ER-Golgi Flashcards
Proteins destined for the Golgi apparatus, endosomes, lysosomes, and the cell surface all first enter the ?
Endoplasmic reticulum (ER)
All these compartments are?
‘Connected’ but ‘physically distinct’
Membrane traffic of vesicles
What is the Molecular Machinery of Membrane Traffic ?
- Sorting
- Budding
- Targeting
- Fusion
Transport Vesicles Carry ?
Soluble Proteins and Membrane Between Compartments
Vesicle traffic from ?
ER to Golgi
making vesicle:
- a coat: COPII
- and a switch: Sar1
- ‘Sec’ proteins: from yeast genetics
SAR1A (mammalian name) orSar1(yeast name) is ?
Aproteininvolved in membrane trafficking.
What is it and where is it found ?
It is a monomeric small GTPase found in COPII vesicles
It is a member of ?
‘ras superfamily’: in traffic, signalling etc
What does SAR1 regulate ?
It regulates the assembly and disassembly of COPII coats
What does Sar1 protein need ?
Needs Exchange Factor
[GDP]-boundSar1interacts with ?
The membrane-bound exchange factor Sec12 and exchanges its bound GDP for GTP
Explain the beginning of the COP-II coated vesicle process ?
- complete the coat: Sec13/31
- bend the membrane and
- ….release the vesicle
Then what happens next ?
- Sec23/24 GAP now active
- Sar1 hydrolyses GTP
- ….and falls off membrane
- ….and so does the coat
COPII mediated Vesicle Formation at a Glance ?
- Sec12 catalyses guanine nucleotide exchange on Sar1, recruiting Sar1 to the ER membrane. Membrane deformation initiates
- Sar1 at the membrane recruits the inner coat proteins: the Sec23-Sec24 heterodimer
- 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
- 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)
- The outer COPII coat, the Sec13-Sec31 heterotetramer, binds and collects the pre-budding complexes
- Full COPII assembly leads to fission of a vesicle from the donor ER membrane. Soluble bulk flow cargo can be trapped inside a vesicle
- The hydrolysis of GTP bound to Sar1 leads to COPII coat disassembly
Explain Rab proteins ?
- 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
Describe the SNARE complex ?
- 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
What is Anterograde Transport ?
Vesicle transport from ER to Golgi: COPII coated vesicles
What is Retrograde Transport ?
From Golgi to ER: COPI coated vesicles
Explain Retention in the ER?
- 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
What is the functions in the trafficking system of the Golgi complex ?
- concentration: less area than ER
- modification: glycosylation
- sorting: plasma membrane, lysosomes,
back to ER
What is the appearance of the Golgi complex ?
- electron microscope: ‘stacks’
- light microscope: depends on cell type
Explain the Glycosylation steps in the Golgi ?
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)
What are some key points about vesicle traffic ?
- Sorting, budding, targeting and fusion
- Coats formed from subunits in the cytosol
- Coat falls off
- Rab proteins and tethers
- SNARE proteins and membrane fusion
