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Cell Biology > Section 7B: The clathrin and COP coats > Flashcards

Section 7B: The clathrin and COP coats Flashcards

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1
Q

What is the function of a coat?

A

bind to a donor membrane compartment causing a change in shape of the membrane, so that you form a bud, then causing a scission or release of that vesicle from the target protein

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2
Q

______ scaffolds the shape of the budding vesicle

A

Clathrin

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3
Q

Clathrin forms a structure called…

A

Triskelions

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4
Q

Hexagons and pentagons convex to form a basket called…

A

Clathrin coated pit

  • different sizes represent different stages of bud formation
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5
Q

What is the problem with Clathrin?

A

it does not have the ability to bind membranes on its own

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6
Q

Adaptor protein (AP)

A
  • a peripheral membrane protein
  • can associate directly with the lipid bilayer
  • binds clathrin
  • binds cargo: picks what will make it into the vesicle
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7
Q

What does the adaptor protein (AP) make the clathrin do?

A

makes sure that the scaffold or assembly only happens on the surface of the right donor membrane compartment

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8
Q

What does the adaptor protein (AP) have to be able to tell?

A

it has to be able to tell what the membrane is binding to; tell the difference between the plasma membrane and the ER because it binds to the plasma membrane not the ER

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9
Q

Adaptor protein complexes (AP) are intermediates between cargo molecules and clathrin

A
  • during clathrin-mediated endocytosis, cargo is sorted and enriched in budding vesicles
  • cargo may be membrane proteins or soluble cargo
  • if soluble cargo, this is first bound to transmembrane proteins called receptors
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10
Q

Is clathrin located on the inside or outside of the cell?

A

inside; the budding that clathrin does is inward from the plasma membrane

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11
Q

The stages of clathrin coat assembly

A
  1. adaptor protein lands
  2. recruits clathrin: this will begin process of bending the membrane inwards
  3. vesicle formation: eventually, clathrin assembles spontaneously into a sphere
  4. concentration of protein in the middle of the sphere
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12
Q

Membrane scission of clathrin-mediated vesiculation

A
  • dynamin spiral squeezes neck of the vesicle from the donor membrane
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13
Q

What is Dynamin?

A
  • a protein that assembles in a spiral
  • a special GTPase that does not require an external GAP
  • undergoes a conformational change with the hydrolysis of GTP that cuts the neck of the vesicle from the donor membrane
  • the GTPase activity of dynamin is assembly-regulated
  • the hydrolysis of GTP to GDP by dynamin causes scission
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14
Q

Some of the vesicles that dynamin works to make are the same vesicles and neurons that hold neurotransmitters, so if you can’t make the vesicles that hold the neurotransmitters…

A

then they can’t transmit neurotransmitters to the muscle

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15
Q

What controls where and when coat proteins assemble to form a vesicle?

A
  1. Phosphoinositides:
    - controls clathrin coat assembly
  2. Specific GTPases:
    - controls COP coat assembly
    - Arf: controls COPI
    - Sar: controls COPII
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16
Q

What is the major difference of the phospholipid Phosphatidylinositol (PIP2)?

A

The sugar Inositol headgroup has 6 hydroxyl groups (6 carbon sugar)
- rest are the same: glycerol backbone, 2 fatty acid tails, a phosphate

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17
Q

Why is the 6 hydroxyl headgroup important in Phosphatidylinositol (PIP2) important?

A

The lipid headgroup can become phosphorylated or modified with phosphate groups on specific positions (4 and 5)

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18
Q

Where does phosphorylation occur on Phosphatidylinositol (PIP2)?

A

Occurs on the part of the lipid that is already polar

19
Q

Phosphatidylinositol (PIP2)

A
  • phospholipid on cytosolic leaflet of bilayer
  • inositol headgroup has 6 hydroxyl groups
  • several of these hydroxyl groups can be phosphorylated and dephosphorylated
  • the phosphorylation of the inositol headgroup is carried out by specific lipid kinases, and dephosphorylation occurs due to specific lipid phosphatases
20
Q

What are Phosphoinositides?

A

They are Phosphatidylinositol (PIP2) phosphorylated at any position(s)

21
Q

Phosphatidylinositol-4,5-biphosphate

A

Phosphatidylinositol (PIP2) phosphorylated at 4 and 5 positions of inositol headgroup; controls clathrin coat assembly

22
Q

What does Phosphatidylinositol-4,5-biphosphate regulate?

A
  • they are the signal to start assembling clathrin coated pits
  • highly enriched at the plasma membrane
  • allows AP2 (adaptor protein 2), dynamin and other proteins to bind membrane
23
Q

What membrane is PIP2 found on?

A
  • inner leaflet of plasma membrane
24
Q

Why do clathrin coats form selectively on the plasma membrane?

A

because the plasma membrane is the only one that has PIP2

25
Q

PIP2 recruit proteins and the headgroup sticks out on which part of the membrane?

A

the cytosolic face

26
Q

How does AP or AP2 bind to membranes?

A

It has a unique groove on its surface to bind to the 2 lipid

27
Q

What is the Adaptor Protein (AP) controlled by?

A
  • It is controlled by PIP2
  • It’s the ability of the very first AP to bind to the membrane and bind to the cargo at the same time
28
Q

What allows the progression to a couple AP molecules for assembly of the first part of the clathrin coat?

A

PIP2

29
Q

What causes clathrin coat disassembly?

A

PIP2 is removed or degraded in the vesicle once it is released from the donor compartment which causes disassembly

30
Q

How does PIP2 control the “opening” of AP2

A
  • AP2 starts off in a locked position, unable to do anything: good thing because we dont want AP binding to Clathrin creating coats everywhere in the cell
  • As soon as AP interacts with the lipid PIP2, it activates it and opens it
  • Once open it can do two things: bind to cargo, and clathrin
31
Q

What is different about the COPII coat?

A
  • doesn’t use clathrin
  • doesn’t use AP
  • doesn’t use dynamin
  • is not regulated by PIP2
32
Q

What is the similar end product of COP and COPII?

A

the ability to package proteins into a vesicle; in this case from the ER

33
Q

Mechanism of COPII coat formation

A
  1. GTPases
  2. Sar1 (GTPase) and Sar1-GEF
  3. Sec23/24 and Sec 13/31
    (4). Membrane is deformed and buds off from ER
34
Q

Sar1 GTPase regulates assembly of COPII coat on the ER membrane

A
  • material goes from the ER to the Golgi; in order to make that journey, the material from the ER has to get packaged into a vesicle which then fuses with the Golgi
  • COPII is involved in forward membrane trafficking from ER to Golgi
  • it helps to deliver new synthesized proteins and lipids to the Golgi
  • first step of biosynthetic sorting process
35
Q

What type of cargo goes into a COPII coated vesicle or a vesicle made by COPII?

A

any proteins and lipids that the ER makes; wide range of different types of proteins and lipids

36
Q

GTPase is on and off when…

A

ON: when it’s GTP bound
OFF: when it’s GDP bound

37
Q

What causes GTPase to turn ON?

A

the binding of GTP caused by a GEF

38
Q

What causes GTPase to turn OFF?

A

the binding of GDP caused by GAP

39
Q

What would trigger turning the coat ON controlled by SAR?

A

the formation of this coat will be triggered by the binding of GTP caused by GEF

40
Q

Where does the hydrophobic antenna go when Sar1 is GTP bound?

A

it will go and reach into the lipid bilayer

41
Q

What is the breakdown of the first step that causes the COPII coat to begin to form?

A
  1. Sar1-GEF displaces GDP from inactive, cytosolic Sar1-GDP
  2. GTP loads onto Sar1
  3. Sar1-GTP exposes amphipathic helix (antenna)
  4. Sar1-GTP binds strongly to ER membrane
42
Q

Sec23/24

A
  • forms an inner coat that allows the binding to specific cargo molecules for export out of the ER and into Golgi
  • only binds to membranes when Sar1-GTP is there
43
Q

Sec13/31

A
  • COPII subunits form outside coat
  • end result: on the surface of the ER because that’s the only place where Sar1 is bound to GTP

Cell Biology (8 decks)

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