(L7) Protein folding in ER & vesicular transport in Golgi apparatus

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Which Hsp70 chaperone does the ER contain?
BiP
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What are BiP's functions?
Assists folding of proteins as they enter the ER, prevents misfiled proteins from escaping from ER
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What other 2 molecules assist in folding?
Calnexin and Calreticulin
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Describe the method that calnexin aids in protein folding
2 glucoses are removed as proteins fold, C & C recognise N-linked oligosaccharides with 1 glucose, after protein leaves C, the glucose is removed, if normally folded, protein exits from ER
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What happens if the protein is incompletely folded?
A glucosyl transferase adds a glucose to the protein so that they can return to C
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What happens to the proteins that are incorrectly folded?
They are transported to the ER and degraded by proteasomes
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Describe the mechanism of targeting for degradation
Removal of mannose to which glucose would be attached (meaning glucose can't be re-added, so cannot bind to calnexin or calreticulin), removal of the other two mannose terminals (targets protein for export into ER and degradation)
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How are proteins transported to and from the Golgi apparatus?
Vesicles
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What are the 3 stages of vesicular transport?
Budding, movement, fusion
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What is on the vesicles to tell the cell where to move them?
Protein coat
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What protein coat is on the vesicles to be transported from ER to the Golgi?
COPII
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What protein coat is on the vesicles to be transported from Golgi to ER?
COPI
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What protein coat is on the vesicles to be transported from Golgi to lysosomes?
Clathrin
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How is specific docking and vesicle fusion brought about?
SNARE proteins in vesicle membranes in combination with Rab proteins
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Describe docking of a vesicle via SNARE proteins
v-SNARE in vesicle binds to t-SNARE in target membrane, pulling membranes together, Rab proteins (GTPases) assist, GTP hydrolysis means its irreversible, at target membrane, Rab proteins interact with Rab effectors, which act as tethering proteins
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Describe the docking and fusion of a vesicle to target membrane
GDP is bound to inactive Rab in cytosol, Rab binds to guanine-nucleotide exchange factor (GEF) on source membrane, GDP exchanged for GTP and inserts into membrane, vesicle moves to target membrane where Rab binds to Rab effector, membrane fuse
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What are cargo proteins recruited into transport vesicles by?
Poorly-understood signals
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Where do the the membrane proteins bind to?
To the Sec24 (part of the COPII)
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Where do the soluble proteins bind to?
Cargo receptor which binds Cop 24
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What should proteins that remain in the ER have?
Specific sequences at their carboxy-termini
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ER membrane proteins have:
KKXX
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ER soluble proteins have:
KDEL
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What can happen to ER proteins?
They can leak out of the ER
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What happens to the soluble proteins if they leak out?
Soluble proteins bind to a KDEL receptor and are returned via COPI-coated vesicles
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What happens to the membrane proteins if they leak out?
They bind directly to COPI and are also returned in COPI-coated vesicles
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What do the proteins move through in the Golgi apparatus?
A series of compartments
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Name the compartments in order, from entrance from ER to exit to plasma membrane, lysosomes or secretory vesicle
cis Golgi network, Golgi stack- (cis cisterna, medial cisterna, trans cisterna), trans Golgi stack
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Name the two models for organisation fo the Golgi apparatus
Vesicular transport and cisternal maturation
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Vesicular transport
Cisternae are fixed, vesicles transport substances between cisternae
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Cisternal maturation
Cisternae move down the Golgi and change activity as they do so, retrograde transport of vesicles balances the transport of material forward and recycles enzymes (vesicles transport material in the opposite direction to the cisternae to balance)
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Name the two types of glycosylation
N-linked and O-linked
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N-linked
Builds on glycosylation in ER
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O-linked
On serine and threonine, takes place entirely in thee Golgi
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Many cell surface proteins have complex glycosylation, name some functions of this
Cell-cell interactions with specific glycoproteins on cell surfaces (recognition and adhesion), protection and lubrication
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Name some examples of protection and lubrication by glycosylation
Extracellular matrix around animal cells; glycoproteins around plant cell walls, mucus (made from muffins, poly peptides with extensive O-glycosylation)
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What are the three main pathways from the Golgi apparatus?
1) Movement to lysosomes, 2) Regulated secretory pathway (in specialised cells), 3) Constitutive secretory pathway ("default pathway", vesicles move from Golgi to plasma membrane)
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What are the principal proteins of lysosomes?
Acid hydrolases
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What do acid hydrolases do?
Hydrolyse molecules under acidic conditions (intracellular digestion)
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What happens to lysosomal hydrolases in the Golgi?
Mannose-6-phosphate is added to their N-linked oligosaccharides
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What does adding M6P do?
It allows the enzymes to recognise signal patch on the surface of folded proteins
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Describe the process of M6P bound proteins transported to early endosome
Protein with M6P binds to M6P receptor bound with clathrin coat, clathrin-coated vesicles bud from Golgi, loose coats and bind to early endosome, its acidic so receptor dissociates and a phosphate is removed, M6P receptor transported back to Golgi
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What happens to an early endosome?
Mature to late endosomes, which mature to, or fuse with, lysosomes
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Describe regulation secretion
Secretory vesicles wait near the plasma membrane and release contents in response to extracellular signals
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Give some examples of the contents in these vesicles
Small molecules (e.g. histamines, neurotransmitters), peptide hormones, digestive enzymes
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What can happen to the vesicles?
Can become densely packed and so proteins aggregate and clump together
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Name 3 types of signal
Amino-terminal signal peptides (target newly synthesised polypeptides to ER), carboxy-terminal ER retention signals, Internal signal patches (Targeting to lysosomes and possibly regulated secretion)
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Other cards in this set

Card 2

Front

What are BiP's functions?

Back

Assists folding of proteins as they enter the ER, prevents misfiled proteins from escaping from ER

Card 3

Front

What other 2 molecules assist in folding?

Back

Preview of the front of card 3

Card 4

Front

Describe the method that calnexin aids in protein folding

Back

Preview of the front of card 4

Card 5

Front

What happens if the protein is incompletely folded?

Back

Preview of the front of card 5
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