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BMS 242 - Advanced Cell and Molecular > Protein Trafficking > Flashcards

Protein Trafficking Flashcards

1
Q

Where are cytosolic proteins made?

How?

A

In the cytosol

On free ribosomes through a ‘simple cycle’

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2
Q

Where are membrane and secretory proteins made?

How?

A

In the rough endoplasmic reticulum (RER)

Through an SRP cycle

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3
Q

Where are nuclear proteins made?

A

In the outer nuclear membrane

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4
Q

Where do the proteins made in the RER travel to? (in order)

A
  • Smooth ER
  • Golgi
  • Secretory vesicles
  • Membrane
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5
Q

Why is the RER ‘rough’?

A

Due to ribosomes

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6
Q

Where does ALL protein synthesis START?

A

In the cytosol

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7
Q

Describe the SRP cycle

A

ER signal sequence in the mRNA binds to the signal recognition particle and directs the ribosome to the ER membrane

The mRNA then remains permanently bound to the ER as part of a polyribosome (as the ribsomes translate the mRNA - the protein sequence falls into the ER lumen)

Ribosomes move along the mRNA and are recycled at each round of replication to join the free ribsome pool

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8
Q

Where do proteins travel to after they are synthesised in the RER?

A

To the edges of the ER where there are no ribosomes - the SER

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9
Q

Where are LIPIDS made?

A

In the SER

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10
Q

What happens to lipids and proteins in the SER?

A

They are packaged together in a vesicle - to travel towards the golgi and further on

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11
Q

What happens in the SER?

A

Lipids are made and packaged with proteins into vesicles

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12
Q

What is used to separate the heavy and light vesicles?

Describe this process

A

Differential centrifugation:

  • Gradient of sucrose concentration
  • Spin at high speed
  • Low density settles at the bottom
  • High density settles at the top
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13
Q

What are the high density vesicles?

A

Studded ribosomes - ER vesicles that studded with ribosomes

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14
Q

How are proteins sent to the right destination in the cell?

A

Based on their signal sequence

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15
Q

What are signal sequences?

A

The first 10-12 amino acids on the polypeptide chain

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16
Q

What type of signal sequence is needed to direct the protein to the nucleus?

What is this sequence different to?

A

Positive charges

These sequences are different to the signal sequences that destines the proteins to the ER

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17
Q

What does the SRP do?

A

Attaches to and guides free ribosomes to the ER by binging to the signal sequence on the growing poly peptide chain

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18
Q

What does the SRP consist of?

A

Cellular proteins

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19
Q

How does the SRP attach to ribosomes?

A

With its hinge region

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20
Q

What happens after the SRP is bound to the signal sequence?

A
  • Pause in translation
  • SRP receptor in the ER membrane binds the SRP-ribosome complex
  • Directs the complex to the to a protein translocator in ER membrane
  • Signal sequence is captured by the translocator - opening the channel
  • Ribosome is passed to the translocator and translation continues
  • SRP and receptor are released and recycled
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21
Q

What is the topography of the ribosome on the ER membrane?

A

Small ribosome subunit facing to cytosome

Large ribosome subunit is attached to the protein translocator in the ER membrane

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22
Q

What removes the signal sequence from proteins in the ER?

A

ER-based signal peptidases (specific signal peptidases)

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23
Q

What happens if the proteins being synthesised on the ER membrane have a stop-transfer sequence?

A

The protein becomes embedded in the ER membrane

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24
Q

What happens if the proteins being synthesised on the ER membrane don’t have a stop sequence?

What can this protein then be destined for?

A

ALL of the protein goes into the ER lumen

Destined for secretion or into the lysosomes

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25
Q

What 2 kinds of proteins are moved from the cytosol to the ER?

A

1) Water soluble proteins - completely cross the membrane and go into the ER lumen
2) Transmembrane proteins - only partially cross the ER and become embedded

26
Q

What is the signal sequence that directs the proteins to the ER?

A

Small hydrophobic amino acids

27
Q

How can swapping the signal peptide for a lipid anchor in the ER membrane occur?

What does this allow?

A
  • WITHOUT a transmembrane sequence
  • Signal peptide removed and a lipid tail is attached
  • Allows the continuation of membrane association
28
Q

Where are GPI anchors found?

A

In the OUTER leaflet of the lipid bilayer

29
Q

What is a GPI anchor?

A

GlycoslyPhosphatidylInositol

30
Q

What follows the removal of the signal sequence by specific signal peptidases?

What happens during this process?

A

Conformational maturation:

  • Formation of disulphide bridges
  • Protein is glycosylated by a standard carbohydrate sugar chain
31
Q

Where are disulphide bridges formed in the maturation of an ER protein?

What is the function of these bridges?

A

Between cysteine residues

To ‘solidify’ the protein shape - allowing the protein parts to stay together in a harsh environment (outside the cell or acidic pH inside vesicles)

32
Q

In what proteins are disulphide bridges important in?

A

Luminal and extracellular proteins

33
Q

Why are proteins glycosylated in the maturation of an ER protein?

How does this work?

A

For quality control:

  • If the protein is synthesised into the incorrect shape - sugar cannot be attached to the right amino acid
  • Therefore, enzymes inside the ER lumen will recognise these as defective and destroy them
34
Q

Where do glycosylation and disulphide bridges occur?

A

Extracellularly or inside the lumen of the ER

Cytosol is a reducing environment

35
Q

What do ER-resident enzymes carry and why is this important?

A

KDEL sequences

Important for their return back to the ER - captured by the KDEL receptor and brought back to the lumen

36
Q

What is KDEL?

A

Lysine-aspartic acid-glutamic acid-leucine

37
Q

What 3 things are the addition of carbohydrates to proteins important for?

A

1) Protein stability in the harsh extracellular environment
2) Cell-cell recognition
3) Cross-species separation

38
Q

How is the addition of carbohydrates used for cell-cell recognition?

A

Subtle changes can have major affects

39
Q

How are humans separated from other animals using the addition of carbohydrates?

A

Humans use beta-galactose

Other animals use alpha-galactose

40
Q

What is the difference between N-linked and O-linked glycosylation?

A

N-linked:
- Sugar attached to asparagine by a terminal nitrogen

O-linked:
- Sugar attached to threonine through oxygen

41
Q

What does INITIAL addition of carbohydrates occur?

A

In the ER

42
Q

What is the INITIAL function of the addition of carbohydrates?

A

Quality control

43
Q

Where does FINAL addition of sugars and sorting occur?

A

In the golgi

44
Q

What is the ‘protein maturation organelle’?

A

The golgi

45
Q

What is different about the cisterns of the golgi?

A
  • Different cisterns have different enzyme number/type with different modifications occurring at each enzyme
46
Q

How does trimming and growth of carbohydrate chains on protein occur?

A
  • Step-by-step in individual golgi cisternae
47
Q

What does each glycosylation step require?

A

Separate golgi compartments (cisternae)

48
Q

What is the process of glycosylation?

A

Multi-step:

  • Glucose residues removed from the original quality control signal
  • Mannose residues are trimmed down
  • Sugars expand via the attachment of N-acetylglucosamine
  • Then, more complex olligosaccharides added
49
Q

What are limitations in organ transplant due to?

A

Simple sugar modification

50
Q

What is the difference between human and animal cells? (in terms of glycosylation)

A

Human - beta-galactose
(Hydroxyl at pos. 1 faces UPWARDS)

Animals - alpha-galactose
(Hydroxyl at. pos 1 faces DOWNWARDS)

51
Q

How can human rejection of animal transplants be overcome?

A

Genetically modify pigs:
- To lack galactose all together

  • By removing the protein responsible for the synthesis of alpha-galactose sugars
52
Q

How are proteins activated?

A

Trimmed by proteases

53
Q

Where does trimming of hormones and enzymes (in order to activate them) occur?

During when?

A

In immature secretory vesicles (before maturation and secretion)

54
Q

What amino acids need to be removed from insulin in order to make it function?

A

LLALLAL (leucinces and alanines)

55
Q

What is protein trimming also known as?

A

Protein processing/maturation

56
Q

Why does insulin travel freely through the trans-golgi network?

A

No sugars are attached to it

57
Q

What causes type 1 diabetes?

What does this result in?

A

Genetically inherited

  • Miss folding of proinsulin in the ER
  • Protease in secretory vesicle cannot cleave off the C-peptide and mature the protein
  • Pro-insulin can’t bind insulin receptors
  • Blood glucose concentration increase
58
Q

In type 1 diabetes, why is there a destruction of the pancreatic cells?

A

Due to the generation of antibodies against the pancreatic cells that release the deficient pro-insulin

59
Q

How can the same polypeptide be processed into different hormones?

A

By different cells

60
Q

What hormones can opiomelanocortin give rise to?

Where are they released from?

A

Several hormones with completely different functions:

  • ACTH and beta-lipotrophin (from the pituitary gland)
  • Beta-endorphin (from neurons)
61
Q

When is beta-endorphin generated by the neurons?

A

In response to exercise and stress

62
Q

What does insulin trigger?

A

The delivery of vesicles containing glucose transporters to the plasma membrane of muscle cells

BMS 242 - Advanced Cell and Molecular (24 decks)

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