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Cellular Processes > Secretory Pathway and Protein Sorting > Flashcards

Secretory Pathway and Protein Sorting Flashcards

1
Q

What is the function of peroxisomes?

A

They are the site of oxidative reactions

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2
Q

What is the role of the endosome?

A

Recycling and sorting proteins. Also has a role in signalling

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3
Q

What experiments have been done to prove the cell is made up of compartments?

A
  • Pulse chase experiments
  • In Vitro Cell Free Assays
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4
Q

Outline the pulse chase experiments

A
  • Cells are placed in a medium with 3H-leucine
  • Cells are then placed in unlabelled medium
  • Radioactive proteins are made and their position in the cell is tracked
  • Cells are placed in radioactive medium only for enough time that one wave of proteins is labelled
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5
Q

Outline in vitro cell free assays

A
  • Cells are broken upand fractionated by centrigation
  • Organelles are then added back to the cytosol and ATP in order to see how proteins are trafficked
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6
Q

How can yeast mutants be used to help identify machinery of secretory pathways?

A
  • Mutations introduced in yeast
  • If the secretory pathway is interupted this would result in vesicles being stuck inside the cell at specific point
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7
Q

Give an example of a disease which is caused due to trafficking defects

A

Cystic Fibrosis

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8
Q

What causes cystic fibrosis?

A

Mutation in the CFTR chloride channel protein, causing protein to be trapped in the ER

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9
Q

If mRNA has no sorting signal where does the protein end up?

A

In the cytosol

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10
Q

What happens to mRNA with an organelle specific targeting sequence?

A

It is targeted and transported into the specific organelle

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11
Q

What type of sorting sequence leads to post-translational import?

A

Organelle specific

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12
Q

When does a protein have an ER signal sequence on the mRNA?

A

If it is a membrane protein or a protein to be secreted

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13
Q

What type of sorting sequence leads to co-translational import?

A

An ER signal sequence

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14
Q

What happens to proteins with an ER signal sequence?

A
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15
Q

Give an example of an experiment which proves the importance of signal sequences

A

Putting different signal sequences onto a cytosolic protein cause it to be expressed in different compartments in the cell

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16
Q

What is required from an ER signal sequence?

A

A string of hydrophobic amino acids (the actual sequence matters less, the hydrophobic nature is what is important)

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17
Q

What type of translocation is ER translocation?

A

Co-translational

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18
Q

What is the role of the ER?

A
  • synthesis and folding of transmembrane and secreted proteins
  • N-linked glycosylation
  • GPI anchor attachment
  • Lipid synthesis
  • Calcium storage
19
Q

What are the roles of the smooth ER?

A
  • Lipid synthesis
  • Where proteins exit the ER
20
Q

What is the free ribosome cycle?

A
  • There is a common pool of subunits in cytosol
  • mRNA encoding a cytosolic protein remains free in cytosol
  • Once the protein is built ribosomal subunits become free in cytosol again
21
Q

What is the membrane bound ribosome cycle?

A
  • Common pool of ribosomal subunits in cytosol
  • if mRNA has an ER signal sequence then it binds an SRP which bind to SRP receptors, localising the ribosome to ER membrane
  • Ribosomes translate mRNA and then return to cytosol
22
Q

When a ribosome is brought to the ER membrane where is a ribosome localised to?

A

A translocation channel/translocon

23
Q

Why is the translocon normally closed when not bound to the ribosome?

A

ER would otherwise be unable to hold Ca2+ in

24
Q

What happens to the translocon when the ribosome binds?

A
  • The translocon opens and the protein channel in the subunit lines up with it
  • proteins synthesised in the MRNA are then directly cotranslated through the translocon into the ER lumen/membrane
25
Q

What property does the translocon channel have?

A

It is lined with water

26
Q

Why are secretory proteins translated in vitro bigger then expected?

A

They contain amino acids of the signal sequence at the N-terminus that are removed following translocation

27
Q

What cleaves the signal sequence off proteins?

A

Signal peptidase

28
Q

What does SRP stand for?

A

Signal recognition particle

29
Q

What is the function of a singal recognition particle?

A

They recognise the ER sequence

30
Q

What is the structure of SRPs?

A
  • Made up of 6 polypeptide and 1 RNA molecules
  • Inside there is a signal sequence binding pocket
31
Q

What happens when an SRP recognises a polypeptide with a ER sequence?

A

It induces a pause in translation so no more protein is made in the cytosol

32
Q

What happens to SRPs when the translocon binds an ER signal sequence?

A

It is released

33
Q

What is a type I membrane protein?

A

Has the N terminus outside the cell and the C terminus inside

34
Q

What is a type II membrane protein?

A

Has the N terminus inside and the C terminus outside

35
Q

What is a type III membrane protein?

A

A mutispanning protein

36
Q

How does signal peptidase cleave in relation to start and stop signals?

A
  • If the start signal is at the N terminus the enzyme will cleave it off
  • The stop signal will remain within the protein
37
Q

What characteristic do start and stop transfer sequences have?

A

They are hydrophobic

38
Q

What type of transfer sequence does an ER signal sequence act as?

A

A start transfer sequence

39
Q

How do you ensure membrane proteins are incorporated in the correct orientation?

A

Start transfer sequences bind to the translocon and start making polypeptide, until a stop transfer seuqence is reached and the rest of the protein stays on the outside

40
Q

In complex membrane proteins how do you ensure you have the correct orientation?

A

Involves charges on either end of the transfer sequence

41
Q

In the positive inside rule what does inside refer to?

A

the cytosol

42
Q

What is the positive inside rule?

A

C

43
Q

Where is the start sequence found in multispanning membrane proteins?

A

In the centre, not at the N terminus

44
Q

How do multispanning membrane proteins use start and stop transfer sequences?

A
  • Use multiple start and stop sequences
  • The hydrophobic stop and start sequences end up in the protein or ER membrane
  • The N terminus or C terminus of the protein might end up in the cytosol

Cellular Processes (22 decks)

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