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PSA712 Biochemistry > Protein Sorting > Flashcards

Protein Sorting Flashcards

1
Q

The ER membrane is continuous with what structure within a cell?

A

Nuclear envelope.

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2
Q

What are the two types of ER?

State what both sites synthesise.

A
  1. RER - ribosome covered tubules, site for protein synthesis.
  2. SER - shorter tubules, site for lipid synthesis.
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3
Q

What are the four main functions of the SER?

Acronym: sally sea-shells dies.

A
  1. Synthesis of lipids and cholesterol for cell membranes.
  2. Synthesis of steroid hormones.
  3. Storage and release of calcium ions.
  4. Detoxification: getting rid of toxic molecules in the body.
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4
Q

The RER is the site of protein synthesis and processing for which three types of proteins?

A
  1. Transmembrane proteins.
  2. Proteins destined for the ER, golgi body, plasma membrane, lysosomes and endosomes.
  3. Proteins to be secreted to the cell exterior.
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5
Q

Some proteins are not processed in the ER. What are these proteins are there are they instead processed?

A

Cytosolic proteins - processed on cytosolic ribosomes.

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6
Q

How is the cytosolic protein synthesised in the cytosol?

A
  1. Common pool of ribosomal subunits in the cytosol.
  2. Ribosomal subunit binds to mRNA 5’ end.
  3. Formation of a free polyribosome in cytosol (multiple ribosomal subunits synthesise the mRNA).
  4. Protein in synthesised.
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7
Q

How is a protein destined to be a transmembrane protein/hormone synthesised?

simple overview

A
  1. Common pool of ribosomal subunits in the cytosol.
  2. Ribosome subunit binds to mRNA 5’ end.
  3. During protein synthesis, the ribosome-mRNA complex is moved to the ER membrane.
  4. Protein is embedded or transferred into the ER.
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8
Q

Co-translational translocations allows proteins to be imported to the ER while they are undergoing what biological process?

A

Protein synthesis.

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9
Q

How does a protein remain in its primary structure during co-translational translocation?

A

One end of the polypeptide attaches to the ribosome, whilst the other inserts itself into the ER.

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10
Q

What do chaperone proteins do?

Why is it unnecessary for a chaperone protein to be present in co-translational translation?

A

Chaperone proteins prevent target proteins from being damaged and folding into their tertiary structure.

As the target protein in co-translational translocation is in its primary structure and held in it by a ribosome and the ER, a chaperone protein is not needed.

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11
Q

The signal sequence of a protein can be found at what terminal?

A

N terminal.

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12
Q

For co-translational translocation the signal sequence must be recognised by what two structures?

A
  1. SRP (signal recognition particle).

2. ER - must be embedded within the ER membrane.

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13
Q

The SRP recognises the signal sequence on the N terminal of the growing polypeptide.
What two things do the SRPs bind to?

A
  1. Signal sequence.

2. Ribosome.

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14
Q

The binding of a SRP to the signal sequence and ribosome causes what?

A

Synthesis of the polypeptide to pause momentarily.

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15
Q

What part of the SRP pauses synthesis of the polypeptide?

A

Translational pause domain.

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16
Q

SRP consists of RNA and how many proteins?

A

6

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17
Q

Describe how SRPs facilitate the protein synthesis of a protein destined to enter the ER lumen.

complex explanation

A
  1. Protein is synthesised by the ribosome, forming the N terminal (signal sequence).
  2. SRP binds to signal sequence and its translational pause domain binds to the ribosome.
  3. Synthesis pauses.
  4. SRP moves towards the ER membrane and binds to a SRP receptor.
  5. Binding to the SRP receptor (next to the protein translocator) allows protein to move into the central lumen of the protein translocator.
  6. Complex is disassembled and SRP and its receptor are released from target protein.
  7. Ribosome and protein remain bound with the membrane, until protein is released into the ER lumen.
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18
Q

Describe what a polysome is.

A

mRNA remains bound to the ER as it is translated by multiple ribosomes at once.

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19
Q

Once a protein has been translated and is inside the ER, it is folded into its 3D conformation by what bonds?
List 3.

A
  1. Ionic bonds
  2. Hydrogen bonds
  3. Van der Waals attraction
  4. Disulphide bonds
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20
Q

What happens during N-linked protein glycosylation?

A

Pre-formed precursor oligosaccharide, composed of 14 sugars, is added to proteins via the N-terminus (amine group) of an asparagine side chain.

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21
Q

What is the importance of N-linked protein glycosylation?

A

Quality control - ensures the protein has been folded correctly and is hence mature.

22
Q

If a protein has not folded correctly, what two processes may occur?

A
  1. 3x glucose and 1x mannose are cleaved from the oligosaccharide.
  2. Glucosyltransferase enzymes add a single glucose back.
    Addition of glucose binds protein to chaperones to prevent aggregation and gives them another opportunity to fold.
23
Q

Describe the process that happens when a protein tries to fold correctly, include the addition of glucose and use of enzymes.

A
  1. Protein containing one glucose binds to calnexin (chaperone).
  2. Protein released and glucosidase cleaves the glucose.
  3. Protein tries to fold.
  4. Cycle repeats - if protein does not fold correctly, it is degraded.
24
Q

Accumulation of misfolded proteins causes ER stress and triggers an unfolded protein response. What three things could happen?

A
  1. Inhibition of protein synthesis.
  2. Degrading of misfolded proteins at a faster rate.
  3. Increase of transcription of chaperones to look after misfolded proteins.
25
Q

What process occurs in order for a protein to be degraded if it has not folded properly?

A

Apoptosis.

26
Q

What three pathways are there when it comes to intracellular membrane trafficking?

A
  1. Secretory
  2. Endocytic
  3. Retrieval
27
Q

COPII coated vesicles transport things from where?

A

ER

28
Q

COPI coated vesicles transport things from where?

A

Golgi

29
Q

Clarthrin coated vesicles transport things from the plasma membrane between which two structures?

A

Golgi and endosomes.

30
Q

Vesicles will encounter many membranes before finding the right one. What molecule enables vesicles to find their target membrane?

A

Rab

31
Q

Rab GTP binds to a Rab effector protein on the target membrane, this causes what?

A

Tethering/binding of the vesicle to the target membrane.

Membrane fusion occurs.

32
Q

The fusion of vesicular cargo and target organelle membranes is facilitated by what proteins?

A

SNARE proteins.

33
Q

What are the two types of SNARE proteins?

A

V-SNAREs (present on vesicle).

T-SNAREs (present on target membranes).

34
Q

The wrapping of V-SNAREs and T-SNAREs forms what stable structure?

A

trans-SNARE complex

35
Q

When vesicles are in very close proximity with their target membranes, what happens to the phospholipids?

A

Phospholipids can flow between one another = fusion.

36
Q

During fusion, Rab GAP causes Rab to hydrolyse GTP to what?

A

GDP

37
Q

Rab GDP in the cytosol is released from the vesicle and bound by Rab GTP dissociation inhibitor. What does the inhibitor do?

A

Keeps Rab inactive.

38
Q

Multiple vesicles can fuse, as long as SNAREs match, forming what structures?

A

Vesticular tubular clusters.

39
Q

How are vesicular tubular clusters formed?

A
  1. Two separate vesicles leave the ER.

2. V-SNAREs and T-SNAREs from each vesicle wrap around one another and vesicles fuse.

40
Q

Receptors, SNARE proteins and other proteins are returned the the ER via what pathway?

A

Retrieval pathway.

41
Q

The Cis golgi network faces and receives organelles from what structure?

A

ER

42
Q

The trans golgi network faces and secretes in what direction?

A

Outwards

43
Q

An oligosaccharide can further be processed in the ER, what three advantages does this provide?

A
  1. Promotes correct folding of proteins.
  2. Prevents unwanted aggregation.
  3. Acts as signals for sorting and targeting the correct pathway.
44
Q

The acidic environment of lysosomes are maintained by what enzyme?
State what the function of this enzyme is.

A

V-ATPase

Actively pumps proteins into the lumen of the lysosome which makes it more acidic.

45
Q

Anything entering a lysosome must be tagged by what molecule?

A

Mannose-6-phosphate

46
Q

Late endosomes contain material which has been ingested by lyosomes (endocytosed).

Fusion with endolyosomes or lyosomes forms what?

A

Endolyosomes?

47
Q

After endocytosed material is complete, the endolyosomes form what structures?

A

Lyosomes

48
Q

What happens during the process of endocytosis?

A
  1. Vesicles bud off the plasma membrane, containing material.
  2. Several vesicles fuse to form early endosomes.
  3. Early endosome matures and becomes late endosome.
  4. Late endosome fuses with lyosome.
  5. Degradation occurs - material outside the cell is broken down.
49
Q

Exocytosis can occur via two pathways, what are these?

A
  1. Constitutive

2. Regulated Secretory

50
Q

What happens in the constitutive pathway of exocytosis?

A
  1. Protein made in the ER is processed in the golgi.
  2. Passed out the golgi via a vesicle.
  3. Vesicle fuses with plasma membrane and material is exocytosed into the cell and its membrane.
51
Q

What happens in the regulated secretory pathway of exocytosis?

A
  1. Protein made in the ER and processed in the golgi.
  2. Golgi sorts protein into vesicles.
  3. Protein is stored in the vesicle in the cytosol until it is needed.
  4. When needed, vesicle fuses with plasma membrane and contents is released.
52
Q

Give an example of when the regulated secretory pathway of exocytosis would be used?

A

Muscle contraction - release of acetylcholine or calcium.

PSA712 Biochemistry (11 decks)

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