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BC3003: Introduction to Cell Biology & Biomembranes > Protein Targeting: Endoplasmic Reticulum > Flashcards

Protein Targeting: Endoplasmic Reticulum Flashcards

1
Q

Where is the protein ER signal sequence located?

A

N-terminus of proteins destined for the ER

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2
Q

When is the ER signal sequence synthesised?

A

During protein translation

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3
Q

How many amino acids long is the ER signal sequence?

A

16 - 20

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4
Q

What types of amino acids are ER signal sequences comprised of?

A

1 or more charged residues and 6-12 hydrophobic residues

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5
Q

What does the Signal Recognition Particle (SRP) do in the targeting of secretory proteins to the ER?

A
  • Binds to the signal sequence of proteins
  • Slows elongation of the polypeptide
  • Targets polypeptide + ribosome to ER
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6
Q

What is the structure of the Signal Recognition Particle (SRP)?

A
  • Cytosolic ribonucleoprotein (RNA:Protein complex)
  • 300 nucleotide RNA + 6 protein subunits
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7
Q

Which protein subunit of the Signal Recognition Particle (SRP) binds to the signal sequence and also GTP?

A

P54

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8
Q

Which protein subunits of the Signal Recognition Particle (SRP) interacts with the ribosome?

A

P9 / P14

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9
Q

Where is the SRP Receptor located?

A

ER membrane

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10
Q

What is the structure of the SRP receptor?

A

2 subunits (α and β)

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11
Q

Which subunit of the SRP receptor binds GTP?

A

α subunit

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12
Q

What promotes the interaction between the SRP and the SRP receptor?

A

Binding of GTP

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13
Q

What is the Translocon in the ER membrane composed of multiple copies of

A

Sec 61 complex

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14
Q

TRUE OR FALSE:
Translocation of proteins across the ER membrane is CO-TRANSLATIONAL in Eukaryotes?

A

True.
Occurs as peptide is being made by ribosome

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15
Q

Which enzyme cleaves off the signal sequence as the protein passes through the ER membrane?

A

Signal peptidase

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16
Q

TRUE OR FALSE:
Translocation of proteins across the ER membrane is CO-TRANSLATIONAL in Yeast?

A

False.
Post-translational translocation occurs

17
Q

TRUE OR FALSE:
SRP / SRP receptor is involved during translocation of proteins across the ER membrane in both Eukaryotes AND Yeast?

A

False.
Not involved in Yeast

18
Q

Which TWO proteins are involved in bringing the polypeptide chain through the translocon into the ER lumen in Yeast cells?

A
  • Sec 63 complex
  • BiP chaperone protein
19
Q

What determines the orientation of proteins inserted into the ER membrane?

A

Topogenic sequences

20
Q

How many Topological classes of integral membrane proteins are there?

A

4

21
Q

How do Type I and III of the topological classes of integral membrane proteins differ?

A

A signal sequence is cleaved during insertion into membrane in TYPE I

22
Q

What does the translocon do during translocation when an internal sequence of 20-25 hydrophobic amino acids is encountered for Type I integral membrane proteins?

A
  • Stops further transfer of polypeptide through membrane
  • Moves polypeptide laterally to anchor protein in the membrane
  • Referred to as the Stop-Transfer Anchor (STA) sequence
23
Q

TRUE OR FALSE:
Type II and Type III integral membrane proteins lack a cleavable N-teminal signal sequence

A

True

24
Q

Where is the positive charge located of the signal anchor sequence in Type II integral membrane proteins?

A

N-teminal side

25
Q

Where is the positive charge located of the signal anchor sequence in Type III integral membrane proteins?

A

C-teminal side

26
Q

TRUE OR FALSE:
Type IV integral membrane proteins have only one transmembrane helices?

A

False.
They have multipe TM helices

27
Q

What is the location of the Type IV-A N-terminus?

A

Cytosol

Signal anchor sequence of Type II (SA-II)

28
Q

What is the location of the Type IV-B N-terminus?

A

Exoplasmic space

Signal anchor sequence of Type III (SA-III)

29
Q

What is used to predict the topology of a transmembrane protein?

A

Hydropathy plot

30
Q

Which type of integral transmembrane protein are GPI-anchored proteins initially synthesised and anchored as?

A

Type I

31
Q

What is the function of GPI Transamidase?

A

Cleaves off the stop-transfer anchor sequence and attaches the rest of the protein to GPI anchor

32
Q

What are TWO reasons to switch from a protein anchor to a lipid anchor?

A
  • Greater mobility in the membrane
  • Targeting to particular membrane domains
33
Q

What THREE proteins assist the folding of proteins in the ER?

A
  • Chaperone proteins
  • Calnexin
  • Calreticulin
34
Q

What TWO ways do cells deal with unfolded proteins?

A
  • Unfolded Protein Response
  • Endoplasmic Reticulum Associated Protein Degradation (ERAD)
35
Q

What happens during Endoplasmic Reticulum Associated Protein Degradation (ERAD)?

A
  • Misfolded proteins are translocated backwards into the cytosol
  • Degraded by the proteasome
36
Q

Which post-translational modification is performed on most proteins in the ER?

A

Glycosylation

BC3003: Introduction to Cell Biology & Biomembranes (15 decks)

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