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Cell Biology > ER-Golgi > Flashcards

ER-Golgi Flashcards

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1
Q

Proteins destined for the Golgi apparatus, endosomes, lysosomes, and the cell surface all first enter the ?

A

Endoplasmic reticulum (ER)

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2
Q

All these compartments are?

A

‘Connected’ but ‘physically distinct’

Membrane traffic of vesicles

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3
Q

What is the Molecular Machinery of Membrane Traffic ?

A
  1. Sorting
  2. Budding
  3. Targeting
  4. Fusion
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4
Q

Transport Vesicles Carry ?

A

Soluble Proteins and Membrane Between Compartments

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5
Q

Vesicle traffic from ?

A

ER to Golgi

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6
Q

making vesicle:

A
  • a coat: COPII
  • and a switch: Sar1
  • ‘Sec’ proteins: from yeast genetics
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7
Q

SAR1A (mammalian name) orSar1(yeast name) is ?

A

Aproteininvolved in membrane trafficking.

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8
Q

What is it and where is it found ?

A

It is a monomeric small GTPase found in COPII vesicles

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9
Q

It is a member of ?

A

‘ras superfamily’: in traffic, signalling etc

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10
Q

What does SAR1 regulate ?

A

It regulates the assembly and disassembly of COPII coats

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11
Q

What does Sar1 protein need ?

A

Needs Exchange Factor

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12
Q

[GDP]-boundSar1interacts with ?

A

The membrane-bound exchange factor Sec12 and exchanges its bound GDP for GTP

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13
Q

Explain the beginning of the COP-II coated vesicle process ?

A
  • complete the coat: Sec13/31
  • bend the membrane and
  • ….release the vesicle
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14
Q

Then what happens next ?

A
  • Sec23/24 GAP now active
  • Sar1 hydrolyses GTP
  • ….and falls off membrane
  • ….and so does the coat
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15
Q

COPII mediated Vesicle Formation at a Glance ?

A
  1. Sec12 catalyses guanine nucleotide exchange on Sar1, recruiting Sar1 to the ER membrane. Membrane deformation initiates
  2. Sar1 at the membrane recruits the inner coat proteins: the Sec23-Sec24 heterodimer
  3. The Sec24 subunit binds to cargo proteins that need to be transported out of the ER. This group (Sar1, Sec23, Sec24 and cargo) is termed a pre-budding complex
  4. Soluble cargo in the lumen of the ER can be captured by transport adaptors. The formation of pre-budding complexes is concentrated at ER exit sites (ERES)
  5. The outer COPII coat, the Sec13-Sec31 heterotetramer, binds and collects the pre-budding complexes
  6. Full COPII assembly leads to fission of a vesicle from the donor ER membrane. Soluble bulk flow cargo can be trapped inside a vesicle
  7. The hydrolysis of GTP bound to Sar1 leads to COPII coat disassembly
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16
Q

Explain Rab proteins ?

A
  • another branch of the ras superfamily
  • GTP binding/hydrolysis
  • cycle on and off membrane
  • ~1 per stage of trafficking
  • different Rab’s found on different membranes
  • lipid ‘anchor’ in membrane: unlike Sar1
17
Q

Describe the SNARE complex ?

A
  • from nerve terminal
  • Bo: Botulinum
  • Te: Tetanus
  • neurotoxins: specific
    proteases
- SNAREs coil around
each other
- force membranes 
together
- lipid bilayers 'fuse' 
i.e. mix to make one 
membrane
18
Q

What is Anterograde Transport ?

A

Vesicle transport from ER to Golgi: COPII coated vesicles

19
Q

What is Retrograde Transport ?

A

From Golgi to ER: COPI coated vesicles

20
Q

Explain Retention in the ER?

A
  • Problem: some ER proteins (e.g. chaperones) are soluble but ‘stay’ there
  • They have a C-terminal tag, the ER retention signal, usually KDEL
  • If ‘KDEL’ proteins escape, receptor binds in Golgi
  • …….and returns it to ER
21
Q

What is the functions in the trafficking system of the Golgi complex ?

A
  • concentration: less area than ER
  • modification: glycosylation
  • sorting: plasma membrane, lysosomes,
    back to ER
22
Q

What is the appearance of the Golgi complex ?

A
  • electron microscope: ‘stacks’

- light microscope: depends on cell type

23
Q

Explain the Glycosylation steps in the Golgi ?

A

Core N-linked oligosaccharide ‘pruned’ and modified both in the ER and in the Golgi by specific glycosidases (remove sugars) and glycosyl transferases (add sugars)

24
Q

What are some key points about vesicle traffic ?

A
  • Sorting, budding, targeting and fusion
  • Coats formed from subunits in the cytosol
  • Coat falls off
  • Rab proteins and tethers
  • SNARE proteins and membrane fusion

Cell Biology (16 decks)

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