shape
  • Once curvature has happened, can recruit Clathrin
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12
Q

What is the shibire temperature sensitive mutation?

A

“the sensation when a bit of your arm goes numb when you sleep on it”

  • shibire is a mutation that enables this sensation in flies
  • mutation becomes apparent when the temp is shifted
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13
Q

What is the role of dynamin in the shibire temperature sensitive mutation?

A

The mutation affects membrane trafficking

- the vesicle never fully detaches and therefore is unable to release the neurotransmitters

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14
Q

What does Dynamin do?

A

pinches off vesicles from the membrane
- membranes are pushed together so much that they fuse
(other proteins involved: Synaptobrevin, Synaptotaxin and SNAP-25)

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15
Q

What happens in the WT of the drosophila that have the shibire mutation?

A

the synapse releases the neurotransmitter

  • vesicles are trafficked down the neurone
  • reaches surface of the cell by regulated endocytosis
  • fuse with synaptic surface and release the contents
  • recycling of the endosome
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16
Q

How does Dynamin work?

A
  • part of the GTPase family
  • it is circling around the membrane where fusing wants to happen
  • GTP/GDP cycle is present
  • once it is switched OFF, the shape of the vesicle changes, allowing it to detach
17
Q

How does vesicle trafficking occur?

A
  1. Cargo vesicle has no protein coat:
    - Phospholipid bilayer is exposed
    - Rabs is another form of GTPase which is anchoring onto the membrane of the vesicle
    - V-SNARE also present
  2. Vesicle has to associated and be recognised by the target organelle
    - Rab-GTP recognises and binds to a tethering protein in the target membrane (Rab effector)
    ○ Rab will only associate in the GTP phase
    ○ Individual Rab will recognise a particular Rab effector (specificity)
  3. Brought into closer contact to the target membrane via t-SNARE’s in the target membrane
    - Once v-SNARE’s & t-SNARE’s are in close contact to each other, they bring the vesicle down (SNARE complex)
  4. Fusion occurs
    - Rab is no longer needed and so gets hydrolysed to the Rab-GDP state
    - Associates with GDI (accessory proteins) - hold onto them in that inactive state
    - Rab’s dissociated from the membrane of the Cargos phospholipid bilayer
  5. Cargo receptors become part of the target plasma membrane and the cargo goes wherever it is meant to
    - Recycling of the components
18
Q

What are the localisations of Rabs (1-5)?

A 
Rab 1-
effectors: p115, GM130, and as now shown
organelle: Er and early golgi complex
Rab 5-
effector: LET-413
organelle: endosome
19
Q

What is Griscelli Syndrome Type 2?

A

patients have partial albinism and immunodeficiency caused by Rab27A mutation

20
Q

What happens during Griscelli syndrome 2?

A

melanosomes (contain pigment melanin) cannot get transferred from the microtubules to the actin filament as Rab27 is needed and unavailable during Griscelli syndrome 2
- therefore cannot be trafficked into cells and provide protection

21
Q

How do you unravel SNARE’s?

A

Trans-SNARE complex has to be unravelled through recycling events
- requires energy from ATP hydrolysis

22
Q

What carries out the transport between the ER and Golgi?

A

vesicular tubular clusters (not spherical)

23
Q

How do you form vesicular tubular clusters?

A
  • homotypic membrane fusion
  • bud off from the ER as spherical vesicles with COPII coat
  • fusion occurs between vesicles
  • v-SNARES and t-SNARES of each vesicle fuse with each other from the other vesicle
  • a tubular structure is formed
24
Q

What are vesicular tubular clusters?

A

transport vesicles that constantly go from the ER to the cis-Golgi

  • move along microtubules
  • as soon as they form they begin to bud off transport vesicles of their own
  • vesicles are COPI coated rather than COPII coated
25
Q

What are the 2 models of how Golgi is organised and how proteins move through it?

A
  • cisternal maturation model

- vesicle transport model

26
Q

What is the cisternal maturation model of how the Golgi works?

A
  • each Golgi cisternae matures as it migrates outwards (away from the ER)
  • detach from ER -> form vesicular tubular clusters -> moves into the Golgi (no fusion)
  • the cluster becomes a part of the cis Golgi (like moving stacks of Golgi along)
  • as each stack moves forward, the Golgi resident proteins are moved back through the retrieval pathway in COPI coated vesicles
27
Q

What is the vesicle transport model of how the Golgi works?

A
  • the Golgi cisternae are static compartments and forward transport of molecules is achieved through budding and fusion of transport vesicles
  • tubular structures will fuse with Golgi
  • trafficking is via vesicles

Cell and Developmental Biology (16 decks)

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