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From Molecules to Cells BIOL10232 > L5- The ER > Flashcards

L5- The ER Flashcards

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Organic Chemistry 101 flashcards
1
Q

What is the ER’s structure?

A

Continuous with outer membrane, forming a dynamic system of interconnected tubules and sacs.

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2
Q

What is rough ER?

A

When bound ribosomes are present.

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3
Q

How much ER is there in a cell?

A

Amount of ER= cell type specific. e.g. in the pancreas lots of ER because it is secretary

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4
Q

What are the ERs functions?

A
  1. Protein synthesis
  2. Entry point into the secretary pathway
  3. Supplies newly made proteins and lipids to the compartments of the secretary pathway and the plasma membrane. (golgi, lysosome, and pm)
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5
Q

What is the schematic secretary pathway?

A

ER–> Vesicle–> Golgi –> vesicle–> PM

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6
Q

What happens to ribosomes that synthesis proteins without a signal sequence?

A

They remain free in the cytosol

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7
Q

What happens to ribosomes that do make an ER signal sequence?

A

They’re directed to the ER membrane while the protein is still being made. The proteins are then translocated across the membrane into the ER.

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8
Q

What is co-translational translocation?

A

This process of proteins being threaded through the ER membrane while they are being synthesised by the ribosome. (v. different from post translational process like in nucleus)

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9
Q

What is SRP?

A

Signal recognition particle (6 proteins and an RNA backbone)

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10
Q

What does SRP do?

A

Binds to ER targeting signals as they emerge from the ribosome. It has a sting of hydrophobis residues. SRP receptor in the ER membrane recognises and binds the SRP-polypeptide-ribosome complex

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11
Q

What do SRP and the SRP receptor contain?

A

GTP binding proteins

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12
Q

What is the SRP dependent targeting cycle to the ER regulated by?

A

GTP binding and hydrolysis

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13
Q

What happens when the ribosome/SRP complex binds to the SRP receptor at the ER membrane?

A

The SRP is released.
The ribosome is transferred to the protein translocator and the polypeptide is threaded through this channel as it’s being made.
(this translocation channel opens when it binds the ER signal sequence)

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14
Q

What is the protein translocator in the ER membrane?

A

The sec61 complex

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15
Q

What connects ribosomes with ER signalling proteins to available translocation channels in the ER membrane?

A

SRP and the SRP receptor

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16
Q

What happens after targeting for the soluble proteins that will go right into the ER?

A

The signal sequence will remain bound to keep the translocon open. The rest of the protein is threaded through as a large loop into the ER lumen. (translocation)

The ER signal sequence is then cleaved off in the lumenal side by a signal peptidase. (so whole protein is in the ER)

17
Q

What happens after targeting for the simple membrane proteins?

A

Integral membrane proteins have an ER targeting signal and a stop-transfer signal.
The stop-transfer signal is released sideways from the ER translocon, allowing the protein to become embedded in the membrane. = integration.

N-terminal signal sequences are cleaved.

Signal-anchor sequence acts as both a targeting and stop-transfer signal and is not cleaved.

18
Q

What happens after targeting for complex membrane proteins with multiple transmembrane segments?

A

Made up of combinations of hydrophobic start and stop transfer signals. Also referred to as signal-anchor sequences. All of these signal sequences are inserted into the ER membrane via the sec61 complex.

19
Q

Why are tail-anchored proteins not recognised by SRP?

A

Their hydrophobic ER targeting signal is at the C-terminus so as they emerge from the ribosome they’re not recognised by SRP.

20
Q

What does the ER targeting signal in tail-anchored proteins act as?

A

A membrane anchor

21
Q

How are tail-anchored proteins inserted into the ER membrane?

A

Post-translational insertion (as SRP doesn’t attach)

22
Q

What’s an important example of tail-anchored proteins?

A

SNARE proteins

23
Q

How are proteins modified at the ER?

A
  1. Cleavage of signal sequences
  2. Disulphide bond formation
  3. Glycosylation (addition of carbohydrate)
24
Q

How are disulphide bonds formed?

A

Oxidation of cysteine side chains-stabilise folded structure.
Catalyzed by enzyme in ER lumen -> protein disulphide isomerase =PDI

25
Q

How are proteins glycosylated into glycoproteins?

A

Covalent attachment of short oligosaccharide side chains

26
Q

What are the functions of glycosylation?

A
  1. Assists protein folding
  2. Modified to create mannose-6-phosphate tags which act as a lysosome specific sorting signal
  3. Ligand for specific cell-cell recognition events
27
Q

What are preformed oligosaccharide structures donated from in glycosylation?

A

Special lipid donor called dolichol

28
Q

What do the oligosaccharides attach to?

A

The NH2 side chain of asparagine residues (Asn/N), hance N-linked glycosylation

29
Q

Glysocsylation can only occur when Asn is in what sequence?

A

Asn-Xaa-Ser or Asn-Xaa-Thr

where Xaa can be any amino acid except proline

30
Q

What is N-glycosylation catalysed by?

A

The oligosaccharyl transferase complex

31
Q

When does N-glycosylation normally occur?

A

Whilst the protein is still being synthesised and threaded through the translocon. i.e. co-translationally

32
Q

What’s the nature of the 14 sugar oligosaccharide attached to a protein in ER?
(the glycan?)

A

High mannose content

33
Q

What is the glycocalyx?

A

Protective layer of sugars attached to proteins and lipids coating eukaryotic cells. Facing outside the cell.

34
Q

What is protein folding promoted by?

A

Specific molecular chaperones

35
Q

What is BiP?

A

The heavy chain Binding Protein - chaperone that uses ATP to promote protein folding

36
Q

What is Calnexin?

A

An ER chaperone- specifically assists the folding of N-glycosylated proteins

37
Q

How is quality control done by chaperones?

A

Chaperones remain bound to incorrectly folded proteins so they cannot leave the ER

38
Q

If there are too many misfolded proteins in the ER what happens?

A

This triggers the unfolded protein response -> UPR.

The more unfolded proteins, the more ER is made

39
Q

What mediates flip-flop between the 2 halves of the ER bilayer?

A

The enzymes- scramblases

evens out bilayer, only making lipids on one layer

From Molecules to Cells BIOL10232 (17 decks)

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