Protein Targeting: Endoplasmic Reticulum

Question 1

Where is the protein ER signal sequence located?

Answer 1

N-terminus of proteins destined for the ER

Question 2

When is the ER signal sequence synthesised?

Answer 2

During protein translation

Question 3

How many amino acids long is the ER signal sequence?

Answer 3

16 - 20

Question 4

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

Answer 4

1 or more charged residues and 6-12 hydrophobic residues

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

P54

Question 8

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

Answer 8

P9 / P14

Question 9

Where is the SRP Receptor located?

Answer 9

ER membrane

Question 10

What is the structure of the SRP receptor?

Answer 10

2 subunits (α and β)

Question 11

Which subunit of the SRP receptor binds GTP?

Answer 11

α subunit

Question 12

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

Answer 12

Binding of GTP

Question 13

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

Answer 13

Sec 61 complex

Question 14

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

Answer 14

True.
Occurs as peptide is being made by ribosome

Question 15

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

Answer 15

Signal peptidase

Question 16

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

Answer 16

False.
Post-translational translocation occurs

Question 17

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

Answer 17

False.
Not involved in Yeast

Question 18

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

Answer 18

• Sec 63 complex
• BiP chaperone protein

Question 19

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

Answer 19

Topogenic sequences

Question 20

How many Topological classes of integral membrane proteins are there?

Answer 20

4

Question 21

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

Answer 21

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

Question 22

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?

Answer 22

• 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

Question 23

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

Answer 23

True

Question 24

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

Answer 24

N-teminal side

Question 25

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

Answer 25

C-teminal side

Question 26

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

Answer 26

False. They have multipe TM helices

Question 27

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

Answer 27

Cytosol ## Footnote Signal anchor sequence of **Type II (SA-II)**

Question 28

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

Answer 28

Exoplasmic space ## Footnote Signal anchor sequence of **Type III (SA-III)**

Question 29

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

Answer 29

Hydropathy plot

Question 30

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

Answer 30

Type I

Question 31

What is the function of GPI Transamidase?

Answer 31

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

Question 32

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

Answer 32

* Greater mobility in the membrane * Targeting to particular membrane domains

Question 33

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

Answer 33

* Chaperone proteins * Calnexin * Calreticulin

Question 34

What TWO ways do cells deal with unfolded proteins?

Answer 34

* Unfolded Protein Response * Endoplasmic Reticulum Associated Protein Degradation (ERAD)

Question 35

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

Answer 35

* Misfolded proteins are **translocated backwards** into the cytosol * Degraded by the **proteasome**

Question 36

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

Answer 36

Glycosylation